Do you need vitamin D through the winter? New research casts doubt on its benefits as you get older

As the evenings get longer, many people reach for vitamin D supplements. Often called the ‘sunshine vitamin’, it is widely taken in the UK and is considered important for healthy bones, teeth and muscles. The NHS advises adults to take 10mcg a day during the darker months, when sunlight is too weak for skin to produce enough of it.

Older adults often take it all year because the body makes less of it with age and many people spend less time outdoors. More recently, vitamin D has also been linked to immune function, and some research has suggested it could help protect against dementia and cancer.

Yet a new study has raised fresh questions about how much benefit it really provides.

Published in The BMJ, the research found that vitamin D supplements did not make a meaningful difference to fracture risk in older people.

The accepted view is that supplementation matters in winter because the sun sits lower in the sky.

Susan Lanham-New, professor of nutrition at the University of Surrey and a leading vitamin D expert, explains that when ultraviolet rays reach the skin, they trigger 7-DHC, a precursor beneath the surface, to begin producing an active form of the vitamin.

In the UK, around 10 to 15 minutes of midday sun on bare skin in spring and summer is usually enough to produce the recommended 10mcg a day. But in winter, the weaker sunlight does not provide the UVB rays the body needs, according to Professor Lanham-New.

Vitamin D’s main job is to help the body absorb calcium from food. Without enough of it, the body starts pulling calcium from the bones.

Over time, that can make bones more fragile and increase the risk of fractures and osteoporosis, a condition that affects more than three million people in Britain.

Although oily fish, eggs and fortified breakfast cereals contain some vitamin D, food usually provides only about a third of daily needs at best.

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

The NHS recommendation to take a supplement is based on evidence of the nutrient’s role in bone health and follows advice from the Scientific Advisory Committee on Nutrition in 2016.

Earlier this year, however, a group of UK doctors argued in Frontiers in Endocrinology that the definition of vitamin D deficiency is too low. They said the cutoff of 25nmol/l should be raised to 50nmol/l, which is the threshold 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, although uncommon, rickets has not disappeared, with around 480 hospital cases a year in England.

They also argued that obesity, darker skin, and deprivation can all increase the risk of deficiency, especially where diets may not provide enough vitamin D.

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

There is also evidence linking vitamin D to immunity and to 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 respiratory infections.

Researchers analysed data from 36,000 adults and found that those with very low vitamin D levels, defined as below 15nmol/l, were about a third more likely to be admitted with illnesses such as flu, pneumonia and bronchitis than people whose levels were above 75nmol/l. The team concluded that adequate vitamin D intake could help protect against respiratory infections.

The vitamin is also thought to support heart and brain health by helping regulate inflammation, blood-pressure-related hormones and blood vessel function, which could in theory reduce artery damage and high blood pressure.

Laboratory work also suggests it may slow cancer cell growth and reduce harmful inflammation by improving immune cell function, says Eamon Laird, a lecturer in nutrition and health at Atlantic Technological University in 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 reported by Sleep Medicine also found that older adults taking a daily high dose of 125mcg scored better on memory and cognitive tests than those who did not take it.

In addition, vitamin D helps protect cells from damage caused by unstable molecules and may support immune cells in clearing sticky amyloid plaques associated with dementia.

Still, those cognition findings should be treated cautiously, says Paresh Malhotra, 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 added that the study only used a questionnaire to work out who took vitamin D supplements and did not measure participants’ vitamin D levels.

Pills, patches or sprays: which form works best?

Vitamin D comes in several forms, including tablets, liquids, patches and oral sprays, and prices range widely from a few pence per tablet for a supermarket own-brand product to several times that for premium versions.

“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 2020 study in Nutrients found that tablets, capsules and drops were similarly effective when given at the same dose.

Other research suggests sprays work just as well, though patches are less well supported by evidence.

It is also important to choose a supplement containing vitamin D3, the form made by skin exposure to sunlight and found in fish and meat, because it is more effective for the immune system than D2, which comes from fungi and yeast, says Laird. But he warns against taking too much.

“It’s important to prevent vitamin D deficiency — but more is not necessarily better,” says Susan Lanham-New.

High doses, generally above 100mcg a day, can raise calcium levels and cause nausea, constipation, excessive thirst, fatigue and muscle pain. Over time, they can damage the kidneys.

“In rare cases, it is even possible to die from vitamin D toxicity,” says Laird.

More troubling is whether the latest research, which found no meaningful improvement in bone health among older adults, undermines the main reason many people take vitamin D in the first place.

The BMJ analysis pooled results from 69 trials involving 154,000 adults over 65 and found no difference in fracture rates over the next 2.5 years among those taking vitamin D.

The authors concluded that the evidence does not support routine supplementation to prevent fractures and suggested that older adults may be better off spending time and money on resistance training and balance work, both of which have proven benefits.

That aligns with earlier large trials, including a 2019 study in the New England Journal of Medicine, which found that taking 50mcg of vitamin D daily did not prevent fractures in generally healthy middle-aged and older adults. It also made no difference to cancer or heart disease.

A separate study involving 21,000 older Australians found that monthly vitamin D pills of 1,500mcg taken for up to five years did not reduce overall deaths, cancer deaths or cardiovascular deaths.

However, Professor Lanham-New says those trials mostly involved people who already had healthy vitamin D levels, and there is little evidence that extra supplementation helps when levels are already adequate.

So who should take it?

Professor Lanham-New says the standard daily dose of 10mcg is enough to keep most people above 25nmol/l during the winter months.

She adds that people who spend little time outdoors, including many care home residents, and those with darker skin, whose skin produces less vitamin D from sunlight, should take a supplement throughout the year.