Too much vitamin A may increase risk of bone fractures

Vitamin A
Retinol or Vitamin A 3D space model (balls model). Credit: YassineMrabet, Wikipedia.

Consuming too much vitamin A may decrease bone thickness, leading to weak and fracture prone bones, according to a study published in the Journal of Endocrinology. The study, undertaken in mice, 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, and suggests that people should be cautious of over-supplementing vitamin A in their diets.

Vitamin A is an essential vitamin that 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. 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. 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.

In this study, Dr. Ulf Lerner and colleagues from Sahlgrenska Academy at the University of Gothenburg, report that 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 8 days, which progressed over the ten week study period.

Dr. Ulf Lerner commented, "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. 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."

Next, Dr. Ulf Lerner intends to investigate if human-relevant doses of vitamin A affect bone growth induced by exercise, which was not addressed in this study. Additionally, his team will study the effects of vitamin A supplementation in older mice, where growth of the skeleton has ceased, as is seen in the elderly.

Dr. Ulf Lerner cautions: "Overconsumption of vitamin A may be an increasing problem as many more people now take . Overdose of vitamin A could be increasing the risk of weakening disorders in humans but more studies are needed to investigate this. In the majority of cases, a balanced diet is perfectly sufficient to maintain the body's nutritional needs for A."


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More information: The study "Clinically relevant doses of vitamin A decrease cortical bone mass in mice" will be published in the Journal of Endocrinology on Tuesday 09 October 2018.
Provided by Society for Endocrinology
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Oct 17, 2018
The study would be much more palatable if they showed how it translates to human consumption of carrots.
How many carrots would one need to consume to have an intake of 4.5 to 13 times the RDA?
One website give the following quote:
when we are in pristine health, it requires at least six units of carotenes to convert into 1 unit of retinol (source). To put this in perspective, that means one must eat 4 1/2 pounds of carrots to potentially get the amount of useable A as in 3 oz. of beef liver (source).


One carrot contains roughly 20000Re.

So if one's digestive system is fairly OK, it's safe to eat lots of carrots without worrying about an overdose.

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