As autumn turns to winter, many people in the UK start stocking up on vitamin D supplements. Often called the “sunshine vitamin”, it is vital for maintaining healthy bones, teeth and muscles. The NHS advises adults to take a daily 10mcg supplement between October and April, when sunlight is too weak for the skin to produce enough naturally.
Some older adults take vitamin D throughout the year because the skin becomes less efficient at making it with age, while reduced time outdoors can also limit sunlight exposure. In recent years, research has suggested that vitamin D may have wider benefits, including supporting the immune system and potentially lowering the risk of dementia and cancer.
However, fresh research is prompting scientists to reconsider exactly who needs to take a supplement.
A study published in The BMJ last month found that vitamin D supplements did not significantly reduce the likelihood of bone fractures among older people.
The long-standing advice is that people should take vitamin D during the colder months because the winter sun sits too low in the sky to provide sufficient ultraviolet light.
Susan Lanham-New, professor of nutrition at the University of Surrey and a leading vitamin D researcher, says sunlight activates a substance called sterol 7-DHC, a vitamin D precursor found just beneath the skin. This process begins the production of the active form of the vitamin.
Vitamin D, one of the UK’s most widely used supplements, is essential for strong bones, teeth and muscles, with experts advising a daily 10mcg dose during the winter months
During spring and summer, around 10 to 15 minutes of midday sunshine on uncovered skin is generally thought to provide the body’s recommended daily amount of 10mcg in the UK. But Professor Lanham-New says winter sunlight is weaker and lacks enough of the UVB rays required for vitamin D production.
The vitamin’s primary function is to help the body absorb calcium from food. When vitamin D levels are too low, calcium is taken from the bones instead.
Over the years, 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 food typically supplies no more than around one-third of the daily amount needed.
Around one in six UK adults has a vitamin D deficiency, with the proportion increasing to approximately one in three during winter.
The NHS advice is based on vitamin D’s established importance for bone health and follows guidance issued in 2016 by the Scientific Advisory Committee on Nutrition, an independent panel of experts.
Earlier this year, however, a group of UK doctors argued that the current advice may not go far enough. Writing in Frontiers in Endocrinology, they said vitamin D deficiency should be defined as a blood concentration below 50nmol/l, rather than the current UK threshold of 25nmol/l. The higher level is used by the World Health Organisation, the US National Academy of Medicine and the European Food Safety Authority.
The doctors pointed to the fact that rickets, a condition in which vitamin D deficiency causes weak bones, has not disappeared entirely. About 480 cases are still recorded in English hospitals each year.
They also highlighted groups considered more vulnerable to deficiency, including people living with obesity, those with darker skin—which absorbs less vitamin D from sunlight—and people in deprived communities who may have limited access to vitamin D-rich foods.
Dietary intake may also be declining as more people reduce their consumption of meat and fish, two important food sources of vitamin D.
Beyond bone health, vitamin D is involved in immune function and has been linked to 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 respiratory infections.
Researchers examined health data from 36,000 adults and found that those with vitamin D levels below 15nmol/l had a one-third higher risk of hospital admission for illnesses including flu, pneumonia and bronchitis than people whose levels exceeded 75nmol/l. The team concluded that maintaining adequate vitamin D levels could help protect against respiratory infections.
Vitamin D helps the body absorb calcium from food. When levels are too low, calcium is drawn from the bones, gradually making them weaker and more vulnerable 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.
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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.
Vitamin D is available in various forms (for example, tablets, liquids, patches and oral sprays) and prices can vary dramatically – from just a few pence per tablet for a High Street own brand to ten times that for a high-end product.
‘But a daily 10mcg dose in a cheap pill is enough for most people and works just as well as other formulations,’ says Eamon Laird, a lecturer of nutrition and health at Atlantic Technological University, Sligo.
A 2020 study published in the journal Nutrients found that tablets, capsules and drops were similarly effective at equal doses.
Other research suggests sprays work equally well. But patches are less proven.
It is also important to choose supplements containing the D3 form of the vitamin – the type our skin makes from sunlight and which is found in fish and meat – as it is more effective for the immune system than D2, the form found in fungi and yeast, says Eamon Laird. But don’t overdo it.
‘It’s important to prevent vitamin D deficiency – but more is not necessarily better,’ says Susan Lanham-New, a professor of nutrition at the University of Surrey.
High doses (considered to be above 100mcg a day) can raise calcium levels, leading to nausea, constipation, excessive thirst, fatigue and muscle pain. Over time, it can damage the kidneys.
‘In rare cases, it is even possible to die from vitamin D toxicity,’ says Eamon Laird.
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.