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Summer is supposed to refill the body’s vitamin D stores. Surely, especially in the midst of these heat waves, all that sunshine should restore our vitamin D. However, at least in Britain, the seasonal rescue may never arrive for many people who need it most.
A new study found that low vitamin D remained common from winter through summer among older adults and people with darker skin who were not taking supplements. Of those screened, 54.8 percent of participants aged 65 or older and 72.1 percent of those in the darker-skinned group had blood levels below 50 nanomoles per liter, the cutoff the researchers used for insufficiency. The rates did not fall as daylight lengthened.
That’s not to say all this extra sunshine is useless. Rather, the findings suggest that the usual summer rebound may not reliably lift some high-risk groups — particularly at northern latitudes and in people whose age or skin pigmentation reduces vitamin D production.
“What’s striking about these findings is that vitamin D levels didn’t improve, even in the summer months when we would usually expect them to recover,” said Bernard Corfe, a professor of human nutrition and health at Newcastle University who co-led the work.
The Summer Rebound That Never Came
The team recruited 299 people across northern Britain between December 2024 and August 2025. Participants used a finger-prick kit, and a specialist laboratory measured 25-hydroxyvitamin D, the circulating form commonly used to assess vitamin D status. The older volunteers lived independently rather than in care homes or sheltered accommodation.
Among older adults, the proportion below the study’s threshold averaged 55.6 percent from June through August. In the darker-skinned group, at least 64 percent fell below it in every month examined.
The researchers offer several plausible explanations. Aging skin produces vitamin D less efficiently, and older people may spend less time outdoors. Melanin absorbs ultraviolet radiation, protecting skin from damage but also slowing vitamin D production. Darker skinned people have more melanin. Diet can help offset some of the effects — oily fish, egg yolks and fortified foods provide some vitamin D — but the researchers mention natural food sources can only help so much.
Earlier results seem to concur. A 2021 UK Biobank analysis found that low vitamin D was more common among people of color, during winter and spring, and in northern parts of the country. Other studies reported especially high rates among South Asian and African-Caribbean adults.
But the new paper adds a new and potentially important point: in these recruited, unsupplemented groups, summer did not erase the risk.
“For people living in places like the North of England, this shows that sunlight alone may not be enough, particularly for older adults and those from minoritised ethnic backgrounds,” Professor Corfe said.
What the Study Can and Cannot Tell Us
This was not a representative survey of Britain’s overall population. Volunteers joined the screening phase of a supplementation trial, and people already taking vitamin D were excluded. The researchers also did not follow the same participants from winter into summer; they compared different people screened in different months.
Then, there’s the chosen cutoff. The study classified levels below 50 nanomoles per liter as insufficient or deficient. British public-health surveys often reserve “deficiency” for levels below 25 nanomoles per liter. Its headline figures therefore should not be directly compared with the often-cited estimate that roughly one-fifth of British adults have low vitamin D.
Perhaps the most important caveat is that the research was wholly funded by BetterYou Ltd., a company that sells nutritional supplements. The authors said the sponsor had no role in designing, conducting or interpreting the study.
Still, the study’s practical conclusion largely reinforces existing policy rather than overturning it. UK guidance advises most people to consider 10 micrograms, or 400 international units, daily during autumn and winter. Adults with risk factors, including darker skin or little sun exposure, are advised to consider it throughout the year.
Vitamin D’s main role in the human body is maintaining bones and muscles. Claims that supplements broadly prevent cancer, heart disease or other chronic illnesses remain contentious. The large VITAL randomized trial, for instance, found no reduction in invasive cancer or major cardiovascular events among generally healthy adults taking vitamin D.
“The message is simple but important,” Professor Corfe said. “If you are in a higher-risk group, you can’t assume that spending more time outdoors in summer will solve the problem.”
The findings appeared in the European Journal of Clinical Nutrition.
This story originally appeared on ZME Science. Want to get smarter every day? Subscribe to our newsletter and stay ahead with the latest science news.
