Vitamin A and bone health

Abstract

The main sources of Vitamin A are preformed vitamin A (retinol or retinyl esters) from animal food, fortified foods and pharmaceutical supplements, and precursors as beta-carotene and other Vitamin A forming carotenoids from green plants, carrots and some fruits. Human hyper-vitaminosis A clearly involves the skeleton. However, studies evaluating the association between serum retinol level or retinol intake and skeletal health in humans showed inconsistent results. This inconsistency may be related to the difficulty in obtaining an accurate assessment of vitamin A intake, and to the use of serum retinol level, which is an unstable marker and a poor indicator of vitamin A status. There is some evidence that high vitamin A intake accelerates bone loss and increases fracture risk. The usual levels of intake are often close or beyond the approximate safety limit, and furthermore the proposed safety limit of 3,000 mcg retinol (10,000 IU) is not based on the hip fracture risk. Skeleton-related adverse effects of vitamin A are sometimes observed and only when supplement intakes reach 25,000 IU. Vitamin A seems to have a relatively narrow optimal dosage window with low and high intakes associated with negative effects, especially in elderly persons. In one study very high and low intakes were associated with a decreased BMD. In the elderly, the balance which ensures a sufficient retinol intake and simultaneously protects against excessive supplementation, is very delicate.