Too much vitamin A may increase bone fracture risk: Study

Consuming too much vitamin A may decrease bone thickness, leading to weak and fracture prone bones, according to a study conducted in mice.

The study, published in the Journal of Endocrinology, found 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.

It suggests that people should be cautious of over-supplementing vitamin A in their diets, said researchers at the University of Gothenburg in Sweden.

Vitamin A is important for numerous biological processes including growth, vision, immunity and organ function.

Our bodies are unable to make vitamin A but a healthy diet including meat, dairy products and vegetables should be sufficient to maintain the body's nutritional needs.

Some evidence has suggested that people who take vitamin A supplements may be increasing their risk of bone damage, researchers said.

Previous studies in mice have shown that short-term overdosing of vitamin A, at the equivalent of 13-142 times the recommended daily allowance in people, results in decreased bone thickness and an increased fracture risk after just 1-2 weeks, they said.

This 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.

Mice given lower doses of vitamin A, equivalent to 4.5-13 times the RDA in humans, over a longer time period, also showed thinning of their bones after just eight days, which progressed over the ten week study period, researchers said.

"Previous studies in rodents have shown that vitamin A decreases bone thickness but these studies were performed with very high doses of vitamin A, over a short period of time," said Ulf Lerner from the University of Gothenburg.

"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," Lerner said.

Researchers intend to investigate if human-relevant doses of vitamin A affect bone growth induced by exercise, which was not addressed in this study.

They will also study the effects of vitamin A supplementation in older mice, where growth of the skeleton has ceased, as is seen in the elderly.