Jan. 22, 2003 -- Getting too much of a good thing may be partially responsible for the rise of osteoporosis and brittle bones among older people in the U.S. and other developed countries. A new study shows excessive intake of vitamin A can increase a person's risk of bone fracture by as much as seven times, and current fortification levels may need to be reassessed.
Prior studies have suggested that vitamin A plays a role in the gradual weakening of bones frequently associated with age, but this study is the first to show a link between levels of vitamin A in the blood and the long-term risk of bone and hip fracture.
Researchers say the findings suggest that excessive vitamin A intake may explain, in part, the high rates of hip fractures in Scandinavia and the United States where the use of vitamin supplements and vitamin A fortification is common.
Fortification of dairy products and cereals with vitamin A began in Scandinavia and the U.S. several decades ago in an effort to prevent night blindness, which is often the first sign of vitamin A deficiency. Vitamin A deficiency is also a major cause of blindness in developing countries.
In the study, Swedish researchers measured vitamin A (retinol) levels in blood samples from 2,300 men who were followed for up to 30 years and then compared those levels with fracture rates among the group.
They found men who had the highest levels of vitamin A at the start of the study were significantly more likely to suffer a bone or hip fracture, and the overall risk of fracture increased 64% in men with the highest levels compared with those with average levels. The risk of fracture among those with the highest levels of vitamin A was seven times higher than those with the lowest levels.
The findings are published in the Jan. 23 issue of The New England Journal of Medicine.
Laura Tosi, MD, chair of the women's health issues committee for the American Academy of Orthopaedic Surgeons, says the findings could have a huge impact on efforts to prevent osteoporosis. She says statistics show that an older person who breaks a bone is very likely to break another one and is at high risk of losing their independence due to a hip or vertebrae (back) fracture.
"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."
"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.
"The study by Michaëlsson and colleagues suggests that vitamin A supplementation and fortification of food with vitamin A may be harmful in Western countries, where the life expectancy is high and the prevalence of osteoporosis is increasing," writes Lips. "One may conclude from such data that supplements containing vitamin A should not be routinely used by men or women and that fortification of cereals with vitamin A should be questioned."