An analysis found that some older people and some darker-skinned adults may not move far out of vitamin D deficiency even as summer daylight increases.

On July 28 local time, online media outlet Gigazine reported that researchers at Newcastle University in Britain analysed pre-screening data from a clinical trial of northern England residents. They found the share of people whose vitamin D levels fell short of a benchmark stayed high even in summer.

The analysis used pre-screening data from a separate Newcastle University team’s clinical trial comparing ways of administering vitamin D supplements. The participants were 299 people who took part in screening from December 2024 to August 2025. Eligibility was either adults aged 65 and over, or adults aged 18 and over who belonged to an ethnic minority and had relatively dark skin. The latter group had to fall into Fitzpatrick skin types IV to VI, which classify skin response to sun exposure.

Participants gave a small blood sample from a fingertip, and a specialised testing organisation measured 25OH vitamin D concentration, which indicates vitamin D status. The researchers classified values below 50 nanomoles per litre (nmol/L) as below the benchmark, 31 to 49 nmol/L as insufficient and below 30 nmol/L as deficient.

In the group aged 65 and over, the below-benchmark share fell to 25.0 percent in May 2025 but then rose again. It was 63.4 percent in June, 47.8 percent in July and 55.5 percent in August, with a June-to-August average of 55.6 percent. This differs from the usual expectation that vitamin D levels recover in summer.

In the group of ethnic minority adults with relatively dark skin, the improvement trend was even less clear. The below-benchmark share was at least 64.0 percent in every month. It was tallied at 69.3 percent in June, 75.0 percent in July and 83.3 percent in August, showing no tendency to decline from winter to summer.

The researchers explained that vitamin D can be consumed through food, but it is also synthesised in the body when skin receives ultraviolet B. They said the ability varies widely by individual. Older people produce less in the skin, and darker-skinned people need more sunlight to make the same amount of vitamin D because melanin blocks more ultraviolet B.

Co-researcher Bernard Koff pointed to the lack of clear improvement in monthly screening results even in summer as the most notable part of the findings. He viewed the results as showing no sign of levels improving even in summer, when vitamin D recovery is usually expected. He also said that in places such as northern England, it may be difficult for older people and ethnic minorities in particular to make enough vitamin D from sunlight alone.

The study’s limitations are also clear. This survey did not track the same people from winter to summer, but compared data from different people tested in each month. It also did not measure participants’ actual sunlight exposure. The researchers said these factors mean they cannot conclude at the individual level that vitamin D did not increase despite summer sunlight.

The findings therefore are read as a signal that an increase in summer daylight does not necessarily lead directly to normalised vitamin D levels. They also showed that among older people in northern regions and adults with darker skin, more precise follow-up is needed to see whether deficiency is resolved by seasonal change alone.