July 22, 2026, 11:04 am | Read time: 4 minutes
Today, the belief persists that sunlight automatically provides the body with enough vitamin D in the summer. In fact, the body produces most of the vitamin itself–provided the skin gets enough UVB radiation from the sun. But does this really apply to everyone? Scientists have now investigated this question. Their findings could at least partially challenge the widespread assumption.
More Than Half Had Low Vitamin D Levels
The study results were clear. Among participants aged 65 and older, 54.8 percent had a vitamin D level below the threshold of 50 nanomoles per liter of blood, considered adequate. The percentage was even higher among adults with darker skin: 72.1 percent were below this level.
Sun Alone Isn’t Always Enough
With the first sunny days, the topic of vitamin D often fades from view for many people. After all, the body produces the vitamin with the help of the sun’s UVB radiation. Therefore, summer is often seen as the season when vitamin D stores naturally replenish. But does this really apply to everyone?
Previous studies had already shown that this is not the case for everyone.1,2 Older people and those with darker skin, in particular, often have lower vitamin D levels than the general population. However, it was unclear whether these levels significantly improve during the summer months. Scientists at Newcastle University specifically investigated this question. They examined adults aged 65 and older, as well as adults with darker skin living in northern Great Britain.
How the Study Was Conducted
The recently published work did not examine the effects of vitamin D supplements. Instead, the scientists analyzed data collected before the start of a larger study.3 Before participants were enrolled in this study, researchers first determined their vitamin D levels. The results of this preliminary investigation have now been scientifically evaluated.
How the Study Group Was Assembled
Between December 2024 and August 2025, a total of 936 people signed up for the study. However, many did not meet the criteria or were already taking vitamin D supplements. In total, 299 people were examined: 168 participants were 65 years or older, and 147 had darker skin pigmentation. Since some older participants also had darker skin, they were included in both analyses.
To determine vitamin D levels, a small drop of blood from the fingertip was sufficient. This was then analyzed using a particularly precise laboratory method. Participants with a vitamin D level of at least 50 nanomoles per liter of blood were considered adequately supplied. Levels below this were classified as insufficient.
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Even in Summer, Levels Remained Low
It was also notable that this pattern hardly changed over the seasons. It would have been expected that vitamin D levels would rise with increased sun exposure. However, this expected effect was largely absent in the current study.
Among the older participants, the proportion with low vitamin D levels averaged about 56 percent between June and August. Among adults with darker skin, researchers found no noticeable change between winter, spring, and summer. Monthly evaluations showed that low vitamin D levels remained common throughout the study period.
According to the authors, the results suggest that sunlight alone is not sufficient for many people in the two groups studied to raise their vitamin D levels to an adequate level by summer.
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What the Results Mean
The results suggest that older people and adults with darker skin in northern Great Britain may not be able to rely solely on summer sun to achieve adequate vitamin D levels.
The authors see this as an indication that recommendations for vitamin D intake could more strongly consider these population groups. In their view, additional measures such as vitamin D supplements year-round could be beneficial. However, whether this is indeed the best strategy was not investigated.
The study also does not answer why vitamin D levels remained low for many participants despite the summer. The authors cite diet, darker skin pigmentation, and cultural or lifestyle influences as possible explanations. However, these were not directly investigated.
Where the Study’s Significance Ends
The study shows how frequently low vitamin D levels occurred in the two population groups examined. However, it cannot clarify the causes of these levels or prove that sunlight is generally insufficient or that taking vitamin D offers health benefits.
Moreover, only people from northern Great Britain who volunteered for the study were examined. Therefore, the results cannot be easily generalized to other population groups or countries. Additionally, the study did not account for some possible influencing factors–such as liver function or the simultaneous intake of multiple medications.
The study was fully funded by BetterYou Ltd. According to the authors, the company had no influence on the study design, execution, or evaluation of the results.