Vitamin D deficiency is common among adults in Wallonia (Belgium, 51°30′ North): findings from the Nutrition, Environment and Cardio-Vascular Health study

Abstract

Data on the vitamin D status of the population of Wallonia (Belgium, 51°30′ North) are scarce. This study was carried out to estimate vitamin D deficiency, identify potential determinants, and analyze their relationship with vitamin D supplementation. We tested the hypothesis that vitamin D deficiency is common in the general population, particularly among subjects without supplementation. Vitamin D deficiency was defined as a serum level of 25-hydroxyvitamin D (25(OH)D) concentration less than 50 nmol/L. Data were analyzed from 915 participants of the Nutrition, Environment and Cardio-Vascular Health cross-sectional survey. The median (interquartile range) 25(OH)D level was 53.1 (37.8-69.9) nmol/L, and 44.7% of the subjects were vitamin D deficient. Subjects without vitamin D supplementation were more concerned by vitamin D deficiency than those with supplementation (odds ratio [OR], 3.35; P < .0001). From a multivariate standpoint, the potential determinants of vitamin D deficiency among subjects without vitamin D supplementation were season, specifically spring and winter (OR, 7.36 and 6.44, respectively), obesity (OR, 2.19), low household income (OR, 1.73), and lack of solarium use (OR, 1.79). For subjects with supplementation, the only determinant observed for vitamin D deficiency was obesity (OR, 5.00). This work evidenced the high prevalence of 25(OH)D deficiency in the general population, especially among nonsupplemented subjects with obesity, low household income, and/or lack of light. Vitamin D supplementation looks effective in our population, especially via a stabilization of vitamin D coverage throughout the seasons. The best dietary strategy to achieve optimal 25(OH)D concentrations all year round in the general population requires more research.

Introduction

Vitamin D deficiency is a worldwide problem that impacts all age, sex, socioeconomic, and educational and ethnic groups, with major human and financial consequences [1], [2]. Vitamin D status can be assessed by the determination of serum 25-hydroxyvitamin D (25(OH)D) concentration, the predominant circulating form of vitamin D. Although experts agree on the use of 25(OH)D to determine vitamin D status, the 25(OH)D threshold level defining vitamin D deficiency is still a matter of much debate. The Institute of Medicine (IOM) recently reported that a 25(OH)D level of at least 50 nmol/L (20 ng/mL) is largely sufficient and that the Recommended Dietary Allowance of 600 IU/d for ages 1 to 70 years should meet the requirements of 97.5% of the population. In addition, the IOM expert panel indicated that the only effects of vitamin D for which a “reasonably” evidence-based target value may be currently proposed are those on the bone health [3]. By contrast, according to the Endocrine Society group, vitamin D deficiency corresponds to 25(OH)D concentrations less than 50 nmol/L (20 ng/mL) and insufficiency to concentrations in the range of 50 to 75 nmol/L (20-30 ng/mL) [4]. Furthermore, experts of the Endocrine Society recommended a 25(OH)D level higher than 75 nmol/L to maximize the effect of vitamin D on musculoskeletal health and mineral metabolism and to provide most noncalcemic benefits for health. The discordance between the 2 conclusions is not surprising given the target difference between the guidelines. The IOM threshold is intended for public health policies, whereas the Endocrine Society group makes recommendations for clinical health care.

If severe vitamin D deficiency characterized by rickets in children and osteomalacia in adults is a rare condition in developed countries, subclinical vitamin D deficiency/insufficiency is highly prevalent and a potential risk factor for osteoporosis, falls, and fractures in the elderly [1], [5], [6]. Besides the important skeletal functions of vitamin D, a multitude of ecologic and observational studies have shown associations between low vitamin D and potential “nonskeletal” effects, notably increased risk of some cancers, cardiovascular disease, autoimmune diseases, infections, asthma and allergy, and all-cause mortality [7], [8], [9].

Diet and sunlight are the 2 main sources of vitamin D in humans. Only a limited number of foods naturally contain vitamin D. The well-known and significant one is marine fatty fish, barely consumed by Belgians [10]. Although several foodstuffs are enriched with vitamin D in the United States and Canada, the Belgian legislation on food fortification is quite restrictive. Margarine is the only food item that requires to be enriched with vitamin D [11], [12].

The major source of vitamin D comes from exposure to solar ultraviolet B (UVB) radiations (wavelength, 290-315 nm). Vitamin D is produced endogenously from 7-dehydrocholesterol in the epidermis of the skin after adequate exposure [1]. However, in Belgium (latitude 51° North), the amount of UVB photons reaching the surface of the earth is insufficient for almost 6 months of the year (October-March) to allow the skin photosynthesis of vitamin D [13]. The determinants of vitamin D status are multifactorial and include environmental and personal characteristics. Many environmental factors including latitude, season, time of day, and ozone pollution in the atmosphere diversify the number of solar UVB photons reaching the earth and influence the cutaneous production of vitamin D. Likewise, personal factors including age, skin pigmentation, body fat content, clothing, and sunscreen use inhibit cutaneous vitamin D synthesis and bioavailability [14], [15]. Low-vitamin-D diet coupled with insufficient exposure to sunshine is indicative of poor vitamin D status in Belgium. Contemporary lifestyle is also associated with reduced sun exposure (decreased outdoors activities) and use of UVB-blocking sunscreen. Obesity, another determinant of vitamin D status, is also highly prevalent in developed countries.

Despite numerous potential health effects reported, data on vitamin D status at a population level in Belgium are limited. Three studies have focused on 25(OH)D levels, but none was carried out in a representative sample of adults and determinants of vitamin D status were only briefly studied. In 2004, MacFarlane et al [16] conducted a study on 126 presumably healthy volunteers aged 21 to 65 years but during the late winter months. The prevalence of vitamin D deficiency was also assessed among 401 subjects aged 40 to 60 years, and the only determinant linked with serum 25(OH)D was ethnicity [17]. Serum 25(OH)D₃ concentrations were also determined in 542 adults in the 1980s [18]. As noted recently by Cashman and Kiely [2], there is an ongoing need to assess vitamin D status in representative populations within Europe. Because of the paucity of Belgian data, the present research based on data collected from the Nutrition, Environment and Cardio-Vascular Health (NESCaV) cross-sectional survey was conducted to assess the vitamin D status in the general population of the province of Liège (Wallonia, Belgium) and to identify its potential determinants and relationship with vitamin D supplementation. First, it was hypothesized that vitamin D deficiency is common in the general population, particularly among subjects without vitamin D supplementation. Second, several determinants such as winter conditions, age, obesity, and lack of sun exposure are supposed to be indicative of poor vitamin D status. To test these hypotheses, the research objectives were (1) to determine the vitamin D status, using serum 25(OH)D concentrations, not only globally but also in subgroups of vitamin D supplement users and nonusers, and (2) to test the potential effects of subject characteristics on the prevalence of vitamin D deficiency in the 2 subgroups.