Vitamin D requires some dietary fat in order to be absorbed in the small intestine. People with conditions that cause fat malabsorption, such as cystic fibrosis, celiac disease, Crohn's disease, whipple disease, and chronic liver disease, are more prone to vitamin D deficiency. People with kidney disease may not be able to convert vitamin D to its active form.
Infants who are exclusively breastfed
Vitamin D requirements for infants can't be met by human breast milk alone. Consult your pediatrician before using vitamin D supplements in infants.
Side Effects and Safety Concerns
Vitamin D is a fat-soluble vitamin. This means that, if consumed in excess, it can build up in the body and cause toxic symptoms, unlike vitamin C and other water-soluble vitamins. Because the buildup is slow, it can take months or years before toxic levels are reached.
Excess vitamin D may result in too much calcium in the blood (hypercalcemia), which causes confusion, muscle weakness, and vomiting, and can lead to complications such as kidney stones and kidney failure.
The safe upper limit of vitamin D is 2,000 IU (50 mcg) for men and women. For infants (up to 12 months), the safe upper limit is 1,000 IU (25 mcg).
There is growing evidence suggesting that these upper limits may be too low. Until a new limit is set, however, it's recommended that you not exceed the upper limit unless under physician supervision.
People with hyperparathyroidism or sarcoidosis should not consume vitamin D, unless they have consulted their physician.
Possible Drug Interactions
The combination of vitamin D and calcium should not be taken with thiazide diuretics, because it could lead to excess calcium levels in the body. Consult your physician before taking either.
People taking calcium-channel blockers should not take vitamin D and calcium, unless under a doctor's supervision, because it may interfere with the effect of the medication.
Certain medications, such as phenytoin (Dilantin), primidone (Mysoline), phenobarbital, valproic acid (Depakene), corticosteroids, cimitidine (Tagamet), heparin, isoniazid (INH) and rifampin may interfere with the absorption or activity of vitamin D.
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Heaney, Robert P. “The Vitamin D Requirement in Health and Disease.” The Journal of Steroid Biochemistry & Molecular Biology 97 (2005):13-9.
Holick MF. Vitamin D. In: Shils M, Olson J, Shike M, Ross AC, ed. Modern Nutrition in Health and Disease, 9th ed. Baltimore: Williams and Wilkins, 1999.
National Institutes of Health Office of Dietary Supplements. Vitamin D: Dietary Supplement Fact Sheet.
University of Ottawa Evidence-based Practice Center. Effectiveness and Safety of Vitamin D in Relation to Bone Health. Agency for Healthcare Research and Quality. Aug 2007: 07-E013.
Wilkins, Consuelo H. and Yvette I. Sheline, et al. “Vitamin D Deficiency Is Associated with Low Mood and Worse Cognitive Performance in Older Adults.” American Journal of Geriatric Psychiatry 14 (2006): 1032-40.