Study warns vitamin D deficiency remains widespread despite summer sun

The common belief that summer sunlight naturally restores vitamin D levels after the darker winter months seems to be misleading, according to a new study that found vitamin D insufficiency remained widespread even during the brightest months of the year.

Researchers from Newcastle University in the United Kingdom tested 299 people using a simple finger-prick blood test and found that vitamin D deficiency remained common among participants despite measurements being taken from winter through spring and summer, as the Science Alert outlet highlighted.

Among adults aged 65 and older, nearly 55% had vitamin D levels below the recommended threshold. The situation was even more pronounced among adults with darker skin, with more than 72% recording insufficient levels.

Researchers found that these figures changed little as the seasons progressed, challenging the assumption that increased exposure to summer sunlight automatically restores vitamin D levels.

Their findings were published in the European Journal of Clinical Nutrition.

Why vitamin D matters

Vitamin D plays an important role in maintaining bone health by helping the body absorb calcium, a function that becomes particularly important as people age. It is also involved in muscle function, immune system regulation and may influence mental health.

A large meta-analysis published in 2022 in Critical Reviews in Food Science and Nutrition found an association between higher vitamin D levels and fewer depressive symptoms among people diagnosed with depression.

However, scientists continue to debate some broader health claims associated with vitamin D. While some studies have suggested links between higher vitamin D levels and reduced risks of conditions such as cancer or diabetes, other researchers argue that those findings have been overstated. Further research is ongoing.

Unlike most vitamins, vitamin D can be obtained both through diet and produced naturally by the body. When skin is exposed to ultraviolet B (UVB) rays from sunlight, it begins producing vitamin D.

This process has led to the widespread belief that deficiency is primarily a winter problem, as shorter days and weaker sunlight reduce the body's ability to produce the vitamin.

Why summer may not solve vitamin D deficiency

Researchers identified several reasons why warmer months do not necessarily eliminate vitamin D insufficiency.

Age is one major factor. As people get older, their skin becomes less efficient at producing vitamin D from sunlight. Older adults may also spend less time outdoors, further reducing their exposure.

Skin pigmentation also plays a role. Melanin, the pigment responsible for skin colour, provides natural protection against UV radiation but can also reduce the amount of UVB available for vitamin D production. As a result, people with darker skin generally require more sunlight exposure to produce similar amounts of vitamin D compared with people with lighter skin.

Geography is another important factor. Northern regions, including much of the UK, receive relatively weak UVB exposure for much of the year. Even during summer, sunlight may not always be strong enough to produce adequate vitamin D for everyone.

The researchers also noted that increasing sun exposure is not always a straightforward solution, as excessive ultraviolet exposure carries other health risks.

Supplementation may be needed year-round

The study's authors say their findings challenge the assumption that summer sunlight alone can maintain adequate vitamin D levels, particularly among groups at higher risk of deficiency.

"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," said Bernard Corfe, a nutrition researcher at Newcastle University.

"The message is simple but important. If you are in a higher-risk group, you can't assume that spending more time outdoors in summer will solve the problem."

The researchers concluded that year-round vitamin D supplementation may be necessary for people who are particularly vulnerable to deficiency, rather than relying solely on seasonal sunlight exposure.

By Nazrin Sadigova