Increased Micronutrient Requirements during Physiologically Demanding Situations: Review of the Current Evidence

Every day, the human body is exposed to physical and psychological challenges that upset its internal equilibrium. Strenuous activities, daily defense against pathogens or the response to infection, seasonal changes, and recurring natural biological processes (e.g., the menstrual cycle) can all disturb homeostasis. The body brings its internal environment back into balance by the constant interaction of its many regulatory processes, allowing it to adapt to the ever-changing environment. This ability to adapt and respond (referred to as ‘phenotypic flexibility’) is fundamental to maintaining good health. Micronutrients (vitamins and minerals) have key roles in numerous homeostatic processes, enabling the body to produce enzymes, hormones and other substances that are essential for energy production, cell maintenance and repair, immune function and recovery from illness, blood formation, and maintenance of vital organs. Micronutrients are thus crucial to facilitate adequate responses to stressors that may challenge the body’s homeostasis. Micronutrients are generally not produced by the human body, necessitating an adequate daily intake at levels that have been recommended by various governing bodies. However, micronutrient requirements to optimally support homeostasis during daily demanding situations have not been clearly established. This review examines the roles of micronutrients during some of these demanding situations, to help determine whether there may be a rationale for increasing micronutrient intake during these periods to address any increased needs and potentially aid recovery.


Introduction
Homeostasis within the body is a dynamic condition in which the body's equilibrium can shift in response to changing and often demanding conditions, modulated by the constant interaction of the body's many regulatory processes. Homeostasis is continuously being disturbed by physical and psychological stresses, from the demands of work and school or running to catch a bus in time, intense physical exercise, seasonal exposure to temperature or climate extremes, or exposure to bacteria or viruses causing an immune challenge. In most cases, the disruption of homeostasis is mild and temporary and quickly restored by the responses of body cells, but in some cases the disruption may be intense and prolonged. The internal environment of the body is usually brought back into balance by its regulating systems, thus allowing the body to adapt to the ever-changing environment. This ability to respond to a physiological challenge (recently termed 'phenotypic flexibility') is essential to maintaining good health and may be a good indicator of health status [1].
Micronutrients have key roles in numerous homeostatic processes including, for example, those regulating energy metabolism, redox systems, inflammatory responses and immune function [2][3][4]. Inadequate functioning of these processes reduces the phenotypic resilience to daily challenges, with potential detrimental effects on health [1]. It is well-established that chronic and severe micronutrient deficiencies can increase the risk of poor growth, cognitive development, morbidity, and ultimately mortality [1]. However, the health implications of marginal nutritional inadequacies that may occur when the body is in demanding 'everyday' situations or life stages are less established, largely due to a lack of clear clinical indicators or measures that may point towards a suboptimal nutritional status. Essential micronutrients facilitate adequate responses to stressors that may challenge the body's homeostasis via their impact on immune function or energy production, for example, and thus help to maintain homeostasis [2,5]. Inadequate functioning of these homeostatic processes may reduce phenotypic flexibility, initially leading to common symptoms such as tiredness, fatigue, and an impaired immune response [6][7][8]. In the longer term, this may lead to a potentially increased risk of chronic disease [1]. Certain situations can place the body under additional stress, for example when it is exposed to pathogens or succumbs to infection, when extra energy is required during and after strenuous activities, when seasonal changes affect our behavior, or when recurring natural biological processes place the body under extra demand, such as during the monthly female reproductive cycle. In each of these situations, micronutrients play a central role in supporting the biological processes that help maintain and restore homeostasis [1][2][3][4][5] (Figure 1). Therefore, it is of paramount importance to ensure micronutrient intake is sufficient to account for the increased needs or losses associated with these demanding situations. Individuals that are often exposed to increased physiological demands and that don't adapt their micronutrient intakes accordingly may be at increased risk of suboptimal nutritional status, ultimately leading to an impaired health status. This review will provide a detailed overview on the specific micronutrient needs associated with many of these physiologically demanding situations.

Micronutrients are essential for body functions
Micronutrients are vitamins and minerals that are required in miniscule amounts by the body, but which are involved in virtually all metabolic and developmental processes (Table 1).
Vitamins are organic nutrients that generally cannot be synthesized by the body (although some are produced by bacteria in the gastrointestinal tract), and should be ingested daily. Water-soluble vitamins (the B vitamins and vitamin C) are absorbed with water in the gastrointestinal tract and travel freely throughout the body; for the most part, they do not have specific storage sites in the body and are readily excreted once the renal threshold is exceeded. The body therefore needs to be frequently resupplied with water-soluble vitamins. Fat-soluble vitamins (vitamin A, D, E, and K) are absorbed along with other dietary lipids by the small intestine, may be stored in cells, and are not as easily excreted as water-soluble vitamins. Minerals (e.g., calcium, magnesium, zinc, etc.) are inorganic substances that cannot be made by the body and should also be supplied on a frequent basis. They are stored mainly in teeth and bones, in the liver, skeletal muscle, and other tissues. Concentrations of many of the minerals (e.g., calcium) are tightly controlled; when plasma levels are low, the minerals are mobilized from the reserves.
Micronutrients perform a variety of functions that are essential to homeostasis. They enable the body to produce enzymes, hormones and other substances that are required for energy production (e.g., in mitochondrial function), cell maintenance and repair (including division, replication, and growth), immune function and recovery from illness, blood formation, and maintenance and function of the brain, heart, lung, skin, bone, muscle, etc., Vitamins are key to the regulation and coordination of these homeostatic processes, mostly acting as coenzymes (as do minerals such as zinc) or hormones (e.g., vitamins A and D). Vitamins are essential to bone health [9], wound healing [10], the function, development and differentiation of the immune system [11][12][13], and microsomal drug metabolism and detoxification [14].
Apart from the established consequences of chronic vitamin deficiency (e.g., neural tube defects in pregnancy with folate deficiency, pernicious anaemia with low vitamin B12 or folate status, rickets or osteomalacia with vitamin D or calcium deficiency, etc.), a growing body of evidence is beginning to demonstrate the importance of an adequate vitamin status in the maintenance of good health and in the prevention of diseases [1,15,16]. In their ionized form, essential minerals (e.g., calcium, iron, zinc, magnesium, selenium, etc.) have functions that are vital to life. They are structural components of enzymes, neuropeptides, hormones and hormone receptors, with various roles in numerous enzymatic and metabolic reactions, nerve transmission, and maintaining the structure of bones and teeth [17]. Table 1 provides an outline of some of the main roles of essential micronutrients, where they can be sourced, and their storage within the body [7,11,18].

Vitamins
Fat soluble (all require bile salts and some dietary lipids for adequate absorption) A (formed from provitamin beta-carotene, and other provitamins) Maintains general health of epithelial cells, maintaining innate barriers; acts as an antioxidant to inactivate free radicals; essential for formation of photopigments (light-sensitive chemicals in photoreceptors of retina); aids in growth of bones and teeth; important for innate, cell-mediated immunity and antibody response Yellow and green vegetables (beta-carotene), liver and milk (vitamin A). Stored in liver (approx. 900 mg) Some produced by bacteria of gastrointestinal tract; kidneys, liver, yeast, green vegetables, cereal. Stored primarily in the liver and kidneys B6 (pyridoxine) Essential coenzyme for normal amino acid metabolism; assists circulating antibody production; may function as coenzyme in triglyceride metabolism; helps release energy from food; modulates cellular immunity through involvement in nucleic acid and protein biosynthesis Synthesized by bacteria of gastrointestinal tract; salmon, yeast, tomatoes, yellow corn, spinach, whole-grain products, liver, yoghurt. Mostly stored in muscle (but also the liver and brain) (approx. 40-250 mg) -but not readily available B7 (biotin) Essential coenzyme for conversion of pyruvic acid to oxaloacetic acid and synthesis of fatty acids and purines, and utilization of B vitamins Synthesized by bacteria of gastrointestinal tract; yeast, liver, egg yolk, kidneys B12 (cobalamin) Coenzyme necessary for red blood cell formation, formation of amino acid methionine, entrance of some amino acids into the citric acid cycle, manufacture of choline (used to synthesize acetylcholine); essential for metabolism of fats and carbohydrates and the synthesis of proteins; interacts with folic acid metabolism; modulates cellular immunity through involvement in nucleic acid and protein biosynthesis, aids in antibody production Only B vitamin not found in vegetables; liver, kidney, milk, eggs, cheese, meat. Approx. 3-5 mg is stored, mostly (80%) in the liver B9 Folate Component of enzyme systems essential for DNA and RNA; essential for normal production of red and white blood cells; essential metabolic pathways involving cell growth, replication, survival of cells; modulates cellular immunity through involvement in nucleic acid and protein biosynthesis, aids in antibody production Synthesized by bacteria of gastrointestinal tract; green leafy vegetables, liver. Approx. 5-10 mg is stored, 50% in the liver (represents at least 2 months' supply) Zinc Essential component of certain enzymes, including the copper/zincsuperoxide dismutase (a key enzyme in the defense against reactive oxygen species); important in carbon dioxide metabolism and energy metabolism; necessary for normal growth and wound healing, maintenance of epithelial barrier, normal taste sensations and appetite, normal sperm counts in males, and antibody production; involved in protein digestion Found in many foods, especially meat. Approx. 2-3 g stored, 60% in skeletal muscles, 30% in bone, 4-6% in skin, and the remainder in cells; not easily mobilized from reserves and regular intake is important Copper Required for synthesis of haemoglobin, along with iron; necessary for the function of over 30 proteins; a component of coenzymes in the electron transport chain; part of the copper/zinc -superoxide dismutase (a key enzyme in the defense against reactive oxygen species)

Minerals
Eggs, whole-wheat flour, beans, beets, liver, fish, spinach, asparagus. Some stored in liver and spleen Selenium Acts as an antioxidant, and has a role in cellular immunity and antibody production; prevents chromosome breakage and may play a role in preventing certain birth defects Seafood, meat, chicken, grain cereals, egg yolk, milk, mushrooms, garlic. Approx. 30 mg is stored, depending on geographical location (selenium can be found in soil, and thus accumulates in plants); found in all tissues, bound to amino acids and proteins; regular intake needed to maintain adequate reserves Chromium Potentiates normal activity of insulin in carbohydrate and lipid metabolism, promoting glucose uptake by the cells Brewer's yeast, wine, some beers Molybdenum Biological form is molybdenum cofactor, known to function as a cofactor for four enzymes, particularly sulphite oxidase which is crucial for human health (necessary for the metabolism of sulphur-containing amino acids) Legumes (beans, lentils peas) are the richest source, grain products and nuts also good sources; content is dependent on soil content, so varies by geographical location

Recommended vs. actual daily micronutrient requirements
Clearly, an adequate supply of micronutrients is vital to maintain health. But what does adequate mean? The Institute of Medicine (IOM) [19] provides a set of reference values called dietary reference intakes (DRI) that are used to plan and assess nutrient intakes of healthy people, based on age and gender ( Table 2).
These values include the recommended dietary allowance (RDA), which is the average daily level of intake that is deemed sufficient to meet the nutrient requirements of nearly all (97%-98%) healthy people. Other guidelines, usually country specific, provide their own recommendations [20][21][22][23]. It should be noted that these values are recommended to avoid deficiency -they often don't give an indication of the optimum levels required (Figure 2).
There is a scarcity of data to define optimum micronutrient intake levels, which ultimately will vary for different individuals and depend on several factors including genetic differences and lifestyle factors, as well as intended benefit or outcome measures.
In the developed world, it is often assumed that the easier availability of fresh, nutritious foods means that our diets will be varied enough to meet daily micronutrient requirements. However, it appears that levels regularly fall short of those that are recommended. For example, although the mean daily intake reported across Europe was generally within the recommended range [24], the levels reported within each country varied considerably and frequently fell short even of the somewhat conservative levels recommended both within Europe and by the IOM (Table 2).
In particular, it was recognized that reference intake levels for vitamins D and E and folate were not met, and that in most participating countries intakes of calcium, magnesium and iron (women only) were also below recommended levels. Similar findings have been made elsewhere [25,26]. Nutritional status is a complex issue that is dependent on many circumstances related to lifestyle (e.g., level of physical activity, alcohol consumption, vegetarianism/veganism), socioeconomic factors (e.g., poverty/low income, the need to work and/or a hectic, stressful lifestyle, and so an inadequate time for shopping or cooking, an increased consumption of convenient, and often nutritionally-poor food), health (e.g., dieting, medication use), and geographical or mobility factors (e.g., no easy access to nutritious food, areas where food choice is dependent on season). Figure 2: Optimum levels of micronutrients may be much higher than the recommended dietary allowance (RDA) recommended by the Institute of Medicine [19].  It is well established that inadequate micronutrient intake and status increase the risk of adverse health effects [15,16], with the severity of these health effects largely depending on the extent and the duration of the inadequacy. The next sections will describe in detail the various situations which may increase the risk of suboptimal micronutrient status, ultimately leading to impaired health when not addressed appropriately.

The varying requirements for micronutrients in demanding situations
An inadequate intake of micronutrients is known to result in low energy and fatigue [7,27], and can weaken our immune system and reduce resistance to infections [11,12]. Thus, in everyday life it is essential that we consume sufficient amounts of micronutrients to maintain basic health. Furthermore, optimum micronutrient levels are required in the long term to help prevent chronic diseases such as osteoporosis, coronary heart disease, and cancer [15,16]. Yet for a significant proportion of the population, a gap already exists between micronutrient intakes and minimal requirements and recommended levels are not always achieved [24,25]. This gap is likely to become even wider when our bodies are in demanding situations, leading to additional micronutrient needs. Micronutrients have a fundamental role in so many physiological processes that there is an increased need for certain vitamins and minerals when the body is coping with demanding situations (e.g., related to energy metabolism, immunity, seasonal demands, or during periods of hormonal fluctuations such as the menstrual cycle). A closer look at the role and requirements of micronutrients in such situations can help to evaluate the need for an increased intake of certain micronutrients during these times.
Roles of micronutrients in coping with increased energy demands and combating fatigue: Several micronutrients are required in a number of key reactions during the production and metabolism of energy, which is stored in ATP (adenosine triphosphate), the body's energy storage molecule [6] (Figure 3).
The body's preferred dietary source for synthesizing ATP is glucose, which undergoes a series of reactions mainly in the mitochondria, collectively known as cellular respiration: 1) glycolysis, where glucose is broken down to produce pyruvic acid (and ATP); 2) acetyl coenzyme A (CoA) production, where pyruvic acid is prepared for entrance into the citric acid cycle (also known as Krebs cycle); 3) the citric acid cycle, a set of reactions that oxidize CoA to produce carbon dioxide, ATP, nicotinamide adenine dinucleotide plus hydrogen (NADH+H + ) and flavin adenine dinucleotide (FADH 2 ); 4) electron transport chain (ETC) reactions, which oxidize NADH+H + and FADH 2 and transfer their electrons through a series of electron carriers, enabling small amounts of energy to be released to form many molecules of ATP.
During each step of cellular respiration, certain micronutrients play an essential role. In particular, the B vitamins thiamine, niacin, riboflavin and pantothenic acid act as coenzymes and precursors in many cellular functions, and are cofactors in energy production via their role in the citric acid cycle, the ETC and the formation of ATP [7,28]. Thiamine and riboflavin are involved in the citric acid cycle and complexes I and II of the mitochondrial respiratory chain, coenzyme A contains pantothenic acid, biotin is involved in heme biosynthesis (an essential component of the cytochromes involved in the respiratory chain), FAD is derived from riboflavin, vitamin C has a role in the citric acid cycle, converting fat and amino acid to pyruvic acid and thus ATP, iron and sulphur are essential for electron transfer during the ETC, and pyroxidine, cobalamin and folate are necessary to maintain the mitochondrial one-carbon transfer cycles by regulating mitochondrial enzymes [6,28,29]. The importance of micronutrients is clear, and an adequate intake is necessary for optimal energy production.
There are times when we require more energy (e.g., during exercise or other metabolically-demanding tasks such as during physical or even mental work). These increased energy requirements can put us at increased risk of feeling tired and even fatigued. Fatigue, tiredness and low energy are common symptoms in the general population, with a reported prevalence of between 5% and 45% [30,31], and can lead to impaired quality of life and loss of productive work time [30]. Although common causes of tiredness and fatigue include viral illness, upper respiratory infections, iron-deficiency anaemia (for example due to menstrual blood losses), and depression [32], inadequate nutrition also plays a role. Given the importance of micronutrients in energy production and metabolism, it is possible that in the absence of an identifiable underlying disease, an insufficient intake of key micronutrients could cause decreased enzymatic activity, with consequent impaired cellular energy metabolism and an inability to meet metabolic demands, ultimately leading to feelings of tiredness and fatigue [6,7].  Increased energy expenditure means that it is necessary for our bodies to produce more energy in the form of ATP, and the demand for micronutrients involved in this process will be greater; more of the body's already limited stores of micronutrients will be utilized unless they can be replenished. In fact, there is some evidence of a shortage of certain vitamins and minerals among active individuals, including the B vitamins, vitamin C, calcium, iron, zinc, and magnesium [33][34][35][36][37][38][39][40][41][42]. A suboptimal micronutrient status may affect recovery after activities that cause increased energy expenditure. For example, it has been observed that during periods of low vitamin intake there is a significant 10% decrease in maximal aerobic power -a physiological parameter to quantify the body's capability to uptake, transport and utilize oxygen [43]. A deficiency in vitamin C is associated with impaired folate metabolism, which can lead to fatigue and anaemia [33]. Furthermore, vitamin C depletion is associated with reduced work efficiency during submaximal exercise [44]. The exercise intolerance experienced by some individuals may be related to inadequate vitamin C status due to their lower carnitine levels; vitamin C is a required cofactor for carnitine synthesis, while carnitine is needed to transport fatty acids into the mitochondria, where they are oxidized to release cellular energy [44]. Calcium deficiency affects both bone density and muscle contraction, both of which are detrimental to physical activity [34,45]. Magnesium deficiency can occur after prolonged heavy exercise, causing muscle weakness and neuromuscular dysfunction [46,47]. This may be a function of both sweat losses and redistribution of serum magnesium into working muscles [34,37,48]. A low zinc intake can result in functional disturbances in energy metabolism [49], and is associated with impaired muscle function, including reduced strength and increased propensity to fatigue, decreased power output during peak work capacity testing, and decreased cardiorespiratory function [38,[50][51][52].
Supplementing with micronutrients may help to boost energy production and increase energy levels [7,27,28]. For example, daily intake of a multivitamin/mineral supplement for two months in healthy adults significantly increased energy production during demanding tasks compared to placebo [28]. An adequate intake of micronutrients is necessary to cover increased needs for building, repair, and maintenance of lean body mass in physically-active people [33,34], thus aiding recovery from exercise or other strenuous activities [45]. There is evidence to suggest there is a need to increase the intake of B-complex vitamins beyond the RDA after exercise [40]. Several studies have investigated the effects of single micronutrients on fatigue, in particular thiamine and iron, as well as supplementation with a combination of these micronutrients. Supplementation with thiamine led to significantly increased wellbeing, decreased fatigue and increased activity in elderly women [53], and to a positive association with feeling clearheaded, composed and more energetic in a young female population [54]. Although the results in the young females didn't quite reach significance (possibly because they may have had an adequate thiamine status before supplementation, while marginal deficiency would be more common in the elderly female population), supplementation with thiamine does still show a trend in combating fatigue in an adequately nourished population. Iron deficiency, even in the absence of anaemia (i.e., a sign of clinical deficiency), is associated with decreased activity of iron-dependent enzymes, and supplementation with iron in non-anaemic women with unexplained fatigue led to a reduction in fatigue, particularly in women with the lowest baseline levels of serum ferritin [55], the most sensitive marker of iron status [56]. Supplementation with multiple micronutrients also has a positive effect on fatigue. For example, a supplement containing vitamin C, B vitamins, calcium and magnesium has been shown to improve fatigue by 82% [57]. Use of a micronutrient supplement for two months in healthy females was shown to reduce fatigue and improve the speed and accuracy of their multi-tasking ability, indicating that multiple micronutrient supplementation may have a beneficial effect on fatigue and can ultimately help in the completion of demanding tasks [58]. In a subgroup of these women, there was also a significant reduction in homocysteine after supplementation [58]. Homocysteine is the potentially toxic amino acid by-product of onecarbon metabolism, and is an indicator of poor health [58]. Vitamins B6 and B12 and folate play an essential role in lowering homocysteine levels, and the fact that homocysteine levels were high in this seemingly healthy population suggests that marginal micronutrient deficiencies commonly occur even in a relatively normal cross-section of healthy females [58]. Another study showed that in healthy people suffering from self-perceived fatigue and vitamin D deficiency, a single high-dose supplement of vitamin D3 significantly improved fatigue, which also correlated with a rise in 25-hydroxyvitamin D levels, the major circulating form of vitamin D [59]. Magnesium supplementation has also been shown to have effects on the economy of respiratory metabolism and exercise tolerance [37,45,50,[60][61][62], while zinc supplementation has been shown to increase muscle endurance [63]; these findings suggest they may have a role in delaying tiredness and fatigue.
Micronutrient supplementation also has potential benefits in sufferers of chronic fatigue syndrome (CFS), a medically unexplained persistent or recurring fatigue lasting at least 6 months [64]. Evidence suggests that oxidative stress plays a pivotal role in CFS, where excess oxidants (reactive oxygen species) are free to attack cell components [65]. These oxidants are normally neutralized by antioxidants such as vitamins C or E, or endogenous antioxidant enzymes such as glutathione peroxidase or superoxide dismutase (which require copper, zinc, and selenium for proper functioning) [3]. It is possible that the illness itself (rather than an inadequate diet) could be causing CFS patients to be marginally deficient in these micronutrients [66]. Consequently, oxidative stress occurs as a result of diminished antioxidant capacity and/or decreased activity of antioxidant enzymes [65]. In CFS patients, multiple micronutrient supplementation significantly reduced fatigue and superoxide dismutase activity (suggesting that antioxidant activity improved with supplementation) [65]. There were also significant improvements in sleep disorders, autonomic nervous system symptoms (e.g., dizziness, anxiety, etc.), frequency and intensity of headaches, and subjective feelings of infection [65]. It has been suggested that it is rational to consider micronutrient supplementation in CFS patients, at least for a trial period [66].
Clearly, micronutrients have essential roles in the homeostatic regulation of physiological processes involved in energy metabolism and ultimately production. To ensure minimal perturbation to homeostasis following increased levels of metabolic stress, as in the case of energy-demanding activities, a sufficient intake of micronutrients is necessary to reduce tiredness and fatigue and aid an adequate response to recovery.
Roles of micronutrients in the recovery from periods of illness: The human immune system is usually well equipped to fend off attacks from pathogens and protect against infection [12,67]. A crucial factor modulating immune function is nutritional status, and essential micronutrients work in synergy in many of the processes involved in the development, maintenance and expression of the immune response. In particular, vitamins A, D, C and E, folic acid, vitamin B6, vitamin B12, zinc, copper, iron, and selenium all contribute to the body's natural defences by supporting physical barriers (skin and mucosa), cellular immunity, and antibody production [11,12].
If nutritional status becomes compromised, immune functions that are fundamental to protect the host from infectious agents such as bacteria and viruses can be suppressed [68,69]. Pathogens are then more easily able to evade the immune defences, predisposing us to infection and illness. Once the body succumbs to illness, there may be a greater need for micronutrients to support the immune response and allow convalescence. However, during infection the absorption of micronutrients may be impaired, direct micronutrient losses can occur, metabolic requirements or catabolic losses may be increased, and transport of micronutrients to target tissues may be impaired [70]. As a result, there may be a loss of minerals (e.g., potassium), trace elements (e.g., zinc, copper) and vitamins (e.g., vitamins A and B12, folate) [71]. It has been shown that with the onset of infection, serum and plasma levels of calcium, zinc, iron, vitamin A, E and C fall rapidly and return to normal when symptoms improve [72]. Recovery from infection depends on the intensity of the acute-phase immune response [73]. This is a complex early-defines system activated by trauma, infection, stress, and inflammation, and is the basis of the innate immune response involving physical barriers (skin and mucosa) and responses that serve to prevent infection, clear potential pathogens, initiate inflammatory processes and contribute to recovery and the healing process [72,73]. Micronutrients have essential roles in all of these processes [11,12]. Yet during infection a decrease in food intake can also occur [70], which may prevent effective recovery as deficiencies of specific micronutrients will impair the ability of cells in general and the cells of the immune system in particular to function adequately and effectively [71]. Thus, micronutrient supplementation could be beneficial for the recovery process.
The vitamin that has been studied most regarding a role in managing infection is vitamin C in common cold. Vitamin C is found in high concentrations in leukocytes and is readily mobilized during infection; in fact, it may be defined as a stimulant of leukocyte functions, particularly movement of neutrophils and monocytes [11]. Although its role in antibody production isn't clear, supplementation with vitamin C has been demonstrated to stimulate the immune system by enhancing T-lymphocyte proliferation in response to infection increasing cytokine production and synthesis of immunoglobulins [11]. High supplemental intakes of vitamin C stimulate phagocytic and T-lymphocytic activity [13]. Regular supplementation trials [74], and two controlled trials of high-dose vitamin C supplementation [75], have shown that vitamin C can reduce the severity of cold symptoms and the duration of a cold. Controlled trials have also shown treatment benefits of vitamin C in pneumonia and tetanus patients [75]. Given the consistent effect of vitamin C on the duration and severity of colds in regular supplementation studies, and the low cost and safety, it may be worthwhile for common cold patients to test on an individual basis whether therapeutic vitamin C is beneficial for them [74]. Likewise, zinc lozenges may also shorten the duration of a cold by almost 3 days [76], with the greatest effect for high doses of zinc [77], and improve the recovery rate in common cold patients [78]. There is some evidence to suggest that vitamin E may have the potential to reduce the duration of common cold in elderly patients, but the results were nonsignificant [79].
Although few clinical studies have evaluated the effects of supplementation with other micronutrients to reduce the severity of an infection such as common cold, there is a rationale for ensuring that micronutrient levels are at least optimal to aid recovery during an infection. For example, vitamin A deficiency suppresses the activity of natural killer cells, eosinophils, macrophages and neutrophils involved in the innate immune responses that help to clear infection, while supplementation appears to reverse these abnormalities [11,13]. Vitamin A deficiency can also impair the antibody-mediated adaptive immune response [12]. Even marginal vitamin B6 deficiency impairs lymphocyte maturation and growth and is associated with decreased numbers and function of T lymphocytes, which can be corrected with short-term supplementation [11,12]. Vitamin B6 also acts as a coenzyme in the metabolism of antibodies, and deficiency is associated with decreased antibody response [11]. Low levels of vitamin B12 lead to a significant reduction in cells that have a role in cell-mediated immunity [11,13]; because vitamin B12 supplementation reverses these effects, it is thought that it may act as a modulatory agent for cellular immunity [17,80]. Vitamin B12 deficiency may also result in an impaired antibody response [11]. Vitamin D modulates the responses of macrophages, preventing them from releasing too many inflammatory cytokines and chemokines, and increasing the 'oxidative burst' potential of macrophages [17,72,81]. Vitamin D deficiency impairs the immune capabilities of macrophages, including their antimicrobial function, effects that can be mitigated by the addition of calcitriol, the hormonally-active metabolite of vitamin D [81].
A final point to consider is the use of antibiotics during severe colds or infections. Antibiotics have a negative effect on the gut microflora [82,83], which may be ameliorated by the use of probiotics [84,85]. They can also deplete certain micronutrients, including folic acid, iron, vitamins D, K, B1, B2, B3, B6 and B12, calcium, magnesium, and potassium [14,[86][87][88]. Several mechanisms may be involved, including a reduction in micronutrient absorption, complex formation, chelation, enzyme induction, mucosal block or damage, and decreased endogenous production [88]. Thus, it is important to ensure that micronutrient intake is sufficient if antibiotics are used during infection, to avoid adverse effects on homeostasis and potentially aid recovery. This is also the case with many other classes of medication that can also have adverse effects on micronutrient status (e.g., some analgesics may reduce iron concentrations, antacids can reduce vitamin D and folate, while certain hypertensive agents lead to a reduction in vitamin B6) [14,86,87]. In the context of medications affecting micronutrient status, it should also be mentioned that various nutritional components can affect each other via interactions e.g., in the gastrointestinal tract, especially among the minerals: For instance, interactions of nutritional significance include sodium-potassium, calcium-magnesium, manganese-iron, iron-copper, and zinc-copper [89]. However, these interactions often depend on the level of mineral administered with only large amounts administered over longer periods of time impairing nutrient status. For example, most human studies investigating effects of dietary calcium on magnesium absorption have shown no negative effect. Only intakes of calcium in excess of 2,600 mg/day have been reported to decrease magnesium balance [90]. Similarly, only large quantities of zinc (>50 mg/d) over a period of weeks have been shown to interfere with copper bioavailability and ultimately status [91]. It is therefore important to keep track of the levels of minerals ingested over longer time periods.
Seasonal demands on the body: Throughout the year, the temperature and climate in many countries can fluctuate considerably. In the heat of the summer, our bodies may sweat more during thermoregulation and we tend to eat lighter, smaller meals. During the winter, adverse weather means that more time is spent indoors, light levels are lower, and colds and flu are more common. These seasonal variations can place additional demand on the body and have an impact on micronutrient needs.
Winter: During the winter months, the human body is placed under several kinds of stress. For example, most people should be able to get all the vitamin D they need from sunlight, synthesized in the skin from 7-dehydrochlesterol by UV-B radiation. But the effectiveness of vitamin D production is dependent on the intensity and duration of sunlight to which the skin is exposed. Consequently, most people produce less vitamin D during the darker, winter months when not only is there a low ambient UV radiation level, but we spend more time indoors; in some cooler climates, vitamin D synthesis can even cease [92]. Yet even in the summer or in hotter climates, vitamin D status may be inadequate because of excessive use of sunscreen (which blocks the necessary UV-B radiation) or covering up exposed skin, or because of darker skin pigmentation [92].
Both vitamin D deficiency and insufficiency are of growing concern because they are becoming more common in developed countries. It has been estimated that between 20-80% of US, Canadian, and European adults are vitamin D deficient [92], and affects people of all ages and populations [93,94]. There are well-known and much documented clinical consequences of hypovitaminotic D, including muscle weakness, rickets and osteomalacia [92,95], and an increased intake of vitamin D may be necessary to compensate for these potential complications. Vitamin D also has a vital role in innate and adaptive immunity, and most cells of the immune system express vitamin D receptors [96]. A reduction in vitamin D levels can increase the risk of respiratory infections such as influenza and pneumonia across all age groups [97]. The results from several trials suggest that supplementation with vitamin D can reduce the risk of upper respiratory tract infections and influenza [97].
Insufficient vitamin D levels also have a role in mood disorders such as seasonal affective disorder (SAD) [98,99]. SAD is a type of recurring depression with a seasonal pattern; feelings of low mood, irritability, lethargy and an increased amount of time spent sleeping begins and ends during a specific season (usually winter) every year, with full remittance during other seasons [99]. As many as 15-20% of the general population experiencing 'winter blues' , particularly in women and in those in northern latitudes [99]. Vitamin D in particular is thought to have a role in this mood disorder, based on lower vitamin D levels (which fluctuate seasonally in direct relation to the available sunlight) observed in SAD patients and the fact that vitamin D affects the production of both serotonin and dopamine, which both have a key role in SAD [98]. Vitamin D supplementation may help to improve the symptoms of SAD [98], and taking vitamin D before the winter darkness sets in may help to prevent symptoms of depression [100]. Insufficient vitamin D levels may be associated with depression in general, with increasing risk with decreasing vitamin D levels [101], which can be improved with vitamin D supplementation [102].
It is possible that other vitamins (especially the B vitamins and vitamins C, D, and E) and minerals (e.g., calcium, chromium, iron, magnesium, zinc, and selenium) may also have a role in ameliorating mood disorders such as SAD [103]. Although they have not formally been studied in seasonal-dependent mood disorders, micronutrient supplementation has been shown to have beneficial effects on mood in general [103] and can reduce feelings of stress, depression, anxiety and physical or mental fatigue [58,[104][105][106][107][108][109] and increased energy levels, mood, concentration and mental stamina [110][111][112]. Thus, considering their effects on mood in general, it may be that using a micronutrient supplement during the winter months could help to combat the 'winter blues' .
Summer: Contrary to the demands of the darkness and cold of the winter months, summer places other types of strains on the human body. One consideration during the summer months is the increased likelihood of sweating, as the body tries to maintain a constant, optimal temperature of around 37°C -particularly if a person is exercising [113]. Although sweat is composed principally of water, it also contains electrolytes (sodium, chloride, potassium, magnesium, calcium, phosphate, zinc, iron) that cannot be synthesized in the body and need to be replaced via the diet [113,114]. Sweating, whether heator exercise-induced (or both), is associated with a reduced level of several micronutrients (e.g., B vitamins, vitamin C, potassium, calcium, magnesium, iron, zinc) [7,48,[115][116][117][118]. The number of certain micronutrients lost during sweating can be substantial (Table 3) [118], and intake may need to be increased when living and working in a hot environment to ensure that the RDA is met and homeostasis is restored.
Although there does not seem to be an increased requirement for B vitamins above the RDA in such environments, a deficiency could occur if profuse sweating is combined with suboptimal dietary intake [118]. Long-term exposure to a hot environment, especially in those who are not acclimatized, can compromise vitamin C status and supplementation may be useful in such cases [118]. Exposure to hot temperatures can amplify the increased rate of mineral loss that occurs in sweat with exercise, and marked changes in the metabolism of certain minerals (chromium, copper, iron, magnesium, zinc) have been observed after prolonged, strenuous exercise [118]. It is unclear whether mineral losses resulting from chronic heat exposure or exercise, or both, result in compromised health and performance (endurance capacity, immune defines, antioxidant response, or recovery from illness or trauma) [118].

Micronutrient Estimated daily loss in sweat, mg
Vitamin B1 (thiamine) 0-1.  Table 3: Daily micronutrient loss during sweating in a hot environment [118]. * Based on the fact that working in a hot environment can produce sweat losses of up to 10 liters per day.
Nevertheless, as more sweat is lost the body can become dehydrated and the loss of vital electrolytes can lead to symptoms of tiredness and fatigue, as well as weakness and muscle cramps [119]. Although drinking water can ameliorate these symptoms, water alone is not enough to restore the concentration and composition of the lost electrolytes and cannot replenish micronutrients. Instead, it is important to increase micronutrient intake in situations where there is an increased likelihood of micronutrient loss through intense sweating. Although the increased nutritional demands after sweating are normally met by professional athletes, for example (whose diets are strictly controlled) [120], this may not be true after moderate physical exercise or in the general population when the weather becomes warmer. Eating appears to be a major contributor to maintaining body heat, and if normal food intake continues under conditions of heat stress, the additional heat that must be dissipated may lead to a breakdown in the body's heat mechanisms [118]. Thus, a reduction in food intake, which is often observed in hotter climates, may actually be a mechanism to cope with hot conditions [118]. Ultimately, the combination of increased micronutrient losses due to intense sweating and a decreased micronutrient intake due to lower overall food intake in hot conditions may lead to an increased risk of micronutrient inadequacies [118].
In people working hard or exercising during the summer months or in hot conditions, supplementation may be required to replenish the electrolytes and micronutrients that may be lost via sweating (e.g., sodium, chloride, potassium, the B vitamins, vitamin C, iron, iodine, magnesium, zinc, copper), particularly if dietary intake decreases, to help restore micronutrient status to homeostatic levels. There may also be a role for antioxidant vitamins (A, C, and E) in reducing lipid peroxidation induced by exercise in a hot environment [118].
Additional demands during the female reproductive cycle: Besides regular seasonal changes placing demands on the body's homeostatic mechanisms to resist various stressors, females of reproductive age are also confronted with regular cyclical changes due to endocrine control of the female reproductive system. This cycle, commonly referred to as the menstrual cycle, is characterized by fluctuations in various hormones that have a broad influence on overall wellbeing, mood, energy levels, and ultimately micronutrient needs. Most women will recognize the feelings of tiredness and fatigue that can occur during menstruation. These are commonly linked to a low iron status [121,122]. It is relatively difficult to remove iron from the body pool once it has entered the body, apart from bleeding -which naturally occurs in women during their period every month [121]. All menstruating women carry an increased risk for iron deficiency due to monthly blood loss [123]. Iron status varies throughout the menstrual cycle [124], but around 10-40 mg of iron is lost in the blood every month [121] and serum ferritin levels are inversely correlated with the duration of menstrual bleeding [125]. Although the prevalence of irondeficient anaemia has slightly decreased in menstruating women over the years, aided by the use of iron-fortified foods and oral contraceptives (which reduces monthly blood loss) for example, iron deficiency still persists in this population [126]. This may in part be due to significantly lower energy and nutrient intakes that are observed [127]. Inadequate iron intakes are also more likely in female athletes and strict vegetarians [123,128]. Iron supplementation may be necessary in many cases, particularly if menstruation is heavy or prolonged [125]. Iron, of course, is also essential for blood formation, with an essential role in the formation of red blood cells (haematopoiesis) and haemoglobin; about 70% of iron in the body is bound to hemoglobin and facilitates transport of oxygen from the lungs to the rest of the body [129]. An adequate intake of vitamin C is also necessary, as ascorbic acid is known to enhance iron absorption [130]. The metabolism of copper is intertwined with that of iron, and copper deficiency generates cellular iron deficiency [131]. Several other micronutrients have roles in restoring the blood and tissue loss experienced during menstruation. Folate is essential for metabolic pathways involving cell growth, replication and the survival of cells in culture and contributes to normal blood formation [132]. Vitamins B6 and B12 also contribute to the formation of red blood cells: vitamin B12 is required for normal erythrocyte production [133,134], while vitamin B6 also contributes to heme biosynthesis [132]. Vitamin D contributes to normal cell division, and influences the action of many genes that regulate the proliferation, differentiation and apoptosis of cells [135]. Magnesium serves as a regulator of many physiological functions including the maintenance of cellular membrane stability [37], while zinc is also required for normal cell division [136].
Cumulatively all these micronutrients influence red blood cell formation and are therefore critical at times of increased blood losses, as is the case during menstruation.
Menstruation can also induce symptoms of premenstrual syndrome (PMS). Over 150 symptoms have been associated with PMS, but irritability, depression, and fatigue are the most important and most frequently reported affective symptoms [137]. Between 8-20% of women are thought to be affected by PMS [138,139], which is associated with a high and probably underestimated socioeconomic burden [140,141]. Although the causes of PMS are not fully understood, it is likely that micronutrients play a role. For example, calcium intake may contribute to symptoms associated with PMS, and PMS shares some of the same symptoms as hypocalcaemia, including fatigue, depression, anxiety and muscle cramps [142]. A higher intake of both calcium [143][144][145] and vitamin D (which is necessary for calcium absorption, and which is often deficient in many women) [146,147] has been associated with a lower risk of PMS symptoms. There is also substantial evidence for a role of B vitamins in the pathophysiology of PMS, although the mechanisms are not fully understood. Proposed theories for the pathophysiology of PMS include an interaction of ovarian hormones with neurotransmitters such as serotonin and gamma-amino butyric acid (GABA) and other endocrine systems [148][149][150]. It is thought that thiamine, riboflavin, niacin, vitamin B6, folate, and vitamin B12 are potentially involved in the numerous metabolic pathways of GABA and serotonin [151,152], and their deficiency may play a role in the development of PMS. Supplementing with vitamin B6 has been shown to alleviate PMS symptoms [153][154][155], but studies of other vitamins of the B complex are restricted to those that contain a wide variety of micronutrients and the individual effects of each vitamin cannot be isolated [156][157][158]. During micronutrient supplementation, the menstrual cycle becomes more regular indicating that one of the factors contributing to irregular cycles might be subclinical micronutrient insufficiencies [159]. Low levels of magnesium [160] and zinc [161] have been reported in affective disorders such as PMS which, along with vitamins D and E and manganese, may also play a role in the symptoms of PMS, in reducing blood loss during menstruation, and the severity and duration of menstrual pain [162][163][164][165]. An increased intake of minerals and vitamins has been recommended for prevention and treatment of PMS [166,167].
Use of oral contraceptives can reduce blood and tissue loss and symptoms of PMS during menstruation. However, it should be noted that oral contraceptives can have adverse effects on the plasma levels of several micronutrients, including vitamins E and B12, folate, betacarotene and coenzyme Q10 [168][169][170]. Therefore, for women on oral contraceptives regular supplementation with multiple vitamins and minerals may help to restore micronutrient levels and aid homeostasis.

Summary
Clearly, micronutrients have a fundamental role in numerous physiological functions, many of which help to regulate the internal environment and enable the body to respond to stressors that may challenge homeostasis. For example, B vitamins are involved in energy production and metabolism, along with vitamin C, iron and several other essential micronutrients. Increased energy expenditure during strenuous activities can lead to a shortage of B vitamins, vitamin C, calcium, iron, zinc, and magnesium, and a suboptimal micronutrient status may affect recovery from such activities. Supplementing with micronutrients during these times can help to boost energy production and increase energy levels. When we become ill, the absorption and transport of micronutrients may become impaired, metabolic requirements may be increased, and direct micronutrient losses can occur. Given the established importance of micronutrients in immune function, it is likely that suboptimal levels during infection will have an impact on recovery. Supplementation with vitamin C or zinc has been shown to reduce the duration and/or severity of common colds, but few other studies have evaluated the effects of supplementation with other micronutrients. Nevertheless, there is a good rational for ensuring that micronutrient levels are at least optimal to aid recovery during an infection. During the winter, vitamin D supplementation may be necessary to ameliorate the clinical consequences of a low vitamin D status, and reduce the risk of respiratory infections, for example, or season-dependent mood disorders such as SAD. Again, there is a rationale for ensuring adequate levels of micronutrients other than vitamin D, particularly the B vitamins, which may have a beneficial effect on mood. In the heat of summer, the body sweats more, particularly during demanding activities, and vital micronutrients and electrolytes are lost (e.g., sodium, chloride, B vitamins, vitamin C, potassium, calcium, magnesium, iron, zinc). Intake may need to be increased when living and working in a hot environment to ensure that homeostasis is restored. During the challenges faced by women every month as part of the reproductive cycle, tiredness and fatigue that are commonly experienced may be caused by a low iron status. Adequate levels of vitamin C, copper, magnesium, zinc and several B vitamins are also essential to maintain homeostasis during this demanding time. Furthermore, suboptimal levels of calcium, manganese, vitamins D and E, and certain B vitamins may play a role in the development of PMS symptoms. Micronutrient supplementation may be necessary in many women to aid in the restoration of homeostasis and to help combat tiredness, fatigue and PMS.
Therefore, when the body is under additional stress from any external factors that can adversely impact homeostasis, micronutrient supplementation may facilitate numerous physiological processes that occur in response to the homeostatic challenges, helping to restore the internal environment and aid recovery.