During the bleak winter months of short days and cloudy skies, many people may seek out the benefits of sunshine in bottle form and reach for supplements of vitamin D. Made naturally by the skin when exposed to sunlight, this vitamin is needed to maintain healthy bones, teeth and muscles and to prevent them from becoming brittle and at risk of fracture.
But a review of evidence from clinical trials on the impact of supplements has found that attempting to get vitamin D through supplements is not so beneficial.
"We conclude that current evidence does not support the use of vitamin D supplementation to prevent disease," said Mark Bolland, associate professor of medicine at the University of Auckland, New Zealand, in a statement. Bolland led the research with Alison Avenell, a clinical chair in health sciences research at the University of Aberdeen.
According to the team, clinical trials have failed to show that supplementation reduces the risk posed by falls and fractures to bones and muscles. But they recognize that it may be beneficial in people who are at high risk, such as those in nursing homes and darker-skinned people living in colder climates.
To those at risk, researchers do recommend supplements during autumn and winter but also suggest getting advice on how best to get vitamin D naturally. "Vitamin D will protect people who are at high risk," Avenell said.
In spring and summer in the far regions of the Northern and Southern Hemispheres, such as the northern US and New Zealand, people tend to produce enough vitamin D through sunlight on their skin and foods in their diet. The vitamin helps the body absorb calcium to promote bone growth.
Getting enough vitamin D, calculated as 15 micrograms for ages 1 through 70 in the United States, also prevents rickets in children and osteomalacia -- bone softening -- in adults. "We're taking about preventing these diseases," said Avenell.
But during autumn and winter, levels drop. It helps to eat the right foods, such as oily fish, egg yolks, red meat and liver, to keep levels high inside the body -- which not everyone can achieve adequately. Fortified foods, including milk, cereals and spreads, provide the most vitamin D in the American diet, according to theNational Institutes of Health.
"In the US, vitamin D supplementation (through food) is higher," Avenell said.
In countries like the UK, however, foods are not fortified as often, and supplements are therefore recommended. Until recently, recommendations were mainly to people at high risk of rickets and osteomalacia, but this summer, Public Health England advised that everyone take the equivalent of 10 micrograms per day.
"That's a big change," Avenell said. "We don't think the evidence supports the necessity for that during winter."
In a separate article, also published Wednesday, Dr. Louis Levy, head of nutrition science at Public Health England, argued that the recommended dose is backed by the Scientific Advisory Committee on Nutrition, which also reviewed the evidence.
"When the days are darker and shorter and sun exposure is minimal, people should consider a daily 10 microgram vitamin D supplement, as it's difficult to get enough through diet alone," he said in a statement.
But Avenell's research argues that the supplements make no difference.
"They're unlikely to do any harm," she said. "But in the adult population, supplementing at the Public Health England levels does not prevent falls or fractures."
Research was also inconclusive on other benefits claimed to be associated with vitamin D. "We didn't see any conclusive evidence of vitamin D protecting against other conditions like heart disease or cancer (either)," she said.
An ongoing debate
The pros and cons of vitamin D have long been debated, but with this review questioning their benefit -- except among those at high risk -- many experts have pushed back in fear of the consequences if people stop taking them.
"Failure to address low vitamin D status during childhood, adolescence, for women of childbearing age and in the elderly could have serious long-term implications for public health," said David Richardson, visiting professor of food bioscience at the University of Reading. "Action is needed now in the face of growing evidence of a high prevalence of vitamin D insufficiency."
Martin Hewison, professor of molecular endocrinology at the University of Birmingham, agreed.
"It is clear that people in the UK are at high risk of vitamin D deficiency, particularly in winter," he said, though he highlighted Avenell's point that supplements are particularly "relevant for people at high risk of deficiency: those with darker skin from African, Afro-Caribbean and South Asian backgrounds; people who are confined indoors; and people who cover up their skin while outdoors."
Hewison also highlighted various levels of confusion and unknowns that persist around the issue. "There is still some debate about what levels of vitamin D count as deficiency, and this varies according to the disease being studied," he said.
"(But) the take-home message is that we need to know more about how vitamin D works before planning new clinical trials to assess its health benefits. In the meantime, Public Health England's vitamin D recommendations are conservative and sensible, and people should be following them."