Solgar Vitamin D-3 10 mcg/400 IU contains the most effective form of vitamin D. Vitamin D increases the absorption of calcium in the bones. Vitamin D has a positive effect on the immune system. Extra vitamin D is used to supplement the diet or in case of increased need (such as in children, the elderly and pregnant women). People with dark skin or people who for some reason do not sufficiently expose their skin to daylight, also have an increased need.