Vitamin D status in Kancheepuram District, Tamil Nadu, India

Vitamin D deficiency (VDD) is very prevalent in many parts of the world [1]. The major cause of VDD is lack of adequate exposure to sunlight due to various lifestyle factors and practices [1]. The amount of sunlight exposure needed to sustain adequate vitamin D levels varies with the skin colour of the individual. For white Caucasians it has been estimated that exposing the face, hands, forearms and legs to sunlight for 9 min daily during lunchtime in the UK summer would ensure vitamin D sufficiency [2], whereas an individual with brown skin would require a 25 min exposure under the same conditions [3]. Vitamin D can be obtained from dietary sources but few foods naturally contain vitamin D. Foods that are fortified with vitamin D are often not consumed in quantities sufficient to meet the body’s vitamin D requirement [1]. Overt VDD is linked to bone disease such as rickets in the very young and osteomalacia in adults. In addition, lesser degrees of VDD are associated with an increased risk of diseases as diverse as cancer, diabetes, skin disease and neuropsychiatric disease [4–7], thus making VDD a problem of public health importance.

Vitamin D3 or cholecalciferol is produced in the skin from 7-dehydrocholesterol through the action of ultraviolet (UVB) rays from the sun. Both UVB intensity and skin pigmentation contribute to the rate of D3 formation [8]. Melanin in the skin blocks UVB from reaching 7-dehydrocholesterol as do clothing and sun-screen. The intensity of UVB from sunlight varies according to season and latitude, being maximal close to the equator. Vitamin D2 or ergocalciferol can be obtained from plants and fungi, principally mushrooms, if they have been exposed to UVB. Vitamin D3 and D2 undergo 25-hydroxylation in the liver to form 25-hydroxy vitamin D (25OHD), and this further undergoes 1α hydroxylation in the kidney to become the active form of vitamin D [8]. The plasma level of 25OHD, (which is the long-lived circulating metabolite of vitamin D) is used most commonly as the marker of an individual’s vitamin D status. It has been shown that the gut microbiota have the capacity to regulate 25 hydroxylation of vitamin D in the liver [9], thus influencing circulating 25OHD concentration.

Rickets in children is associated with circulating concentrations of 25OHD of less than 20 ng/mL (50 nmol/L) [10], as is osteomalacia in adults [11]. Circulating levels of 25OHD of less than 20 ng/mL have been advocated as a cut-off to define VDD [12], ,but the more restrictive Institute of Medicine recommendations suggest the use of circulating 250HD levels less than 12 ng/mL to define VDD, and levels between 12 and 20 ng/mL to define vitamin D insufficiency (VDI) [13]. In the present study, we use the more restrictive definition of VDD.

Social, economic and cultural factors determine an individual’s access to resources, including material goods, healthcare, and educational opportunities. Population health and nutritional status is influenced by socioeconomic status and other social determinants, and these are used in risk predictive modeling of disease [14, 15]. In India, the Kuppuswamy scale and its modifications are used to classify the socioeconomic status of a family using a composite score of education and occupation of the head of the family along with monthly income of the family [16].

Vitamin D status is poor in many countries, with less than 50% of people worldwide being vitamin D sufficient. Deficiency is more common in the Middle East, China, Mongolia and India [17]. It is reported that only about 20% of healthy Indians have vitamin D sufficiency [18, 19], and this is a matter of particular concern given that the country has adequate sunshine. This may indicate a major role for social and cultural factors in the determination of an individual’s vitamin D status.

The present study was undertaken to assess the vitamin D status of a rural adult south Indian population and to identify its associations with socioeconomic and cultural factors.

The present study, from a rural area of Tamil Nadu, indicates that only 44.8% of the population was sufficient in vitamin D. The study further identified that vitamin D status was independently associated with time to which the body was exposed to sunlight very day, the gender of the participant, the level of education of the individual, the consumption of fatty fish, and presence of a closed toilet system at home. Each of these associations is discussed at greater length.

Both VDD and VDI were more common in individuals who had studied beyond primary school. We could not find any description of a similar observation previously in Indian studies of vitamin D status. It is possible that those individuals with higher educational attainments were involved in occupations that allowed less exposure to sunlight. The lack of vitamin D fortification in foods consumed in this location probably contributed to this. While occupational status was associated with VDD and VDI in univariate analysis, it was not significantly associated with vitamin D status in the multivariable analysis. In Western countries, several studies have uniformly identified higher education as being associated with better vitamin D status [23–25], quite the opposite of what we observed in our study.

Exposure to sunshine is a major factor in generating vitamin D in the body and is important in maintaining sufficiency of the vitamin in the body. The time to which the body needs to be exposed to sunlight varies with skin color. Dark skin color as is common in south India requires a longer time of exposure to sunlight for generation of vitamin D. Although we did not formally assess skin pigmentation in the study population, almost all had Type 5 skin pigmentation based on the Fitzpatrick scale [26]. The melanin pigment in skin absorbs ultraviolet radiation and protects it from damage; however it also reduced the ultraviolet radiation necessary for vitamin D synthesis in the skin. Comparative studies in the United Kingdom have shown that white-skinned Caucasians required only 9 min of daily UK summer sunlight exposure of face, forearms and lower legs to meet their vitamin D needs, while those with brown skin required 25 min of daily sunlight exposure under similar conditions [1, 2]. Our participants had brown skin probably similar to the UK cohort in the second study above. Penetration of UVB radiation through the atmosphere is greater at midday than early in the morning. In addition to skin color and time of day, the use of clothing to cover the entire body and both upper and lower limbs and the use of sun screen were other cultural practices that were associated with VDD. In the multivariable analysis, sunlight exposure time less than 60 min per day negatively impacted vitamin D sufficiency, with 15–30 min exposure having a greater negative effect than 30–60 min exposure per day. Our study suggests that going outdoors in the sun for greater than 60 min per day is required for maintaining vitamin D sufficiency. A study conducted among urban men in Pune, India also suggested that more than 1 h of casual midday sunlight exposure was necessary to maintain vitamin D levels [27]. In the present study, exposure of face, arms and legs to sunlight was associated protectively with VDI compared to exposure of face and arms alone.

The higher prevalence of VDD in individuals drinking protected water such as bottled water or water from bore wells is an association that has not earlier been documented. This and the other variables shown in Table 4 have been used as measures of hygiene in cross sectional association studies, and hygiene is known to influence health outcomes by effects on the composition of the gut microbiota [28]. While the association between hygiene and vitamin D status may be due to chance it is interesting to consider that there may be a direct link between serum 25OHD levels and domestic hygiene. The vitamin D produced in the skin is inactive and undergoes its first hydroxylation (25 hydroxylation) in the liver. The gut microbiota, through fibroblast growth factor 23, regulate 25 hydroxylation of vitamin D in the liver [9]. In germ free mice, plasma 25OHD levels were low, andincreased when gut microbiota were introduced. Since we (and other investigators) used plasma 25OHD levels to define vitamin D status, it was reasonable to include variables relating to hygiene (which greatly influences the gut microbiome) in this study evaluating associations of vitamin D status.

Sufficiency of vitamin D is necessary not only for bone health but for a variety of other metabolic and immune processes including effects mediated through vitamin D receptors distributed ubiquitously in the body. In the present study we identify associations of lifestyle and cultural practices with vitamin D status. The newly identified interaction between circulating vitamin D, vitamin D receptors in the gut and the gut microbiome [29] underscores the emerging importance of vitamin D in human physiology and its role in the maintenance of health. It is very likely that poor or marginal vitamin D nutrition is an important determinant of ill health at both population and individual level in this population.

Limitations of the present study include that we did not consider a design effect in the initial sample size calculation, and that we did not assess the gut microbiome. The latter is a complex and sometimes unrewarding exercise, but could potentially have thrown light on the relationship between hygiene parameters and serum 25OHD concentrations.

Vitamin D deficiency and insufficiency were highly prevalent in this adult rural south Indian community, with educational status, time of exposure to sunlight, gender, fish consumption and hygiene being factors that independently determined the vitamin D status of individuals. These findings should help in designing and targeting interventions to improve the vitamin D status of the individuals residing in these communities.