The modest levels of vitamin A present in supplements may reduce cortical bone thickness and increase fracture risk if consumed long term, animal studies suggest.

While it is known that high doses of vitamin A decrease bone thickness when given for short periods, Swedish researchers have now shown in mice models that prolonged exposure to modestly increased vitamin A intake may also have an adverse impact on cortical bone.

In a study published in the Journal of Endocrinology they showed that  sustained intake of vitamin A, at levels equivalent to 4.5-13 times the human recommended daily allowance (RDA), caused significant weakening of the bones.

Previous studies have shown that short-term overdosing of vitamin A, at the equivalent of 13-142 times the RDA in people, results in decreased bone thickness and an increased fracture risk after just one to two weeks.

Dr Ulf Lerner and colleagues from Sahlgrenska Academy at the University of Gothenburg say their study is the first to examine the effects of lower vitamin A doses that are more equivalent to those consumed by people taking supplements, over longer time-periods.

They observed time- dependent decreases in periosteal circumference with the vitamin A supplemented dose. The reductions in cortical bone resulted in a significant time-dependent decrease of predicted strength and a non-significant trend towards reduced bone strength.

Dynamic histomorphometry demonstrated that bone formation was substantially decreased after one week of treatment at the periosteal site with the supplemental dose.

Increasing amount of vitamin A decreased endocortical circumference, resulting in decreased marrow area, a response associated with enhanced endocortical bone formation.

However vitamin A had no effect on cortical bone in the presence of bisphosphonate, suggesting a role for osteoclasts, the researchers noted.

“In our study we have shown that much lower concentrations of vitamin A, a range more relevant for humans, still decreases rodent bone thickness and strength,” Dr Lerner commented.

“Overconsumption of vitamin A may be an increasing problem as many more people now take vitamin supplements. Overdose of vitamin A could be increasing the risk of bone weakening disorders in humans but more studies are needed to investigate this,” he concluded.

Dr Lerner add that his team now intends to investigate if human-relevant doses of vitamin A affect bone growth induced by exercise, and the effects of vitamin A supplementation in older age, where growth of the skeleton has ceased.