Vitamin A is a vital nutrient that supports the body's development and strengthens the immune system. Because our bodies do not naturally produce vitamin A, some choose to take supplements. However, too much vitamin A is likely to harm bone health, researchers warn.

Normally, we derive vitamin A from the food we eat, such as carrots, sweet potatoes, beef liver, salmon, and several dairy products.

Having a balanced, healthful diet should ensure that we have enough vitamin A in our systems.

How much vitamin A someone needs depends on their age, as well as other factors.

The National Institutes of Health (NIH) state that the ideal daily intake of vitamin A is 900 micrograms retinol activity equivalents (mcg RAE) for men and 700 mcg RAE for women aged 19–50.

For example, half a cup of raw carrots contains about 573 mcg RAE, and 3 ounces of pan-fried beef liver contain 6,582 mcg RAE, according to the NIH.

Despite the fact that we can derive enough vitamin A from food, some individuals choose to boost their levels of vitamin A by taking supplements.

However, over time, this might lead to an overload of this nutrient, which can actually increase a person's risk of experiencing bone fractures. This is what researchers from the Sahlgrenska Academy at the University of Gothenburg in Sweden have found in a recent study.

The study's results—reported in the Journal of Endocrinology—indicate that taking too much vitamin A can make bones "thin out," thereby putting them at risk of fracturing easily.

The impact of too much vitamin A

The researchers conducted the study in mice, and it came on the heels of another project that also looked at the effect of oversupplementing vitamin A on bone health.

Previous studies in mice, the study authors explain, have tested the effects of short-term vitamin A overdosage.

Those studies found that rodents that took the equivalent of 13–142 times the recommended daily amount of vitamin A for humans had poorer bone health and an increased risk of fracturing after only 1 or 2 weeks.

This time, though, the team wanted to test vitamin A oversupplementation in conditions that more closely resembled those to which a person may be exposed when taking supplements over long periods of time.

So, study coauthor Dr. Ulf Lerner and team administered lower vitamin A doses—the equivalent of 4.5–13 times the recommended daily allowance for humans—for 1, 4, or 10 weeks.

The scientists saw that after only 8 days of oversupplementation, the mice's bone thickness had started to decrease. Over 10 weeks, the rodents' bones became increasingly fragile and prone to fracturing.

"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," explains Dr. Lerner.

"In our study," he adds, "we have shown that much lower concentrations of vitamin A, a range more relevant for humans, still decreases rodent bone thickness and strength."

In the future, Dr. Lerner and team would like to see whether oversupplementation of vitamin A can also impact bone growth related to exercise, as well as the effects of overdosing in older mice, hoping to simulate the impact of too much vitamin A in aging humans.

"Overconsumption of vitamin A may be an increasing problem as many more people now take vitamin supplements," warns Dr. Lerner.

"Overdose of vitamin A could be increasing the risk of bone-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 vitamin A."

–Dr. Ulf Lerner