Impact of Supplementation and Nutritional Interventions on Pathogenic Processes of Mood Disorders: A Review of the Evidence

This narrative review was conducted using searches of the PubMed/Medline and Google Scholar databases from inception to November 2019. Clinical trials and relevant articles were identified by cross-referencing major depressive disorder (and/or variants) with the following terms: folate, homocysteine, S-adenosylmethionine (SAMe), L-acetylcarnitine, alpha-lipoic acid, N-acetylcysteine, L-tryptophan, zinc, magnesium, vitamin D, omega-3 fatty acids, coenzyme Q10, and inositol. Manual reviews of references were also performed using article reference lists. Abnormal levels of folate, homocysteine, and SAMe have been shown to be associated with a higher risk of depression. Numerous studies have demonstrated antidepressant activity with L-methylfolate and SAMe supplementation in individuals with depression. Additionally, the amino acids L-acetylcarnitine, alpha-lipoic acid, N-acetylcysteine, and L-tryptophan have been implicated in the development of depression and shown to exert antidepressant effects. Other agents with evidence for improving depressive symptoms include zinc, magnesium, omega-3 fatty acids, and coenzyme Q10. Potential biases and differences in study designs within and amongst the studies and reviews selected may confound results. Augmentation of antidepressant medications with various supplements targeting nutritional and physiological factors can potentiate antidepressant effects. Medical foods, particularly L-methylfolate, and other supplements may play a role in managing depression in patients with inadequate response to antidepressant therapies.


Introduction
Major depressive disorder (MDD) is among the most common and debilitating medical conditions [1]. Since the 1950s, the monoamine theory influenced the development of mainstay antidepressant treatments, including selective serotonin reuptake inhibitors (SSRIs) and serotonin norepinephrine reuptake inhibitors (SNRIs) [2]. However, more recently, other biochemical and physiological factors have been shown to contribute to the pathophysiology of MDD [3]. As a result, there are a growing number of novel MDD treatment options, including nutritional augmentation strategies, that have become available based on this evolving knowledgebase [4].
This narrative review will discuss how nutritional imbalances, in particular, affect pathogenic processes in MDD and other mood disorders and will review evidence for nutritional interventions with an emphasis on essential and conditionally essential nutrients found in the body, including established medical foods, amino acids, and other supplements. well beyond infancy, particularly with inflammatory conditions that result in psychiatric disorders, including depressive disorders [10,30,31]. Inflammation and depressive symptoms have a bidirectional relationship, facilitating and promoting one another, as evidenced by elevation in various inflammatory biomarkers in a subgroup of susceptible MDD patients. [19,22,32] From a mechanistic standpoint, inflammation can trigger activation of microglia and subsequently release proinflammatory cytokines, which induce depressive symptoms by altering production, metabolism, and transport of neurotransmitters that affect mood (e.g., dopamine, glutamate, serotonin) [33,34].
Folate deficiencies may cause depressive symptoms by elevating homocysteine and intracellular one-carbon metabolism ( Figure 1) [10,28]. The one-carbon cycle is critical to the numerous transmethylation processes occurring in the CNS and is involved in metabolizing monoamine neurotransmitters such as serotonin, norepinephrine, and dopamine [35,36]. Folic acid must first be converted by 5,10-methylenetetrahydrofolate reductase (MTHFR) to the metabolically active form, L-5-methyl-tetrahydrofolate (MTHF), which is the only form of folate that crosses the blood-brain barrier (BBB). [28,35,37] Upon entering the CNS, MTHF acts as the methyl donor in numerous methylation-dependent processes, including the methylation of homocysteine to form methionine and S-adenosylmethionine (SAMe). Methionine is an essential amino acid and acts as the substrate for SAMe, a methyl group donor in more than 100 methylation reactions in the body [28]. Methionine also is a precursor to glutathione, a naturally occurring antioxidant that modulates glutamate activity and maintains cellular oxidative balance by scavenging and neutralizing reactive oxygen and nitrogen species [38][39][40]. ents 2021, 13, x FOR PEER REVIEW 3 of 25 connection to brain function [29]. Altered vitamin B12 levels also are associated with issues well beyond infancy, particularly with inflammatory conditions that result in psychiatric disorders, including depressive disorders [10,30,31]. Inflammation and depressive symptoms have a bidirectional relationship, facilitating and promoting one another, as evidenced by elevation in various inflammatory biomarkers in a subgroup of susceptible MDD patients. [19,22,32] From a mechanistic standpoint, inflammation can trigger activation of microglia and subsequently release proinflammatory cytokines, which induce depressive symptoms by altering production, metabolism, and transport of neurotransmitters that affect mood (e.g., dopamine, glutamate, serotonin) [33,34]. Folate deficiencies may cause depressive symptoms by elevating homocysteine and intracellular one-carbon metabolism ( Figure 1) [10,28]. The one-carbon cycle is critical to the numerous transmethylation processes occurring in the CNS and is involved in metabolizing monoamine neurotransmitters such as serotonin, norepinephrine, and dopamine [35,36]. Folic acid must first be converted by 5,10-methylenetetrahydrofolate reductase (MTHFR) to the metabolically active form, L-5-methyl-tetrahydrofolate (MTHF), which is the only form of folate that crosses the blood-brain barrier (BBB). [28,35,37] Upon entering the CNS, MTHF acts as the methyl donor in numerous methylation-dependent processes, including the methylation of homocysteine to form methionine and S-adenosylmethionine (SAMe). Methionine is an essential amino acid and acts as the substrate for SAMe, a methyl group donor in more than 100 methylation reactions in the body [28]. Methionine also is a precursor to glutathione, a naturally occurring antioxidant that modulates glutamate activity and maintains cellular oxidative balance by scavenging and neutralizing reactive oxygen and nitrogen species [38][39][40].  [38]. BHMT, betaine:homocysteine methyltransferase; CBS, cystathionineβ-synthetase; DMG, dimethylglycine; GSH, glutathione; Hcy, homocysteine; MAT, l-methionine S-adenosyltransferase; MTHF, 5-methyltetrahydrofolate; MTHFR, 5-methyltetrahydrofolate reductase; SAH, S-adenosylhomocysteine; SAHH, S-adenosylhomocysteine hydrolase; SAMe, S-adenosylmethionine; THF, tetrahydrofolate.
Augmenting antidepressants with folate may benefit MDD patients who are predisposed to or already have low folate levels. Folic acid supplementation can improve depressive symptoms, cognition, and oxidative imbalances, and induce hippocampal neurochemical changes [41]. Despite these positive outcomes, L-methylfolate (i.e., L-5methyl-tetrahydrofolate) may be a better alternative than folic acid due to its ability to pass the BBB [28,37]. Unlike folic acid, L-methylfolate does not carry the risk of masking symptoms of vitamin B 12 deficiency and may have fewer drugs interactions that inhibit dihydrofolate reductase [37].
There was an inverse correlation between severity of MDD based on 17-item HAM-D scores and ALCAR concentrations (p = 0.04, r = 0.35). LAC also was shown to be predictive of moderate to severe MDD (p = 0.04). Furthermore, earlier age of onset of depression correlated with lower concentration of LAC (p = 0.04). Additionally, patients with MDD and a history of TRD were associated with a decrease in LAC levels. Randomized, open-label, crossover study evaluating the effects of magnesium supplementation on symptom improvement in mild-tomoderate depression 126 patients Unadjusted PHQ-9 depression scores improved with magnesium supplementation (−4.3 points, 95% CI −5.0 to −3.6), with a net improvement of −4.2 points. Unadjusted GAD-7 anxiety scores also improved with magnesium (−3.9 points, 95% CI −4.7 to −3.1), with a net improvement in anxiety of −4.5 points.

N/A
The most common side effect was diarrhea, which was reported by 8 participants.  MTHFR polymorphisms are also implicated in the development of depressive symptoms and reduce the ability to adequately synthesize monoaminergic neurotransmitters; thus the effectiveness of SSRIs and SNRIs may be limited in these patients [62][63][64]. Realworld evidence of adjunctive L-methylfolate has demonstrated increased response and symptom improvement in patients with treatment-resistant depression [65], particularly in those with the MTHFR gene mutation [66], with a favorable safety profile [67]. However, it is important for clinicians to exercise caution when using MTHFR gene testing to guide their prescribing strategy, but to rather use it in a confirmatory fashion, as the gene tests may not always be accurate in all cases and may lead to over-methylation with folate supplementation. An American Psychiatric Association research council task force published the following statement in the American Journal of Psychiatry in 2018: "They find that, at present, there are insufficient data to support the widespread use of combinatorial pharmacogenetic testing in clinical practice, although there are clinical situations in which the technology may be informative, particularly in predicting side effects [68]." L-methylfolate is generally safe and well tolerated, and lacks the side effects of other standard-of-care therapies for depression [46]. As an adjunct, L-methylfolate has been shown to cause minimal adverse events [42,46]. Potential candidates for L-methylfolate augmentation include treatment-resistant patients with signs of inflammation, hyperhomocysteinemia, and genetic polymorphisms, or low serum folate values.

Homocysteine
Although homocysteine is not considered a nutritional factor, it is an inflammatory marker that may indicate vitamin B 12 and folate methylation disturbances [69]. Homocysteine is an amino acid that cannot be obtained through diet and can only be formed and removed through the methylation cycle ( Figure 1) [38]. In the methylation cycle, S-adenosylhomocysteine (SAH) is converted to homocysteine by SAH-hydrolase within the cell. This conversion is reversible; however, under normal physiologic conditions, homocysteine is quickly removed and concentrations are usually low. One way that low intracellular homocysteine concentrations are maintained is through methylation of homo-cysteine via receipt of a methyl group from 5-MTHF, which then forms methionine and tetrahydrofolate. This requires vitamin B 12 for transferring the methyl group. Notably, methylation of homocysteine in the CNS can only occur with MTHF as the methyl donor.
Homocysteine is an indicator of select B vitamin deficiencies, as folate and vitamin B 12 are involved in the conversion of homocysteine to methionine [38,70]. Hyperhomocysteinemia (i.e., an increase in serum homocysteine concentration above the normal plasma or serum homocysteine level) confers risk for vascular disease, impaired bone remodeling, cancer, Parkinson's disease, Alzheimer's dementia, mental retardation, and signs and symptoms of neurological dysfunction, including MDD and schizophrenia [38,[70][71][72][73]. Normal blood homocysteine levels are 4-15 µmol/L, while levels above 15 are considered high and levels below 12 are considered low. Optimal homocysteine levels are below 10-12 [74]. Hyperhomocysteinemia will ultimately result in neurotoxicity secondary to impaired methylation, excitotoxicity, oxidative stress, and CNS ischemia [38]. Homocysteine is critical for producing neurotransmitters; therefore, altered homocysteine levels may impact mood [70]. Additionally, hyperhomocysteinemia can increase the permeability of the BBB, and can contribute to numerous cerebrovascular pathologies [75].
Elevated mean (SD) plasma homocysteine levels (17.7 [5.4] µmol/L) have been correlated with higher Hamilton Depression Rating Scale (HAM-D) scores, indicating a link between homocysteine dysregulation and depressive symptoms [10]. These patients also had significantly lower red cell and serum folate levels. Plasma homocysteine levels ≥15.0 µmol/L have been linked with depressive symptoms [76][77][78][79]. Testing homocysteine levels in depressed individuals can identify those with folate or vitamin B 12 deficiency and determine whether patients would benefit from folate supplementation. Expert recommendations include testing serum levels of vitamin B 12 , folate, and methylmalonic acid (MMA) [80]. Folate and/or vitamin B 12 supplementation is expected to decrease homocysteine, which may contribute to improved methylation and neurotransmitter metabolism and release [38].

S-adenosylmethionine (SAMe)
SAMe can be considered an initial MDD treatment for patients who prefer complementary or alternative approaches; however, it can induce mania in depressed patients on the bipolar spectrum [28,[81][82][83][84]. SAMe is a naturally occurring methyl donor involved in over 100 methyltransferase reactions for critical metabolic pathways, including methylation of DNA bases, proteins, phospholipids, free amino acids, catecholamines, and neurotransmitters. [81,82,84] DNA methylation acts to turn off gene transcription, while demethylation is linked to transcriptional activation; therefore, aberrant methylation can negatively impact CNS disorders [81,85]. SAMe is generated from L-methionine in the one-carbon cycle, which is dependent upon sufficient levels of folate and vitamin B 12 , both of which are linked to depressive symptoms [83].
There are several mechanisms potentially responsible for the antidepressant activity of SAMe. As a donor of methyl groups, SAMe may exert antidepressant effects through the methylation of plasma phospholipids, which results in alteration of the neuronal membrane fluidity and function of proteins in the membrane, including monoamine receptors and transporters [84]. Lower SAMe levels may cause a decrease in monoamine synthesis, thereby increasing risk of depression. In animal models, increased SAMe levels were positively correlated with monoamine neurotransmitter concentrations in the brain [28]. SAMe may be taken orally up to 1600 mg/d, a significantly bioavailable and non-toxic dose [49]. Patients receiving SAMe 800 mg/day in addition to their normal antidepressant have shown significantly greater changes in HAM-D scores from baseline and higher remission rates compared with placebo (p < 0.05) ( Table 1) [47]. In a systematic review, all studies demonstrated a significantly positive effect for SAMe up to 1600 mg on the HAM-D in MDD [48]. SAMe produces a rapid onset of action, with nearly a 5-point difference in HAM-D scores from baseline to week 1 [47]. In a Cochrane systematic review, 8 trials that included 934 participants investigated the effects of SAMe versus placebo or SSRIs in MDD [49]. Compared with placebo, SAMe was more efficacious as an adjunctive treatment in terms of response and remission; however, the level of evidence was low.
SAMe is generally well tolerated and has a favorable safety profile. Frequently reported adverse events associated with SAMe are nausea, diarrhea, and abdominal discomfort [81]. As previously mentioned, SAMe may induce mania and hypomania, even in patients without a previous history of bipolar disorder [49].

L-acetylcarnitine (LAC)
L-carnitine is an essential nutrient found in almost all tissues of the human body, including the brain [86]. Intracellular carnitine levels are depleted under specific circumstances such as diabetes, hemodialysis, and carnitine deficiency secondary to genetic conditions that requires supplementation [86,87]. L-carnitine facilitates the transfer of activated long chain fatty acids (LCFA) through the carnitine shuttle, which is a series of reactions that transport fatty acids into the mitochondria, as acyl-carnitine ester, for β-oxidation. The carnitine shuttle prevents buildup of harmful LCFA and long chain acyl-coenzyme A (acyl-CoA) [86]. Carnitine also assists with transferring toxic compounds out of the mitochondria [87].
LAC is the acetyl derivative of L-carnitine with 2 carbons in the acyl moiety and is commonly found in plasma and body tissue [87]. Carnitine, along with LAC, passes through the BBB and accumulates in the cerebral cortex, of which 10-15% is the LAC moiety. LAC is important for metabolic processes, such as modulating glucose metabolism, stimulating glycogen synthesis, increasing plasma adenosine triphosphate (ATP) concentration, and improving neurological function.
Although the exact mechanism of LAC for managing depressive symptoms is unclear, it has been hypothesized that its neuroplasticity effect, neurotransmitter regulation, and metabotropic glutamate (mGlu) receptor upregulation likely contribute [86,88]. LAC has neuroprotective, anti-inflammatory, and antioxidant properties that also may improve depressive symptoms. In animal depression models, LAC rapidly improved depressivelike behaviors, restored glutamate levels, and increased type 2 mGlu (mGlu2) expression through epigenetic modification, specifically histone acetylation [25]. In a recent study, lower LAC levels were reported in MDD patients compared with controls (Table 1) [50]. Furthermore, lower LAC levels in patients with MDD were associated with greater severity and earlier onset of depressive symptoms. A significantly greater proportion of patients with treatment-resistant depression had a decrease in LAC levels (p = 0.01), indicate a potential for augmenting antidepressants with LAC. A meta-analysis of studies investigating the effects of LAC on depressive symptoms showed that LAC was significantly decreased depressive symptoms compared with placebo (standardized mean difference [SMD] −1.10; 95% confidence interval [CI] −1.65, −0.56; p < 0.001) [51]. Furthermore, patients taking LAC had a comparable rate of adverse events (AEs) as those taking placebo/no interventions (odds ratio [OR] 0.86; 95% CI: 0.46, 1.63; p = 0.648) and a significantly lower risk of AEs compared with antidepressants (OR: 0.21; 95% CI: 0.12, 0.36; p < 0.001), with lower rates of gastrointestinal and nervous system AEs [51]. Further investigation is needed to fully characterize the safety of LAC due to the limited data [87]. Individuals who may benefit from LAC supplementation are likely those with lower LAC levels and elevated inflammatory markers.

Alpha-Lipoic Acid (ALA)
ALA is a lipoamide that is synthesized from octanoic acid in the mitochondria and can be obtained through the diet [89]. Once absorbed from the diet, ALA is reduced to dihydrolipoic acid (DHLA) and accumulates mainly in skeletal muscle, liver, and heart, and can cross the BBB [89,90]. ALA plays an important role in mitochondrial energy metabolism by acting as a necessary cofactor for mitochondrial α-ketoacid dehydrogenase reactions [89,91,92]. Additionally, ALA functions as an antioxidant by scavenging reactive oxidative species; chelating transition metals (e.g., iron, copper); and enhancing activity and synthesizing endogenous antioxidants or antioxidant enzymes. DHLA is among the most potent naturally occurring antioxidants and can regenerate other endogenous antioxidants, neutralize free radicals, and chelate metals that contribute to oxidative stress [89]. ALA also significantly reduces cytokine-induced inflammation by decreasing production of IL-6, IL-1β, and TNF-α (p < 0.05) [93,94]. Individuals receiving ALA 300 mg experienced a 15% significant reduction in IL-6 levels compared with placebo (p < 0.001) [89,95].
ALA has been investigated as augmentation therapy for MDD, given its biological properties and potential role in the pathophysiological factors involved in mood disorders ( Table 1). Augmenting desvenlafaxine with ALA in mice was associated with significantly greater improvement in depressive symptoms compared with either treatment alone, demonstrating a potentiating effect of these products together [96]. Combination treatment of ALA and LAC also has been studied in MDD in light of their role in modulating mitochondrial function and metabolism, and neuroprotective effects; however, studies have reported inconsistent results [87]. Together, LAC and ALA have been shown to reduce the number of damaged neuronal mitochondria and increase intact hippocampal mitochondria, thereby improving brain function [97,98]. In preclinical studies, LAC and ALA reversed age-related increase in oxidants, reduced oxidative damage in the brain, and improved metabolic rate and physiological activity without causing further oxidation [99]. Conversely, a randomized controlled trial of LAC plus ALA versus placebo in humans reported no significant differences in Montgomery-Åsberg Depression Rating Scale (MADRS) score between groups [52].

N-acetylcysteine (NAC)
NAC, the acetyl derivative of cysteine, is a glutathione precursor that is known as an antidote for paracetamol overdose [100,101]. A critical role of NAC is restoring cellular glutathione concentrations by providing cysteine in glutathione production [39]. The brain is susceptible to various reactive oxygen species that can cause oxidative cellular dysfunction; thus, oxidative stress is implicated in the pathogenesis of mood disorders [101]. In addition to glutathione replenishment, NAC has been shown to have anti-inflammatory activity by reducing inflammatory cytokines in the brain, which is a potential mechanism for how NAC exerts antidepressant effects [39,102]. NAC plays a role in neurotransmission by modulating glutamate pathways and regulating dopamine release [39].
Recently, NAC has emerged as a potential supplemental treatment for psychiatric and neurological disorders, including MDD (Table 1). A significantly greater proportion of MDD patients receiving NAC responded to treatment and reached remission (p < 0.05) in a study comparing adjunctive NAC with placebo [55]. In a meta-analysis conducted by Fernandes et al., NAC improved depressive symptoms and functionality compared with placebo over a follow-up of 12-24 weeks; however, NAC did not improve quality of life [54]. In another meta-analysis of adjunctive NAC in MDD, there was no significant difference in efficacy between NAC and placebo, but individuals treated with adjunctive NAC showed a positive trend towards efficacy, especially in those with higher MADRS scores [53]. Notably, patients in the NAC group reported more gastrointestinal (33.9% vs. 18.4%; p = 0.005) and musculoskeletal complaints compared with placebo (3.9% vs. 0%; p = 0.025). Furthermore, adjunctive NAC has been demonstrated to improve symptom severity, function, and quality of life in MDD and major depressive episodes in bipolar disorder [55,56]. Doses ranging from 1-3 g have been studied in patients with MDD [54,55,101,103] Doses of 2400-3000 mg/day have been found to be safe and effective for obsessive-compulsive and related disorders [103], NAC is generally well tolerated with no severe AEs reported in studies [101]. The most common AEs associated with NAC were gastrointestinal, neurological, psychological, musculoskeletal, and dermatological. Individuals with glutathione deficiency may be potential candidates to supplement their current antidepressant treatment with NAC.

L-tryptophan
L-tryptophan is an essential amino acid used for protein synthesis and serotonin biosynthesis [104]. Tryptophan undergoes degradation through the kynurenine pathway, where it is converted to kynurenine and ultimately serotonin. It has been suggested that an impairment of neuroprotective components of the kynurenine metabolic pathway plays a role in depression, as evidenced by lower tryptophan availability, higher tryptophan breakdown, and lower mean plasma kynurenic acid concentration in MDD patients [105]. Alterations in tryptophan levels can impact serotonin synthesis and mood [106]. An increase in HAM-D score was reported following acute tryptophan depletion in patients with a history of MDD [107]. Another study also reported significantly increased HAM-D scores (p < 0.0009) in medication-free remitted MDD patients [108]. Low tryptophan levels and increased levels of its detrimental catabolites, kynurenine and quinolinic acid, in plasma are associated with the development of depressive disorders [104,109]. Additionally, elevations in IL-6 in models of depressive symptoms are due to increased HPA activity, thereby cortisol and activating tryptophan 2,3-dioxygenase (TDO), which produces more tryptophan catabolites and less serotonin [110]. In the presence of inflammation, tryptophan produces kynurenic and quinolinic acid; therefore, it must be used with caution in patients with inflammation.
High L-tryptophan doses can result in mild nausea, tremor, dry mouth, and dizziness [111,112]. Tryptophan should be used cautiously with monoamine oxidase inhibitors and SSRIs, which increase the risk of serotonin syndrome, and occurs when there is excessive synaptic serotonin in the brain [112,113]. Serotonin syndrome typically presents with tremor, hyperreflexia, autonomic irregularities, and change in mental status (e.g., agitation, restlessness, delirium, confusion) [113]. Other serotonin-elevating drugs include SSRIs, SNRIs, tricyclic antidepressants, St. John's wort, and pain medications.
Despite the compelling evidence from tryptophan depletion studies that suggest that tryptophan is associated with depressive symptoms, [104,107,109] the actual relationship between tryptophan and the pathophysiology of MDD has not been established. Furthermore, tryptophan must be used with caution when taken with medications that increase serotonin, a common mechanism of action of numerous standard-of-care treatments for MDD. In addition to being mindful of concomitant medications, prescribers and patients must consider the 2-to 3-times daily dosing that may pose a compliance issue.

Zinc
Zinc is an essential trace element that is involved with a number of vital CNS biochemical and physiological processes, thereby facilitating proper brain development and function [114,115]. Zinc primarily acts as a cofactor for over 300 enzymatic processes and is involved in gene transcription and replication, DNA repair, cell growth, neurogenesis, neuronal development, maintaining oxidative balance, and protein synthesis [115,116]. Additionally, zinc is a modulator of immune and inflammatory processes, and affects inflammatory cytokine levels [115]. Importantly, zinc has been identified as an antagonist of the N-methyl-D-aspartate (NMDA) receptor, thereby downregulating glutamate response. [114,116,117] Maintaining appropriate zinc levels is critical in brain regions involved in depressive symptoms including the cerebral cortex, hippocampus, and amygdala [114]. Depleted zinc levels enhance HPA activity, which causes a surge in glucocorticoids, ultimately inducing hippocampal dysfunction and behavioral abnormalities [118]. Hyperactivation of the HPA axis can cause an imbalance in serotonergic and noradrenergic circuits, affecting mood [33,118]. Zinc deficiency may contribute to developing depressive disorders by increasing cortisol levels, decreasing neurogenesis and neuroplasticity, and disturbing glutamate homeostasis [114].
Zinc deficiency has been shown to be significantly associated with MDD and depression symptom severity [59,115]. In a study evaluating zinc supplementation to imipramine, there was an inverse correlation between zinc concentrations and MADRS score. A greater proportion of patients achieving remission had significantly higher zinc concentrations after 12 weeks of supplementation [57]. Higher zinc intake in patients receiving SSRI resulted in nearly a 50% decrease in depressive symptoms (p = 0.007) compared with those with lower zinc intake [58]. Studies from a meta-analysis also reported an association of greater mean symptom severity with greater differences in zinc between patients with MDD and controls [59]. Furthermore, patients with treatment-resistant depression were found to have lower zinc concentrations than treatment-non-resistant patients. [57,115,117] Given these findings, supplemental zinc may be a beneficial adjunct to antidepressants, and obtaining zinc blood concentrations may potentially be a biological marker for monitoring MDD severity.

Magnesium
Magnesium is an essential mineral that functions as a cofactor for >600 enzymes involved in a variety of physiological processes, including the production of vitamin D [119,120]. Magnesium levels can impact CNS function, as it plays a role in DNA replication, transcription, and translation [114]. Magnesium also is recognized for its ability to antagonize the NMDA glutamate receptor, the mechanism thought to be behind its antidepressant and neuroprotective effects [114,121]. Magnesium deficiency causes NMDA hyperactivity and consequently leads to the development of depressive and anxiety-like symptoms and increased inflammatory markers [122]. Low magnesium intake has been linked to an increased risk of experiencing depressive symptoms (OR 1.73, 95% CI 1.48, 2.02; relative risk [RR] 1.49, 95% CI 1.35, 1.66) [123]. Antidepressant action of magnesium has been reported in both animal and human studies [60,[124][125][126]. Consuming magnesium significantly improved depression and anxiety scores (Table 1) [60]. In addition, 61% of patients reported a positive experience with magnesium and would continue taking it for mood.

Vitamin D
Vitamin D plays a critical role in a number of physiologic processes, such as muscle function, regulating cell growth, cancer prevention, metabolic signaling, inflammation, and autoimmunity [127,128]. Vitamin D is integral in a number of brain processes including neuroimmunomodulation, neuroplasticity, neuroprotection, and brain development, which suggests its potential link to depressive disorders [129]. It is thought that vitamin D may affect brain function by acting on vitamin D receptors (VDRs) located in the CNS [130]. VDRs are found in various regions of the brain involved in depressive symptoms including the hypothalamus, prefrontal cortex, hippocampus, thalamus, and substantia nigra. VDR gene polymorphisms have been shown to be associated with cognitive and behavioral impairment, and increased anxiety [128,130]. Potential actions of vitamin D in the brain include neurotrophin stimulation, antioxidation, and anti-inflammation by inhibiting the release of cytokines and metalloproteinases [128]. Additionally, vitamin D promotes glutathione metabolism in neurons, providing protection from oxidative degeneration [127].
Vitamin D deficiency has been shown to be associated with the presence of mood disorders and reduced cognitive functioning. [127,129,131] In a cross-sectional study, patients with serum 25-hydroxyvitamin D levels <10 ng/mL had a significantly greater likelihood of developing a mood disorder (OR: 11.7; 95% CI 2.04, 66.9) than those with adequate vitamin D levels (Table 1) [131]. Additional support is provided by a meta-analysis that demonstrated that low vitamin D levels are associated with a significantly increased risk for depressive symptoms (HR 2.2; 95% CI 1.4, 3.5; p < 0.001) [129]. Patients with vitamin D deficiencies not only have a higher risk of depression, but also have greater duration and severity of depressive symptoms [132].
Numerous studies have demonstrated the effectiveness of vitamin D supplementation in improving depressive symptoms [61]. In a meta-analysis of randomized controlled trials evaluating the effect of vitamin D supplementation (≥800 IU daily) on depressive symp-toms, 10 of 15 studies reported significant improvement. When the studies were analyzed to exclude those with biological flaws, there was a significant effect size (SMD: 0.78).
Vitamin D toxicity rarely occurs but is caused by consuming excessively high doses. Doses >50,000-100,000 IU/day can cause hypercalcemia and hyperphosphatemia [133,134]. Currently, the tolerable upper intake level of vitamin D is 2000 IU/day; however, doses up to 4000 IU/day have been shown to carry low risk of hypercalcemia [134,135]. Vitamin D toxicity typically manifests as nausea, dehydration, and lethargy [135]. Doses of vitamin D 400-18,400 IU/day have been studied in depression [61].
Although the exact mechanism behind vitamin D deficiency in depressive disorders is not clear, supplementing vitamin D carries low risk, given the overall health benefits of vitamin D and low toxicity at doses of 1000-2000 IU. Experts recommend using 10,000 IU, then retesting and adjusting in order to reach adequate vitamin D levels.

Omega-3 Fatty Acids
Omega-3 fatty acids are recognized for their multitude of health benefits due to their anti-inflammatory, antiarrhythmic, antithrombotic, and hypolipidemic effects [136]. Omega-3 polyunsaturated fatty acids (PUFAs) can only be obtained through the diet and are synthesized by consuming short-chained omega-3 fatty acids, which produce eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) [137]. EPA and DHA are involved with different processes related to brain function [136]. DHA is responsible for maintaining the structural integrity of the phospholipid in neuronal cell membranes. Low levels of DHA cause abnormalities in the brain that impact neuron size, nerve growth factor levels, auditory and olfactory responses, and learning and memory [138]. EPA has important physiological functions such as modulating cytokines that affect neurotransmission and neuromodulation [136]. EPA also is thought to reduce inflammation by decreasing IL-1 and TNF-α levels, and also inhibits the upstream mitogen activated protein kinase (MAPK) pathway [137].
Omega-3 PUFAs are thought to improve depression through its role in the uptake, release, metabolism, and receptor function of serotonergic and dopaminergic transmission [139]. Additionally, the anti-inflammatory actions of omega-3 PUFAs are an important mechanism that may address depression-related inflammation. Omega-3 PUFAs, including DHA and EPA, have been found to modulate and reduce neuroinflammation [140,141]. In a rodent study, omega-3 PUFA deficiency was correlated with increases in the proinflammatory cytokines IL-6 and TNF-α, and CRP [142]. In humans, omega-3 PUFAs also were found to be lower in patients with depressive symptoms compared with non-depressed individuals [143,144]. In a study conducted by Rapaport et al., MDD patients who had high levels of inflammation (as measured by hs-CRP, IL-1RA and IL-6), experienced greater improvement with EPA than placebo or DHA, while those who received DHA experienced less improvement than placebo [145]. Patients without any high inflammatory biomarkers experienced a decreased response to EPA than those receiving placebo or DHA. By week 8, patients with high biomarkers who received EPA had at least an 11-point decrease in HAM-D scores, compared to those who received placebo who were progressively less responsive and had increases in inflammatory biomarkers. Despite these positive findings, in a recent meta-analysis, supplementation with omega-3 PUFAs showed little to no effect on risk of depression or anxiety symptoms (number needed to harm, 1000), and inconclusive findings on its effects on depression symptom severity and risk of remission [146]. Patients with increased inflammation and comorbid inflammatory diseases may benefit from omega-3 PUFAs; however, there is conflicting evidence for its use in depressive disorders.

Coenzyme Q10 (CoQ10)
CoQ10, also known as ubiquinone, is a potent antioxidant that possesses anti-inflammatory and neuroprotective properties [147,148]. It protects cells from reactive oxygen and nitrogen species by regenerating oxidized tocopherol and ascorbate, and also enhances mitochondrial activity in the brain [148]. CoQ10 also is involved in a number of biological roles such as cellular membrane repair, regulation of inflammation, and gene expression. As such, reduced levels of CoQ10 in the body are associated with increased free radicals and free radical damage, along with decreased mitochondrial energy production. Patients with MDD are associated with significantly lower plasma CoQ10 levels than healthy individuals [147]. Furthermore, patients with treatment-resistant depression were found to have significantly lower CoQ10 levels than those who were not treatment resistant. In patients with bipolar disorder, supplementing their psychotropic medications with CoQ10 resulted in a significant decrease in MADRS score from baseline after 4 weeks, with significant changes seen as early as 2 weeks [149]. CoQ10 is generally well tolerated and associated with minimal severe AEs. Clinicians should consider supplementing antidepressant regimens with CoQ10 in patients with treatment-resistant depression.

Conclusions and Future Directions
Conventional antidepressant treatment options do not adequately meet the needs of all patients with depressive disorders, as they do not directly address underlying pathogenic factors including nutritional deficiencies, inflammation, oxidative stress, neuroprotection, and neurogenesis. Additionally, there are limited treatment options for patients with MDD that are resistant to conventional treatments. Targeting nutritional imbalances and signaling abnormalities using medical foods and dietary supplements provides several augmentative strategies to treating MDD in patients who do not sufficiently respond to antidepressants and mood stabilizers. Current evidence suggests that medical foods, particularly L-methylfolate and LAC, and other supplements have a role in the adjunctive treatment of MDD, and may particularly target aspects of inflammation and other factors contributing to the pathophysiology of depressive symptoms (Table 2). [48,51,53,59,65,129,146,149].  • Low serum levels of zinc correlate with cognitive impairment and anxiety ADHD, attention-deficit/hyperactivity disorder; BDNF, brain-derived neurotrophic factor; CoQ10, coenzyme Q 10; MMA, methylmalonic acid; MTHFR, methylenetetrahydrofolate reductase; NAC, N-acetylcysteine; OCD, obsessive-compulsive disorder; SAMe, S-adenosylmethionine; TMG, trimethylglycine.
Augmenting conventional antidepressants with medical foods may be a viable option for individuals with MDD who have tried and failed multiple antidepressant regimens and/or are resistant to conventional antidepressants. In particular, L-methylfolate has the most robust body of evidence to support its use in this area. However, the studies that were reviewed have some potential limitations, including small sample sizes, use of various depression ratings scales, and enrollment of heterogeneous populations, so additional research is needed to fully explore potential synergies between agents during the management of MDD. Of particular interest may be the effect of these interventions on those who are treatment resistant or are experiencing residual depressive symptoms with antidepressant monotherapy. Augmentation strategies that manage some of the underlying factors that are not effectively affected by traditional antidepressant treatments may be of particular interest to this population.
Author Contributions: All authors materially participated in the research, article preparation, and its final submission to the journal. All authors have read and agreed to the published version of the manuscript.
Funding: This work was funded by Alfasigma USA, Inc. The authors did not receive payment for their participation.