The retinol form of vitamin A is always a hot topic when it comes to osteoporosis. Long-term intake of high dose retinol has been shown to cause bone loss and and increased fractures in laboratory animals. In the body, retinol is broken down into retinoic acid which interferes with the active form of vitamin D (1,25 dihydroxyvitamin D) and osteoblast/osteoclast remodeling activity. In humans, observational studies such as the Swedish Mammography Cohort and the US Nurses Health Study both showed increased fracture risk with greater than 1500 mcg/day (5,000 IU) and 1000 mcg/day (3,000 IU), respectively. Other studies have found no relationship between retinol intake and fracture risk.
In order to examine the long-term effects of vitamin A intake, researchers from Australia examined data from a cancer prevention study that gave high doses of retinol and/or beta-carotene supplements to asbestos-exposed individuals between 1990 and 2007. After studying the data, the authors concluded that there was “no increases in fracture risk after supplementation with beta-carotene or retinol for as long as 16 years. As such, this study does not support preveious reports of an increased fracture risk with higher intakes of dietary retinol.”
My recommendation, even with the results of this study, is to be cautious with supplemental vitamin A intake. Most multivitamins provide 5,000 IU (some contain 10,000 IU) of either retinyl palmitate or acetate. When taking multivitamins, look for products with 5,000 to 7,000 IU vitamin A as mixed carotenoids. Keeping retinol intake to no more than 2,500 IU/day (750 mcg) is probably wise especially if you have bone loss.
Ambrosini et al. 2012. No dose-dependent increase in fracture risk after long-term exposure to high doses of retinol or beta-carotene. Osteoporosis International DOI 10.1007/s00198-012-2131-6