A study published in the Jan. 23 issue of The New England Journal of Medicine purports to show a link between high intake of vitamin A, or retinol, and increased risk of bone fractures. The research, taken from a 30-year population study conducted on 2,300 men at University Hospital in Uppsala, Sweden, suggests that vitamin A consumption of as little as 1.5 mg (5,000 IUs)—or less than twice the U.S. RDA for adult men—can have a significant impact in decreasing bone density and increasing the risk of bone fractures.
However, some nutritional experts have cautioned against reading too much into the study, saying that the correlation between high blood levels of vitamin A and high bone fracture rates in a population doesn't necessarily mean vitamin A is at fault. There may be other factors at work.
"We know that vitamin A can inhibit vitamin D from doing what it does, which is to keep bones strong," said Donald Brown, N.D., director of Natural Products Research Consultants in Seattle. The human body receives vitamin D largely through sunlight, and "where this research was done, the average person is not getting much sun exposure." Since vitamin D levels were not measured in the study, one possibility is that the observed correlation between bone density and vitamin A intake is a result of low levels of vitamin D, rather than high levels of vitamin A.
"In clinical reports of vitamin A toxicity—which report long-term vitamin A intake at levels 10 to 20 times higher [than those reported in the study], bone problems are not a major component," said Alan Gaby, senior editor for Healthnotes and former faculty member at Bastyr University. "If clinical vitamin A poisoning doesn't cause bone problems, why should a low dose do it?" Gaby suggested that the high blood levels of vitamin A found in those patients with a higher risk of bone fracture are simply markers for foods in the diet that contribute to poor bone health. "Vitamin A levels are a surrogate marker for three main foods— margarine, fortified cereals and milk," Gaby said. The trans fatty acids in margarine can interfere with good fats that support bone health; fortified cereals are themselves a marker for a high sugar diet, which can have a negative impact on bone health; and high milk intake can upset the body's balance between calcium and phosphorus, also leading to loss of bone density.
Because the Swedish data came from a population study, other reasons for bone density loss are not ruled out. The study also had several methodological shortcomings. Participants' vitamin A blood levels, according to critics, were measured only once during the research, and there was little correlation between participants' reported diet and supplement intake and their blood levels of vitamin A. "The only way to know is to do an intervention study," Gaby said. "Give participants either a low dose of vitamin A or a placebo, and follow them for a number of years."
In the meantime, what should consumers using multivitamins do? Most can simply use a multivitamin that contains beta carotene or other carotenoids—precursors to vitamin A that the body can convert to retinol as needed—rather than supplements containing retinol or vitamin A. In fact, most natural vitamin manufacturers already use beta carotene in their formulations.
But for certain populations, especially smokers, the choice is not so clear. "We know for a fact that synthetic beta carotene increases the risk of lung cancer in smokers, and is likely to increase the risk of prostate cancer," said Steve Austin, a Portland, Ore.-based N.D. The irony here is that beta carotene is thought to protect against cancer in nonsmokers.
Should smokers continue to take supplemental retinol? "Smokers have bones too," said Austin, "and already have more problems with bone loss than nonsmokers because of the anti-estrogenic effects of smoking." The best answer, he said, is for smokers to quit before taking a multivitamin.
Ultimately, further studies will be needed before a positive correlation between low level vitamin A intake and bone fracture rates can be proven. "Nobody knows what's in the crystal ball," said Austin. "That doesn't mean retinol will be proven to cause bone loss. At the moment, it's looking like trouble, and we don't have enough evidence to laugh this off, but in three years we may be thinking, no problem."
Mitchell Clute is a poet, musician and freelance writer based in Hotchkiss, Colo.
Natural Foods Merchandiser volume XXIV/number 3/p. 28