Vitamin A: Bone Poison?
Too Much Vitamin A Can Weaken Your Bones
"What these results indicate is, 'Oh my God we have found a bone poison,'" says Tosi, who is also associate professor of orthopaedic surgery and pediatrics at the George Washington University School of Medicine. "Vitamin A is important in small doses, but at high doses it is bone poison."
Concentrations of vitamin A vary in different supplements, but researchers found that consuming more than 1.5 mg per day of vitamin A from supplements increased the risk of fracture.
"In Sweden, if you take two multivitamins a day you've probably exceeded the limit for having an adverse effect on your bones," says researcher Karl Michaëlsson, MD, of University Hospital in Uppsala, Sweden.
Michaëlsson says more research is needed on exactly how vitamin A obtained from dietary sources like liver, fish, and fortified foods affects levels of vitamin A in the blood. But studies on animals and human bone cells show that high blood levels of vitamin A stimulate a bone-weakening process and decrease new bone production.
Tosi agrees and says it's still unclear how people get these high levels of vitamin A -- whether it's related to how the body stores vitamin A, taking huge doses with supplements, or just eating too much fish and liver.
"We need to understand this better, and then maybe there are some people we can help and prevent them from developing osteoporosis," Tosi tells WebMD.
Researchers say it's still too early to recommend that people get their vitamin A levels tested based on the results of this study. But Michaëlsson says it's a good idea for people -- especially those over age 65 -- to be aware of how much vitamin A is in their daily multivitamin to avoid getting too much.
Other experts say the findings merit serious consideration from government agencies that regulate current vitamin A fortification levels.
In an editorial that accompanies the study, Paul Lips, MD, PhD, of the department of endocrinology at Vrije Universiteit Medical Center in Amsterdam, The Netherlands, says eye disease due to vitamin A deficiency is a risk primarily among malnourished children. But the bone-weakening effect of excessive intake of vitamin A may pose a threat to a growing number of older adults.