A study published last month in the New England Journal of Medicine linked high levels of vitamin A to bone fractures, echoing findings of two other studies published in the last year.
While this study is not considered conclusive in linking high intake of vitamin A to increased risk of osteoporosis, the developing body of research is leading some experts to advise consumers to be more vigilant about abiding by federal intake guidelines for vitamin A and the degree to which foods fortified with it can increase risk of poor bone health.
In an editorial published with the study, Paul Lips of Vrije Universiteit in Amsterdam wrote that "vitamin A supplementation and fortification of food with vitamin A may be harmful" in well-fed populations where osteoporosis is growing.
The latest study, conducted by Karl Michaelsson and colleagues at the University Hospital in Uppsala, Sweden, followed 2,322 men over 30 years and found that those with the highest level of serum retinol, or vitamin A in the blood, suffered the highest number of bone fractures.
The group with the highest level of serum retinol had 1.6 times more fractures of all sorts than the middle group. The men who broke hips were 2.5 times more likely to come from the high-retinol group than from the middle group.
The study found no connection between bone fractures and beta carotene, a nutrient found in fruit and vegetables and vitamin supplements, which turns into vitamin A in the body.
The National Academy of Sciences' Institute of Medicine (IOM) revised its recommended daily allowance for vItamin A downward in 2001 to 2,300 I.U. (international units) per day for women and 2,970 I.U. for men. (Previous recommendations had been about 10 percent higher.) The academy also set its first upper limit on the vitamin, advising people not to exceed 10,000 I.U. per day. This followed publication of earlier studies connecting high levels of A with birth defects, liver problems and a variety of other ills.
Many cereals and milk products are fortified with vitamin A; it's also found in multivitamins, various dietary supplements and in many foods, including meat (especially beef liver), fish (especially cod-liver oil) and eggs.
Some experts fear that eating a diet high in A and rich in fortified foods, then popping a couple of innocuous-seeming supplements, can consistently put people over the recommended daily allowance and even the daily upper limit -- and increase risk of osteoporosis.
The study is considered limited partly because it measured levels of A in the blood but did not gather information about the subjects' diets. Some experts believe that the body, which stores vitamin A in fat, keeps blood levels of retinol fairly constant regardless of dietary intake. If that's the case, this study can connect only high blood A levels with fractures, but not high intake of dietary or supplemental A to those fracturers.
Robert Russell, professor of medicine and nutrition at Tufts University School of Medicine and chair of the panel that wrote the recent IOM report on vitamin A, says high serum retinol levels may have some connection to bone fragility, but he questions the trend toward believing vitamin A in the diet leads to broken bones.
The possible link "is an interesting question, but I'm not sure the question is an entirely dietary one," Russell said.
Earlier studies linked high dietary intakes of A to poor bone health, but did not measure blood levels of the vitamin.
Nevertheless, the study's authors say their findings suggest that current levels of vitamin A supplementation and food fortification be reassessed.
"We're having this flooding of nutrients," said Wahida Karmally, director of nutrition for the Irving Center for Clinical Research at Columbia University and spokeswoman for the American Dietetic Association."They're putting vitamins in bottled water now, and people are getting vitamins from nutrition bars, lozenges. It's possible to get too much of a good thing."