You need vitamin A for a healthy immune system, good vision, bone health and cell division, as well as many other body processes. However, hypervitaminosis A, or vitamin A toxicity, may raise your risk for osteoporosis because excess vitamin A appears to interfere with vitamin D’s ability to maintain the proper calcium balance in your body. Always consult a health care provider before adding supplements to your regimen.
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The form of vitamin A that may cause problems when it comes to calcium balance and bone health is called preformed vitamin A. Your body absorbs it in the form of retinol. It’s easy to reach a retinol intake of more than 5,000 international units a day in the United States if you frequently consume fortified foods like breakfast cereals as well as multivitamin supplements, according to Oregon State University's Linus Pauling Institute. Long-term, a daily intake of 5,000 international units of preformed vitamin A, which is 700 to 900 micrograms, is associated with decreased bone mineral density and raised risk for osteoporotic fracture, according to the institute. Preformed vitamin A is also found in whole milk and liver. Vitamin A in fruits and vegetables is a different form called provitamin A carotenoid, or beta-carotene. Excess beta-carotene is not associated with adverse bone health effects.
Since vitamin A may interfere with vitamin D, it may affect your calcium status. You need vitamin D along with calcium because this vitamin promotes calcium absorption in the gut. Vitamin A appears to hamper the intestinal calcium response to vitamin D, according to an October 2001 study published in “Journal of Bone and Mineral Research.” This may explain why Eastern Europe has the highest incidence of osteoporosis, as vitamin A intake is high there while vitamin D-producing sunlight exposure is low, note study authors S. Johansson and H. Melhus.
Excess vitamin A also may weaken your bones independently of interfering with vitamin D’s job of promoting calcium absorption because it encourages bone resorption, according to a March 2003 study published in the “Journal of Nutrition.” Weakened bones and osteoporosis typically occur when your bone resorption rate exceeds your bone formation rate. Raising dietary calcium intake does not affect the ability of vitamin A, in the form of all-trans retinoic acid, to cause bone resorption, note study authors C.M. Rhode and H. DeLuca. This study was performed on rats, however, so more research is needed to determine whether the results apply to humans.
According to the Linus Pauling Institute, more research is needed to fully understand the relationship between vitamins A and D and calcium. Consuming the recommended daily amount of vitamin A, 700 micrograms for women or 900 micrograms for men, is best for bone health. That’s 2,333 international units if you’re female and 3,000 international units if you’re male. Insufficient vitamin A also is associated with decreased bone mineral density. The experts at the institute recommend choosing multivitamin supplements that have 5,000 international units of vitamin A, with half of that coming from beta-carotene rather than retinol.
The intake associated with weakened bones is lower than the tolerable upper intake level, or UI, set for vitamin A, which is 3,000 micrograms or 10,000 international units for both men and women. Your risk for vitamin A toxicity and adverse health effects sharply increases when you exceed the UI.