Q. Is it true that women with osteoporosis should not supplement with more than 6,000 IU daily of vitamin A? What about the vitamin A in food?
A. Vitamin A is an important nutrient essential for normal vision, growth, and immunity. The vast majority of published studies on vitamin-A supplements show an overwhelmingly positive effect on health. However, recent research indicates that, for both women and men, excessive dietary intake of vitamin A in the form of retinol may be associated with decreased bone-mineral density and increased risk for hip fracture. Fortunately few foods, mainly liver and fatty fish, contain a significant amount of retinol. Instead, most foods provide vitamin A in the form of beta-carotene, and the body will convert it into retinol only if it is needed. Beta-carotene is about 100 times safer than retinol.
There is an upside-down, U-shaped association between retinol intake and bone-mineral density. Bone-mineral density is poor with low intake of retinol, optimal when the retinol intake is 2,000 to 3,000 IU per day, and worse again when retinol intake goes above 7,000 IU per day. Thus, both low and high intakes of retinol are bad for bone-mineral density. Unfortunately for those at risk for osteoporosis, as people grow older they appear to have a lower capacity to dispose of excess retinol and are thus more sensitive to an excess.
In animals, the most common toxic effect of excess retinol is spontaneous bone fracture. Animal studies indicate that excess retinol stimulates bone resorption, inhibits bone formation, and changes enzyme activity and gene expression. Any or all of these effects tend to produce bone loss and to contribute to development of osteoporosis. Similar studies on humans are not ethical, but observations show that the highest incidence of fractures is found in northern Europe, where dietary intake of retinol is unusually high.
Such observations do not constitute proof of a direct link between retinol consumption and osteoporosis. There are many factors that may enter into the interpretation of these observations. Perhaps the most obvious is vitamin D, which is produced in smaller amounts because of the reduced sunlight at northern latitudes. Other possible factors include:
• high intake of animal protein, which is high in phosphorus and increases calcium requirement;
• heavy consumption of caffeine, which contributes to an increased loss of calcium from the body;
• increased use of soft drinks, which are often high in both phosphorus and caffeine;
• lower stomach acid, which is more prevalent in older people and reduces the ability to absorb calcium;
• calcium supplements, which may improve bone density but also reduce the absorption of iron, zinc, manganese, and other trace minerals that are also essential to prevent osteoporosis and other nutritional problems.
Clearly, more research is needed to confirm any link between excessive retinol intake and increased loss of bone mass. Based on current evidence it seems prudent for people at risk of osteoporosis to limit their intake of retinol, but not necessarily that of beta-carotene, to 3,000 IU per day or less.
This article appears in May 23 • 2003.