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.

Recently the investigators of Dvisum assessed Vitamin D status in pre-school children (n=90; mean age 54+/-7.1 mo), all living in northern Sweden (latitude 63°) and half of them with fair skin, half with darker complexion. The study group was examined first in August-September (late summer) and then the following January-February (winter). Skin type, vitamin D intake, anthropometrics, S-25(OH) D and parathyroid hormone (S-PTH) were assessed. The investigators found inadequate vitamin D status in these children living in northern Sweden, especially in dark-skinned children and during the winter despite vitamin D intakes meeting the recommendations, prompting strategies to improve intake of vitamin D in this population. Overall, 25% and 40% of the light and dark skinned had S-25(OH) D <50 nmol/L.

The aim is to examine which amount of vitamin D is needed to maintain or increase S-25(OH)D to ≥50 nmol/L among 97.5% of the participants regardless of skin color or geographic location (northern or southern Sweden). Furthermore the investigators will examine if vitamin D status affects health markers such as bone density, blood pressure, serum lipids, fatty acids and inflammatory and immunological markers and mental wellbeing.

In order to identify whether there are differences depending on the latitude within Sweden, children will be recruited both from northern Sweden (Umeå) and from southern Sweden (Malmo). As it is unclear what levels of the serological marker 25 (OH) D that affect the health of children, different markers of health will be examined before and after the intervention. Children aged 5-8 years, 50% fair-skin 50 % darker skin, in northern Sweden (Umeå) and southern Sweden (Malmö) will be included in a longitudinal, randomized trial. The children are first examined in November-December and randomized to a vitamin D supplement of either 10 or 25 g per day, to be used for three months. At the follow up in February-March all examinations will be repeated.

The investigators will use a 2 × 2 × 2 factorial design to investigate the effects of two different doses of vitamin D (10 µg and 25 µg) in children with dark and light skin color, living in northern (Umeå) and southern (Malmö), Sweden. In order to achieve a moderate difference between groups (effect size 0.25) requires 20 children per group (power>87%, alpha = 0.05). With this group size, we can see a group difference in the S-25 (OH) D of 3.75 nmol/L, S-PTH of 0.35 mmol/L and bone mineral density in the lumbar region of 0.075 g/cm2. In Skåne, but not Umeå also a placebo group will participate. With an estimated dropout of 10%, a total of 220 children will be included.

The study include sampling for analysis of S-25 (OH) D, calcium, phosphate, alkaline phosphatase (ALP), magnesium, PTH and osteocalcin, serum lipids (total cholesterol, HDL cholesterol, LDL cholesterol. ApoA1 and ApoB lipoprotein) and fatty acids as well as inflammatory and immunological markers (CRP, interleukin (IL) -1 and 2, IL-4, IL-6, I-10, Il-17, CD40 ligand, TNF and IFNγ, fibrinogen and antisecretory factor). Before sampling, the children receive a topical anesthetic (EMLA).

Measurements of blood pressure and anthropometric measurements of length, weight, waist circumference and bone densitometry (DEXA) and body composition (fat mass% and fat free mass) using a Air Displacement Plethysmography ( BOD POD) Questions about diet, vitamin supplements, foreign travel, how much time the children spend time outdoors and the use of sunscreen as well as questions about the child's health and family situation will be answered by the parents through a questionnaire. To investigate the possible association between vitamin D status and mental well-being, the investigators will use the Child Behaviour Checklist (CBCL).

This study is national with a multicultural perspective, it is expected to provide knowledge about the needs of vitamin D to prevent vitamin D deficiency. The study is also expected to provide a better understanding of association between vitamin D status and various markers of health among children.

By preventing vitamin D deficiency, poor bone development, susceptibility to infections, and perhaps prone to autoimmune diseases and cardiovascular risk factors could be reduced, and hopefully the mental well-being improved, which reduces costs to both society and the individual, and reduces unnecessary suffering of individuals.