As autumn evenings become darker, many people will start taking vitamin D tablets. The UK’s ‘sunshine vitamin’ is vital for healthy bones, teeth and muscles, and the NHS recommends that all adults take a 10mcg supplement each day from October to April, when sunlight is too weak for the skin to produce enough.
Many older adults take vitamin D throughout the year because the skin becomes less efficient at making it with age and they may spend less time outside. In recent years, studies have also suggested that the vitamin could support immunity and potentially help protect against dementia and cancer.
However, new research has prompted fresh questions over exactly who needs vitamin D supplements.
A study published in The BMJ last month found that taking vitamin D supplements did not make a meaningful difference to the risk of bone fractures among older people.
The longstanding advice is that supplements are particularly important in winter, when the sun sits lower in the sky and its rays are weaker.
Susan Lanham-New, professor of nutrition at the University of Surrey and a leading vitamin D expert, 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.’
Vitamin D is one of the UK’s most widely used supplements. Known as the ‘sunshine vitamin’, it supports healthy bones, teeth and muscles, with experts recommending a daily 10mcg dose during winter
In the UK, around 10 to 15 minutes of midday sunlight on bare skin during spring and summer is generally considered enough to produce the body’s recommended daily 10mcg. During winter, however, sunlight is too weak and lacks the UVB rays needed for vitamin D production, says Professor Lanham-New.
Vitamin D’s primary function is helping the body absorb calcium from food. When levels are too low, the body takes calcium from the bones instead.
Over time, this can weaken the skeleton, increasing the risk of fractures and osteoporosis, a condition affecting more than three million people in Britain.
Oily fish, eggs and fortified breakfast cereals all provide some vitamin D, but even a varied diet is likely to supply no more than about one-third of the daily requirement.
Around one in six UK adults is thought to be deficient in vitamin D, rising to roughly one in three during the winter months.
The NHS advice to take a supplement is based on vitamin D’s established importance for bone health and follows a 2016 recommendation from the Scientific Advisory Committee on Nutrition (SACN), an independent panel of experts.
Earlier this year, however, a group of UK doctors argued that the current definition of deficiency sets the bar too low. Writing in Frontiers in Endocrinology, they said a blood level below 50nmol/l should be considered deficient, rather than the current UK threshold of below 25nmol/l. The higher figure is used by the World Health Organisation, the US National Academy of Medicine and the European Food Safety Authority.
The doctors pointed out that rickets, a condition causing weak bones because of vitamin D deficiency, has not been completely eradicated. Although uncommon, it accounts for around 480 hospital cases a year in England.
They also noted that people with obesity, those with darker skin—which absorbs less vitamin D from sunlight—and people living in deprived areas may be more vulnerable to deficiency because they may have less vitamin D in their diets.
Dietary intake has declined as well, partly because people are eating less meat and fish, both of which can provide vitamin D.
The vitamin has also been linked to immune function and the prevention of cardiovascular disease, cancer and dementia. A recent University of Surrey study found that people with severe deficiency were 33 per cent more likely to be hospitalised with a respiratory infection.
Researchers examined data from 36,000 adults and found that those with severe deficiency—defined as a blood concentration below 15nmol/l—were one-third more likely to be admitted to hospital with illnesses including flu, pneumonia and bronchitis than people whose levels exceeded 75nmol/l. They concluded that maintaining adequate vitamin D levels could help protect against respiratory infections.
Vitamin D helps the body absorb calcium from food. Without enough of the vitamin, calcium is taken from the bones, gradually making them weaker and more prone to fractures
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.
