There is only one supplement that the NHS recommends almost everyone to take: vitamin D.

That’s because a third of us are deficient in this nutrient over winter, when British sunshine is too weak for our skin to manufacture it. Even in summer, many spend too much time indoors in an office, or covered up, for it to reach adequate levels in the body.

“It’s shocking what the average vitamin D level is,” said Professor Cathie Martin, a nutrition researcher at the John Innes Centre.

Vitamin D’s best-known benefit is to our bones because it helps the gut absorb calcium, a crucial mineral needed for the skeleton. When children become deficient, they may get rickets, with bowed legs.

But vitamin D has several other health benefits, with two pieces of research out this week adding to the list. Here’s the science behind it.

Boosting brain power

Long after the 1920s discovery that vitamin D reverses rickets, scientists realised that the nutrient also regulates many aspects of the metabolism and should really be classed as a hormone.

Cells in several areas of the brain have receptors for vitamin D, and people with lower levels of it in the blood have a higher risk of dementia.

It also seems to be involved in brain growth of foetuses in the womb. A study published this week in JAMA Network Open has suggested that if pregnant women take high-dose supplements, their children could turn out smarter.

Vitamin D is already in most prenatal vitamin tablets, at levels of 10 micrograms a day. The trial compared this standard dose to tablets with four times that amount, in about 500 women.

By the time their children were ten, those whose mums got the higher dose did better in some kinds of memory and thinking tests, the study found.

“It could have significant public health benefit in the future if the findings are real,” said Professor Andrew Shennan, an obstetrician at King’s College London who was not involved in the study.

But Professor Shennan also warned the results could be a fluke, as the children were given several types of intelligence tests and the high-dose group did not get better scores in all.

Fighting infections

Vitamin D receptors are also found in many types of immune cells and there is some research to suggest it helps us fight infections.

Several studies have found that people with lower vitamin D levels are more susceptible to respiratory infections, like colds and flu.

In fact, during the Covid pandemic, some researchers argued that vitamin D supplements might help the UK population fight off the virus, and clinically vulnerable people were recommended to take free tablets.

But randomised trials – the best kind of medical evidence – have given mixed results on whether people should use vitamin D as a treatment for infections.

Dampening pain

The immune system consists of many different cells and hormone-like chemical messengers. Vitamin D doesn’t just rev them all up, but seems to selectively tweak the activity of different components to make the body function better.

Sometimes, certain cells of the immune system become too active, a state called chronic inflammation. This can contribute to many different medical conditions, including persistent pain.

Some previous studies have shown people with low vitamin D are more predisposed to long-term pain. Now a study, published this week in Regional Anesthesia & Pain Medicine, has found this may also be the case in women after surgery for breast cancer.

In a group of 184 women, those who were vitamin D deficient were three times more likely to have moderate or severe pain after the operation.

“Vitamin D supplementation in breast cancer patients with [low] vitamin D levels may have a role in modulating postoperative pain,” said Dr Mohamed Hasan Ragab, an anaesthetist at Fayoum University in Egypt and an author of the study.

Fighting multiple sclerosis and rheumatoid arthritis

Another group of conditions caused by overactivity of the immune system are autoimmune diseases – when immune cells attack parts of the person’s own body – like multiple sclerosis (MS), which involves nerve damage, and rheumatoid arthritis, which causes painful joints.

People are less likely to get both of those conditions if they have lower vitamin D levels. And a trial in 26,000 people showed taking the vitamin as a supplement cut people’s chances of developing autoimmune conditions as a group.

How to know if you have low vitamin D

GPs will offer a blood test for vitamin D levels if people have symptoms like bone pain or muscle weakness, or if they are in certain high-risk groups like being 65 or over. Supplements may be given through the NHS or can be bought at pharmacies.

But excessive levels of vitamin D can also be dangerous, leading to high blood calcium levels. So people should follow their physicians’ advice and not take more tablets than recommended.