As the nights draw in, many of us will be reaching for the vitamin D tablets. One of the UK’s most popular supplements, the ‘sunshine vitamin’ is known to be essential for healthy bones, teeth and muscles – and the NHS advises all adults to take a daily 10mcg supplement from October to April, when the sun’s rays are too weak for our skin to make it.

Many older people also take it year-round (as the skin’s production of it declines with age and they spend less time outdoors). More recently, research has suggested it also plays an important role in immunity and may even help to prevent dementia and cancer.

But a new study appears to raise questions about who really needs it.

Published in The BMJ last month, the research showed that vitamin D supplements made no meaningful difference to whether older people suffered bone fractures.

The accepted wisdom is that supplements are vital during winter months because of the low position of the sun in the sky. 

Susan Lanham-New, a professor of nutrition at the University of Surrey and a leading expert on vitamin D, explains: ‘When UV rays hit our skin, it triggers sterol 7-DHC [a precursor of vitamin D] that sits just under the skin, to commence production of an active form of the vitamin.’

One of the UK’s most popular supplements, the ‘sunshine vitamin’ is known to be essential for healthy bones, teeth and muscles and experts recommend a daily 10mcg dose over the winter

In the UK, ten to 15 minutes of midday sun on bare skin in spring and summer is generally considered sufficient to manufacture our body’s recommended daily need for 10mcg. But the weaker rays in the winter months do not contain the UVB rays that our bodies need to make it, says Professor Lanham-New.

Vitamin D’s main role is in helping your body to absorb calcium from food. If you don’t get enough, the body draws calcium out of your bones. 

Over time, this makes them more fragile, raising the risk of fractures and osteoporosis (which affects more than three million Britons).

Although certain foods, including oily fish, eggs and fortified breakfast cereals contain modest amounts, you’re only likely to get about a third of your daily requirement from food at best.

About one in six adults in the UK is deficient in vitamin D – and the figure rises to one in three during winter months.

The NHS recommendation to take a supplement is based on the evidence for the nutrient’s crucial role in bone health and follows a 2016 recommendation by an independent committee of experts, the Scientific Advisory Committee on Nutrition (SACN).

But earlier this year, the recommendation was challenged as being too low by a group of UK doctors. They argued in the journal Frontiers in Endocrinology that the definition of vitamin D deficiency – a blood level of anything under 25nmol/l – should be increased to 50nmol/l, the threshold used by the World Health Organisation, the US National Academy of Medicine and the European Food Safety Authority.

The doctors cited the fact that, although uncommon, rickets – weak bones due to a lack of vitamin D – has not been wiped out, with about 480 hospital cases a year in England. 

Another argument is that people with obesity, those with darker skin (which absorbs less from the sun) and people in deprived areas who may not have sufficient quantities in their diet, were more likely to be deficient.

Vitamin D intake from food has also fallen, partly because people are eating less meat and fish.

Vitamin D also plays a critical role in immunity and in preventing cardiovascular disease, cancer and dementia. People who are severely deficient in it are 33 per cent more likely to be hospitalised with respiratory infections, according to a recent study by the University of Surrey. 

The researchers analysed data from 36,000 adults and found that those with a severe vitamin D deficiency – classed as a blood concentration below 15nmol/l – were a third more likely to be admitted to hospital with infections such as flu, pneumonia and bronchitis, compared to those with levels over 75nmol/l. They concluded that taking adequate vitamin D could protect against respiratory infections.

Vitamin D’s main role is in helping your body absorb calcium from food. If you don’t get enough, the body draws calcium out of your bones which, over time, makes them more fragile

The vitamin is also thought to play a role in heart and brain health, by regulating inflammation, blood pressure-related hormones and the function of blood vessels, which could in theory reduce artery damage and high blood pressure. 

Laboratory research suggests it might also slow cancer cell growth and limit damaging inflammation (by enhancing immune cell function), says Eamon Laird, a lecturer of nutrition and health at Atlantic Technological University, Sligo, Ireland.

As he told Good Health: ‘As we get older, our immune systems become less efficient and produce inflammatory molecules that circulate more freely in the body, potentially causing damage and disease. It’s thought vitamin D might help to regulate this.’

A US study also found that older adults who took high doses (125mcg) of the vitamin daily scored higher on memory and cognitive tests than those who didn’t take it, reported the journal Sleep Medicine in August.

In addition, it protects cells from ‘rusting’ caused by unstable molecules in the body and environment, and may help immune cells to remove the sticky amyloid plaques which build up in those with dementia.

However, these results on cognition should be treated with caution, says Paresh Malhotra, a professor of clinical neurology at Imperial College London. ‘This was an exploratory study: The number of people studied was low and there was no before and after comparison,’ he said.

He adds that the study merely used a questionnaire ‘to determine who took vitamin D supplements’ and did not measure the vitamin D levels in participants.

More concerning is whether the latest research that shows no meaningful improvement in bone health among older people raises questions about the key reason to take vitamin D supplements in the first place.

The BMJ study pooled results from 69 trials, involving 154,000 adults aged over 65, and found that taking vitamin D made no difference to fracture rates over the following 2.5 years.

The authors concluded that the evidence ‘does not support routine supplementation’ to prevent fractures, and they speculated that older people may be better spending their time and money on resistance exercise and balance training, which have a proven track record in this regard.

This echoes earlier large trials, including a study published in the New England Journal of Medicine in 2019, which found that taking 50mcg of vitamin D a day did not prevent fractures in generally healthy middle-aged and older people. It also did not prevent cancer or heart disease.

A study in 2022 involving 21,000 older Australians who took a monthly vitamin D pill (1,500mcg) for up to five years found that it didn’t reduce overall deaths or cancer or cardiovascular deaths.

However, Professor Lanham- New says that these trials largely involved people who already had healthy levels of the vitamin and ‘there’s little evidence that giving extra vitamin D to those with adequate amounts will give extra protective benefits’.

So who should take it?

Professor Lanham-New says the recommended daily supplement of 10mcg is enough to keep most people’s blood level above 25nmol/l in the winter months.

In addition, those who spend little time outdoors, including many care home residents, and people with darker skin tones whose skin makes less vitamin D from sunlight, should all take a year-round supplement, she says.