Vitamin D boosts calcium absorption and is necessary for bone growth and bone maintenance. Inadequate vitamin D intake can cause thin and brittle bones and it contributes to osteoporosis. Vitamin D also is important for neuromuscular and immune system function. People with certain intestinal or kidney disorders may have trouble obtaining enough vitamin D. In contrast, excessive vitamin D in the body is associated with certain kidney and cardiovascular problems. Consult a qualified health care provider before taking vitamin D supplements.
Disorders Affecting Absorption
Vitamin D is fat-soluble and needs dietary fat in the intestines for absorption. If you have a disorder or take medication that decreases your ability to absorb dietary fat, you may need vitamin D supplements, according to the Office of Dietary Supplements. Some conditions that decrease absorption include liver disease, cystic fibrosis and Crohn's disease, a form of inflammatory bowel disease. Vitamin D supplements may not fully resolve the problem, as illustrated by a study published in the October 1985 issue of the "The American Journal of Clinical Nutrition," with lead author C.W. Lo. Patients with intestinal fat malabsorption syndrome had much lower responses to a single oral dose of vitamin D than did healthy participants.
Gastric Bypass Surgery
Unless you obtain plenty of vitamin D from food or supplements, you may become deficient in this nutrient if you have undergone gastric bypass surgery, according to the Office of Dietary Supplements. The surgery bypasses part of the small intestine that absorbs vitamin D. The body retains some vitamin D in fat, but this may not provide enough over a long time frame.
Kidney Disease
The body converts vitamin D to an active form with two processes. In the first process, the liver converts vitamin D to the chemical calcidiol, and in the second, the kidneys are mainly responsible for changing calcidiol into the active chemical calcitriol. If the kidneys cannot change calcidiol to calcitriol, a vitamin D deficiency can develop. Calcitriol levels decline with worsening kidney function, and calcitriol deficiency affects nearly all dialysis patients not being treated for low vitamin D levels, according to an article by Daniel W. Coyne, M.D., published in the October 23, 2007 issue of "Renal and Urology News."
Calcifications
The Food and Nutrition Board at the Institute of Medicine of the National Academies has set an upper tolerable level of vitamin D intake at 4,000 IU per day. Although toxicity symptoms are uncommon even when people take 10,000 IU per day, some studies indicate that consuming more than 4,000 IU per day can cause negative effects over time, according to the Office of Dietary Supplements. Excess vitamin D can increase calcium levels in the blood, which may lead to calcification in the body and resulting damage in the kidneys and cardiovascular system. The Office of Dietary Supplements cites a study in which postmenopausal women taking 1,000 mg of calcium and 400 IU of vitamin D per day was associated with a 17 percent rise in the risk of kidney stones over seven years.
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