Vitamin A and bone health: the balancing act.

The role of vitamin A status as it relates to bone health is historical yet controversial. Population-based studies have linked high dietary intake of preformed vitamin A, which is obtained from animal-source foods, fortified foods, and some supplements, to greater risk of osteoporosis and hip fracture. In contrast, carotenoids, some of which are vitamin A precursors from plants, are associated with improved bone health. Carotenoids may be a biomarker that reflects a generally healthy lifestyle, which includes fruit and vegetable consumption. Current dietary recommendations to increase fruit and vegetable intake in the Dietary Guidelines for Americans will result in greater intakes of provitamin A carotenoids if consumers comply. This could lead to artificially high intakes of vitamin A in dietary analyses. However, multiple factors affect the bioconversion of provitamin A carotenoids to the active form of vitamin A. The human body will strive to maintain vitamin A balance by down-regulating provitamin A carotenoid bioconversion. If high preformed vitamin A intake is associated with poor bone health and provitamin A carotenoids are protective, future studies are needed to clarify the associations between total body stores of vitamin A, dietary intake of the pre- and pro-forms, and bone health throughout the life cycle.