Vitabiotics Ultra Vitamin D3 4000IU Maximum Strength Tablets For Healthy Immune System provides extra strength vitamin D, a nutrient that plays a truly remarkable role in the body and is important for many areas of health. Vitamin D is synthesized by the body on exposure to sunlight. However, many areas of the world receive insufficient sunlight and even in sunny areas, many choose to limit their exposure due to the negative health effects of excessive exposure to the sun's UV rays. These factors have led to widespread low vitamin D levels in many parts of the world. Vitamin D is a fat soluble vitamin found in foods such as mushrooms, oily fish, butter and egg yolk. Vitamin D has functions and benefits that are probably more wide ranging than any other vitamin. It not only contributes to the maintenance of normal bones and teeth but is also known for its role in supporting normal function of the immune system, normal cell division and normal muscle function.