Comparing Different Amounts of Vitamin D Supplementation to Preschool Children Living in Northern and Southern Sweden

Vitamin D has a range of biological effects of public health relevance (Prentice et al, 2008). Besides its well known role in mineralization of bone and teeth, vitamin D also play important roles in metabolic functions, the pathogenesis of certain diseases, e.g. type 1 diabetes, celiac disease, asthma and allergies, as well as in the prevention of cancer (Holick, 2008).

Vitamin D status is assessed on plasma or serum levels of its metabolite 25-hydroxyvitamin D [25(OH)D, calcidiol] as it reflects the sum of vitamin D converted in the skin through sunlight exposure and from dietary sources. Several reports advocate that levels <37 nmol/L denote severe vitamin D deficiency; levels <50 nmol/L insufficient; 50-75 nmol/L suboptimal levels and ≥75nmol/L optimal levels (Dawson-Hughes et al, 2005, Huh et al, 2008, Yetley, 2008). In children, most suggested cut-off values for adequate levels of 25(OH)D are based on the absence of rickets, increased measures of bone mineralization and maximal suppression of parathyroid hormone (PTH) levels (Greer, 2009).

The major source of vitamin D is dermal biosynthesis catalyzed by ultraviolet B sunlight (Cashman et al, 2011). However, during winter, northern Sweden has limited hours of daylight leading to reduced sun exposure. Consequently, the dietary source of vitamin D is of specific importance in this region (Brustad et al, 2007, Edvardsen et al, 2007). Fatty fish, eggs, vitamin D fortified milk and margarines are the main sources, mainly supplying the most active form D3. These are important basic foods which also contain common food allergens. Thus, children with food allergies to milk, fish, and egg can be at increased risk of vitamin D deficiency. Several dairy products are fortified with vitamin D, but in some products in the form of D2 which is not as bioactive as D3.

Skin color affects the capacity to form vitamin D3 as children with dark complexion need 5-10 times more sun exposure to generate the same amount of vitamin D3 compared to fair-skinned children, and therefore are at increased risk of vitamin D deficiency when exposure to sun is limited (Holick, 2005). Recently the recommendations on protecting the skin from sunshine to reduce the risk of skin cancer later in life has been debated as it may increase the risk of vitamin D deficiency (Stechschulte et al, 2011). Obesity in children might be another risk factor for vitamin D deficiency, since an increased proportion of available vitamin D may be stored in adipose tissue thus lowering the S-25(OH)D (Prentice, 2008).

Despite studies indicating that the vitamin D intake among Swedish children and adolescents are significantly below recommendations, little is known of their vitamin D status (Garemo et al, 2007, Enghardt et al, 2006, Öhlund et al, 2010). Furthermore there is a paucity of studies investigating vitamin D intake and status in food-allergic adolescents who may be at increased risk of vitamin D insufficiency due to strict avoidance of vitamin D containing foods.