Taking high doses of vitamin A pills could cause osteoporosis, according to new research.
A study found too much of the nutrient – found in oily fish, meat, dairy products and vegetables – weakens bones.
People should be wary about over supplementing it in their diets, scientists have warned.
Dr Ulf Lerner, of the University of Gothenburg, said: “Over consumption of vitamin A may be an increasing problem as many more people now take vitamin supplements.”
Up to 24 million Britons pop vitamin pills every day, fuelling a £400 million a year industry.
Dr Lerner said: “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.”
Using mice, his team found large quantities of the essential nutrient reduced bone thickness, leaving them prone to fractures.
Vitamin A is fat soluble and stored in the liver, where it can lead to damage in large quantities.
The NHS says the amount adults need is 0.7mg a day for men 0.6mg a day for women – and this should be achieved through the foods they eat.
Dr Lerner and colleagues found sustained intake at levels equivalent to 4.5 to 13 times the human recommended allowance triggered significant loss of bone strength in the lab rodents.
It is the first study to examine the effects of doses similar to those consumed by people taking supplements.
Osteoporosis, or brittle bone disease, affects three million people in the UK with two in three sufferers older women.
The sex hormone oestrogen protects them before the menopause.
Vitamin A is vital for numerous biological processes including growth, vision, immunity and organ function, reports the Journal of Endocrinology.
Our bodies are unable to make it but a healthy diet should be sufficient to maintain the right level.
Previous research has suggested people who take vitamin A pills may be increasing their risk of bone damage.
Earlier tests in mice in have shown overdoses of vitamin A equivalent to 13 to 142 times the advised daily intake in people results in decreased bone thickness and an increased fracture risk after just one to two weeks.
The latest findings published in published in the Journal of Endocrinology provide a more accurate assessment of the dangers humans face because they are based on smaller doses over a longer period of time.
Dr Lerner said the mice also showed thinning of their bones – after just eight days. And this progressed across the ten week trial.
He 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.
“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.”
His team is now set to investigate if human relevant doses of vitamin A affect bone growth induced by exercise, which was not addressed in their study.
Additionally, they will study the effects of vitamin A supplementation in older mice, where growth of the skeleton has ceased, as is seen in the elderly.
The NHS says vitamin A, also known as retinol, helps the body’s natural defence against illness and infection by boosting the immune system.
Good sources include cheese, eggs, oily fish, fortified low-fat spreads, milk and yoghurt and liver.
The latter is particularly rich in the nutrient so should not be eaten more than once a week.
Spinach, carrots, sweet potatoes and red peppers are other good sources as well as yellow fruit like mango, papaya and apricots.
The NHS says according to some research, having more than an average of 1.5mg a day of vitamin A over many years may affect your bones, making them more likely to fracture when you are older.
This is particularly important for older people, especially women, who are already at risk of osteoporosis.
Having large amounts of vitamin A can also harm your unborn baby.
So if you’re pregnant or thinking about having a baby, you should not eat liver or liver products, such as pate – or supplements.
The Department of Health says: “You should be able to get all the vitamin A you need by eating a varied and balanced diet.
“If you take a supplement that contains vitamin A, don’t take too much because this could be harmful.”
“Women who’ve been through the menopause and older men, who are more at risk of osteoporosis, should avoid having more than 1.5mg of vitamin A a day from food and supplements.”
It adds: “Having an average of 1.5mg a day or less of vitamin A from diet and supplements combined is unlikely to cause any harm.”