Vitamin D supplements are dietary supplements available in both D2, or ergocalciferol, and D3, or cholecalciferol, formats; both forms work to increase the amount of vitamin D in the blood, according to the National Institutes of Health. While most people get the vitamin D they need from the sun and food, supplements may be needed if the body's level of vitamin D falls below acceptable limits.
Although treatment goals for individuals vary, in general, levels of vitamin D below 30 nmol/L are too low to sustain bone and overall health, while levels in excess of 125 nmol/L are considered too high. Most people should have a level of around 50 nmol/L or slightly more.
Some people are prone to lower-than-average blood levels of vitamin D. This includes breastfed infants, older people who aren't out in the sun a lot or whose kidneys are less able to convert vitamin D to a form that the body can use and people with dark skin, since their skin is unable to produce vitamin D as much as those with lighter skin. Those with celiac disease or Crohn's disease may also have low vitamin D levels, and the same is true obese people becausetheir body fat prevents the movement of vitamin D in the blood. These people may all benefit from vitamin D supplementation under a doctor's supervision.