Summer is often seen as the season when the body naturally rebuilds its vitamin D reserves. Yet a new study suggests that for some people in Britain, brighter days do not automatically translate into healthier levels.
What the research found
Researchers at Newcastle University examined 299 adults in northern Britain between December 2024 and August 2025. The group included older adults living independently and people with darker skin who were not using supplements. Vitamin D status was measured through 25-hydroxyvitamin D, the standard blood marker for assessing levels.
The results showed that low vitamin D remained widespread across the seasons. Among participants aged 65 and over, 54.8 percent were below the study's insufficiency threshold of 50 nanomoles per liter. In the darker-skinned group, 72.1 percent fell below that level. Even in summer, the pattern barely changed.
Professor Bernard Corfe, who co-led the work, said the findings were notable because vitamin D levels did not recover in the months when they would usually be expected to rise.
Why sunlight was not enough
The study points to several reasons. Aging skin produces vitamin D less efficiently, and older adults may spend less time outdoors. Skin with more melanin also reduces vitamin D synthesis, even while offering natural protection from ultraviolet exposure. Diet can help, but foods such as oily fish, egg yolks, and fortified products can only partially close the gap.
The researchers also noted that northern latitude may limit the strength of sunlight available for vitamin D production, especially for groups already at higher risk.
What the findings mean
The study was not designed to represent the whole UK population, and it compared different participants across months rather than tracking the same people over time. Still, its message is clear: for older adults and people from minoritised ethnic backgrounds, summer sunlight alone may not be enough.
Current UK guidance already advises most people to consider 10 micrograms of vitamin D daily in autumn and winter, while those with higher risk factors may need it year-round. The new findings reinforce that advice and highlight the value of targeted nutrition strategies backed by clinical research.
As science sharpens its understanding of seasonal health, more personalized vitamin D guidance could become an important part of preventive care in the years ahead.
