A new study by Newcastle University in the United Kingdom, published in the “European Journal of Clinical Nutrition,” showed that even during the brightest months of the year, vitamin D deficiency or insufficiency remained very common among most of the people studied.
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“What is surprising is that vitamin D levels did not increase even in the summer months, when we usually expect its stores in the body to be replenished,” says nutrition researcher Bernard Corfe.
Between December 2024 and August 2025, Bernard Corfe and his colleagues conducted vitamin D level tests on 299 people, using a simple finger-prick blood sample. Approximately half of the subjects were over 65 years old, and the other half were people over 18 with darker skin.
Among adults aged 65 and older, almost 55% were found to have lower than recommended vitamin D levels. The situation was even more pronounced among people with darker skin – more than 72% of them had vitamin D levels below the recommended limit.
Although the studies began in winter and continued through spring and summer, these indicators remained almost unchanged.
This is concerning because vitamin D plays an important role in maintaining bone health – it helps the body absorb calcium, which is especially important for older people. Vitamin D is also linked to muscle function, the immune system, and may affect mental health.
A large analysis published in “Critical Reviews in Food Science and Nutrition” in 2022 showed that higher vitamin D levels in the body were associated with fewer symptoms of depression in people diagnosed with depression.
However, scientists’ opinions differ on some claims about the health effects of vitamin D. Some researchers argue that studies linking vitamin D to a lower risk of cancer or diabetes are overemphasized. Scientific research in this area is ongoing.
Unlike most vitamins, vitamin D can be obtained not only through food but also produced by the body itself.
Our skin produces vitamin D when exposed to the sun’s ultraviolet rays. Therefore, vitamin D deficiency is often considered a winter problem – shorter days and weaker sunlight mean that our body’s vitamin D production “factories” do not operate at full capacity.
However, a new study shows that for some people, this summer vitamin D surge simply does not happen. Why? There are three main reasons.
The first is age. As we age, the skin becomes less efficient at producing vitamin D from sunlight. Older people may also spend less time outdoors. Skin pigmentation is also important.
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Melanin, which gives skin its color, acts as a natural protective filter against the sun and blocks UV rays. While this helps protect the skin from damage, it also means that people with darker skin typically need more sun exposure to produce the same amount of vitamin D as people with lighter skin.
Geographical location also has a significant impact on vitamin D production. Northern Britain, where this study was conducted, is at a latitude where sunlight is relatively weak for most of the year, and even in summer, not all people may get enough ultraviolet radiation exposure for the body to produce sufficient vitamin D.
Even if you live at lower latitudes, sunbathing may be unsafe for other reasons.
In the sunny city of Valencia, Spain, scientists found that about 7 minutes in the midday sun in July would be needed for the body to get enough vitamin D.
At first glance, this sounds simple, but in summer in Spain, temperatures in the sun can exceed 38 degrees Celsius – which is not only unpleasant but can also be dangerous to health.
In comparison, in winter, about 2 hours of sun exposure would be needed daily to meet vitamin D requirements.
The authors of the new study state that their findings challenge the common assumption that summer sun is sufficient.
“For people living in areas like Northern England, this study shows that sunlight alone may not be enough, especially for older people and individuals from minority ethnic groups,” said B. Corfe.
“The message is simple but important. If you belong to a higher-risk group, you cannot assume that spending more time outdoors in summer will solve the problem,” he added.
The study authors clearly emphasize that year-round vitamin D intake is “critically important.”
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