Why do we study the consumption of vitamin D supplements?

• Vitamin D is a fat-soluble vitamin essential for the absorption of calcium from the diet. It plays a key role in the growth and maintenance of strong bones and teeth. In addition, vitamin D is important for proper muscle function and the immune system. The most important source of vitamin D is sunlight exposure, which triggers its synthesis in the skin. However, due to Belgium’s northern latitude, sunlight exposure is often insufficient, particularly in the winter months, limiting the body’s ability to produce adequate vitamin D. This increases the risk of deficiency, which can lead to conditions like osteomalacia and osteoporosis. 
• While vitamin D can also be obtained from dietary sources such as fatty fish, fortified dairy products, and eggs, these alone often do not provide enough to meet daily needs. To address this, the Belgian Superior Health Council recommends vitamin D supplementation: 10 µg/day for children and adults, 15 µg/day for those at risk of osteoporosis, and 20 µg/day for pregnant, breastfeeding women, and the elderly. These recommendations may vary depending on individual sun exposure, with lower doses advised for those with more sunlight exposure.
• Despite the benefits of supplementation, overconsumption can be harmful. Vitamin D supplements are widely available over the counter in Belgium, raising concerns about potential overuse.  Therefore, it is essential to monitor supplement intake to ensure both adequacy and safety.

How do we study it?

• Participants (or the parents of the 3-9 year old children) were asked to complete a Frequency Questionnaire on supplement consumption. They reported whether they had used a supplement in the past 12 month.  Supplement users were then asked which type of supplement they had taken and during which months. To analyse seasonal differences, the year was further divided in winter (December-March) and the rest of the year (April-November).
• The brand name and dose of those who indicated using vitamin D supplements was asked to get a more complete picture of Vitamin D supplement use.  The use of dietary supplements was monitored during the two 24-hour dietary recalls. Participants were asked whether they had taken any supplements on the previous day. If so, the interviewer recorded the brand and product name of the supplement.    
• Note that this was a self-reported  questionnaire, the answers were given by the participant based on their memory and their knowledge of the supplement type. Therefore, results should be interpreted with caution.