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We May Be Getting Too Much Vitamin A
The possibility has been raised that long-term high vitamin A intake could contribute to fracture risk in humans. It has been known for a long time that toxic amounts of vitamin A affect bone remodeling and can have adverse effects on bones in experimental animals. Likewise, it is common knowledge in public health circles that a lack of vitamin A is one of the leading causes of blindness worldwide; so the thought of getting too much vitamin A may seem far-fetched.
For 18 years, researchers at the Harvard School of Public Health followed 72,337 women within the Nurses' Health Study, aged 34-77 years. They divided the women into 5 groups from lowest to highest levels of vitamin A intake and assessed the groups for a relationship between vitamin A intake from foods and supplements and the risk of hip fracture among postmenopausal women.
What the researchers found was that from 1980 to 1998, 603 incident hip fractures resulting from low or moderate trauma were identified. After controlling for confounding factors, women in the highest quintile of total vitamin A intake (10,000 International Units or more or 3000 :g/d of retinol equivalents [RE]) had a significantly elevated relative risk of hip fracture compared with women in the lowest quintile of intake (<1250 :g/d of RE or <4167 IUs). This increased risk was attributable primarily to retinol (pre-formed vitamin A). The association of high retinol intake with hip fracture was reduced among women using postmenopausal estrogens.
Beta carotene did not contribute significantly to fracture risk. Women currently taking a specific vitamin A supplement had a nonsignificant 40% increased risk of hip fracture compared with those not taking that supplement, and, among women not taking supplemental vitamin A, retinol from food was significantly associated with fracture risk.
The conclusions from this study are that long-term intake of a diet high in retinal/pre-formed vitamin A may promote the development of osteoporotic hip fractures in women. The amounts of retinol in fortified foods and vitamin supplements may need to be reassessed.
Implications for Extension teaching: Encourage eating fruits and vegetables and other foods with vitamin A in them instead of depending on a dietary supplement. Much of the vitamin A in food is in the form of beta carotene, which can be converted by the body to vitamin A if needed. Retinol is vitamin A, not the precursor. Supplements usually contain retinol, so people taking a dietary supplement can quickly reach high intake levels of the vitamin - far quicker than individuals using food as their vitamin A source.
Source: Journal of the American Medical
Association, Jan 2, 2002;287:47-54 File: 7.5x Human Nutrition - Nutrition Research
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