Vitamin A is an essential nutrient important in bone development and metabolism. It can be consumed through foods containing any of the variants of vitamin A, such as retinol or retinoic acid, or in foods with precursor molecules such as beta-carotene and alpha-carotene, which can be converted into vitamin A within the body. Multivitamins also generally contain vitamin A.
Toxicity
Individuals who get more than 1,500 mcg, or 5,000 IU, per day of vitamin A for long periods of time may be at risk for decreased bone mineral density, a precursor to osteoporosis, as well as being more prone to developing bone fractures. Infants who get too much vitamin A may have slowed growth of the long bones, such as the femur in the leg.
Deficiencies
In older adults, a severe deficiency of vitamin A may also cause a loss of bone density. This indicates that vitamin A should be consumed in amounts close to the RDA of 700 mcg daily for adult women and 900 mcg daily for men. A deficiency of vitamin A during fetal development and infancy may lead to bone abnormalities, such as slow bone development.
Possible Mechanisms
A potential mechanism for the effects of vitamin A on bone is that retinol can stimulate the normal process of reabsorption in bone. Another potential reason is that vitamin A may cause problems with the activity of vitamin D, another essential nutrient that works to maintain the balance of calcium and phosphorus in the body. Vitamin A may prevent vitamin D from aiding in the absorption of calcium by the body. Disruption of vitamin D activity may cause calcium to leach out of the bones, making them more fragile.
Considerations
Only true vitamin A, or retinol, causes toxicity that can affect bone. Beta-carotene does not have the same effect, even when consumed at extremely high levels. Vitamin A toxicity usually comes from supplementation, as few foods contain enough retinol to cause problems. Most foods that supply vitamin A actually contain it as beta-carotene and other carotenoids. Cod liver oil is one of the few foods with high retinol content. Foods fortified with vitamin A may also contribute to an overdose if consumed in conjunction with multivitamins.
Recommendations
Because the negative effects of excess vitamin A are linked to preformed vitamin A, you should strive to take in most of your vitamin A consumption in the form of beta-carotene. Try to avoid daily doses of pure vitamin A over 1,500 mcg and below 700 mcg if you are female or 900 mcg if you are male. Keeping consumption close to the RDA guidelines can help prevent both deficiencies and toxicity.
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