Vitamin D- normally found in food supplements as Vitamin D3- is a fat-soluble vitamin- obtained from the diet and is also synthesised in human skin when exposed to sunshine. It is very important for many aspects of human health- including the maintenance of strong and healthy bones and teeth- as well as the absorption and regulation of calcium and phosphorous. It is also needed for the normal function of the immune system and inflammatory response. Recent studies have shown- however- that sunshine levels in most northern countries are too weak during the winter period for the body to produce enough vitamin D3. Some experts estimate that more than half the population in those areas have inadequate or perhaps even deficient levels of the nutrient.