Influence of external, intrinsic and individual behaviour variables on serum 25(OH)D in a German survey

Highlights

• Ambient UV exposure combined with behavioural factors mainly influence serum 25(OH)D concentrations in a German survey.

• The factor most significantly associated with hypovitaminosis D in Germany, defined here as 25(OH)D < 25 nmol L⁻¹, is winter season.

• In winter 43% of survey participants were vitamin D deficient and 42% insufficient.

• In summer over half the population has insufficient 25(OH)D status with 8% deficient and 47% insufficient.

Abstract

The objective of the present study was to identify external, intrinsic or behavioural factors that significantly influenced serum 25-hydroxyvitamin D (25(OH)D) concentrations in a German survey. Data from 3061 participants in the Cooperative Health Research in the Region of Augsburg, Germany (KORA) F4 survey were used to relate potential determinants to measured mean serum 25(OH)D concentrations using multivariable regression models.

The factors significantly associated with hypovitaminosis D (defined as 25(OH)D < 25 nmol L⁻¹) were season (winter, spring and autumn), urban environment and high body mass index. In contrast, times spent in sunny regions, hours per day spent outdoors in the summer as well as additional oral intake were associated with higher 25(OH)D concentrations. These results suggest that mainly ambient UV exposure but also individual behaviour are the most important determinants for personal 25(OH)D concentrations. The analyses further showed that in winter 43% of subjects were vitamin D deficient and 42% insufficient. Even in summer over half the population has insufficient vitamin D status with 8% deficient and 47% insufficient. Therefore measures to mitigate widespread vitamin D insufficiency such as regular short-term sun exposure and/or improved dietary intake/supplementation recommendations by public health bodies need to be considered.

Introduction

Vitamin D belongs to a group of hormones involved in calcium metabolism and bone mineralization. The principal source of vitamin D is from cutaneous synthesis after exposure to solar ultraviolet radiation (UVB), with smaller contributions coming from diet and supplementation. The best indicator of vitamin D status is considered to be the concentration of serum 25-hydroxyvitamin D (25(OH)D) in the blood [1], [2], [3], [4], [5].

Vitamin D is fat soluble and any excess can be stored for use when needed such as when ambient levels of UV are too low for cutaneous synthesis, as occurs from late autumn to early spring in middle and higher latitudes. This seasonal variation in UVB leads to an annual cycle in 25(OH)D concentrations with the maximum occurring in late summer and a corresponding minimum at the end of the winter. If optimal 25(OH)D concentrations are obtained during the summer months then long-term insufficiency (i.e. ⩽50 nmol L⁻¹) or even hypovitaminosis D, defined here as 25(OH)D concentrations below 25 nmol L⁻¹ [6], [7], [8], [9] could be avoided [5]. This is important since vitamin D insufficiency has been implicated in a number of chronic diseases, such as cancer, osteoporosis, diabetes, various autoimmune disorders and incident cardiovascular disease events [10], [11], [12].

There are a variety of factors that influence personal cutaneous vitamin D synthesis, and these can be grouped into three categories: (i) external, (ii) intrinsic and (iii) behavioural [5]. Among the external factors, solar geometry (e.g. solar zenith angle, season), location (e.g. latitude, altitude, exposure geometry, local albedo, etc.) and atmospheric conditions (cloud coverage, ozone, etc.) affect the ambient UV level. Intrinsic attributes that may influence vitamin D synthesis include gender, age, health, skin type (i.e. pigmentation) and body weight. Personal behaviour that can influence the serum 25(OH)D concentration comprises times spent outdoors, use of sun protection, clothes, holiday destinations, diet, and vitamin D supplementation. Variations in personal behaviour can result in very large differences in UVB exposure, and hence cutaneous vitamin D synthesis [13], but despite their importance these factors are the most difficult to quantify and require further research.

Vitamin D insufficiency has been shown to be prevalent in many countries [7], [14], [15], [16], including Germany which has levels of insufficiency of between 40% and 45% [17], [18], [19]. Causal factors for insufficiency have been investigated in a range of studies in different countries e.g. sun exposure [20], [21], age, body mass index (BMI) [22], [23], gender [23], [24], pigmentation [25], latitude [24], season [22] and use of sun protection such as sunscreens [26].

There have been very few studies which directly examined potential causes of the widespread vitamin D insufficiency in the German population [14], [18], [19], [27], and these have had little or no investigation of personal behaviour and its effect on vitamin D. Therefore, the objective of the present study was to use a population-based dataset to identify which external, intrinsic or behavioural variables had the greatest influence on the serum 25(OH)D concentrations of the population.