Many people assume that spending more time outdoors during summer is enough to restore vitamin D levels depleted over the winter. However, new research from the United Kingdom suggests that this may not be the case for many adults, particularly older people and those with darker skin.
A study led by Newcastle University and published in the European Journal of Clinical Nutrition found that vitamin D insufficiency remained common even during the sunniest months of the year. Researchers followed participants from winter through late summer and observed little overall improvement in vitamin D status, challenging the belief that summer sunshine alone is sufficient to correct low levels.
Who Was Most Likely to Remain Vitamin D Deficient?
The researchers followed 299 adults living in northern England between December and late summer, all of whom had low vitamin D levels at the start of the study. Approximately half were aged 65 years or older, while the remainder were adults with darker skin from minoritized ethnic groups. Individuals already taking vitamin D supplements were excluded.
Among participants aged 65 and older, nearly 55% continued to have vitamin D levels below recommended thresholds throughout the study period. The prevalence was even higher among adults with darker skin, with more than 72% remaining vitamin D insufficient despite the transition from winter into spring and summer. Overall, vitamin D levels changed very little across the seasons.
Lead author Bernard Corfe said the findings were unexpected. Under normal assumptions, increased daylight and stronger sunlight during summer would be expected to stimulate greater vitamin D production in the skin. Instead, the results suggest that for many older adults and people with darker skin living at northern latitudes, this seasonal increase may be insufficient.
Why Summer Sun May Not Be Enough
Vitamin D is unusual because the body can obtain it both through food and by producing it in the skin after exposure to ultraviolet B (UVB) radiation from sunlight. Deficiency is commonly viewed as a winter problem, particularly in countries located farther from the equator, where shorter days and lower sun angles reduce UVB exposure.
The study highlights several reasons why summer sunshine alone may not adequately restore vitamin D levels.
Age is one important factor. As people grow older, the skin becomes less efficient at producing vitamin D. Older adults may also spend less time outdoors, use more sun protection, or have health conditions that limit outdoor activity.
Skin pigmentation also plays a significant role. Melanin, the pigment responsible for darker skin, naturally reduces the amount of ultraviolet radiation that penetrates the skin. Although this provides protection against some forms of sun damage, it also means that people with darker skin generally require longer sun exposure to produce the same amount of vitamin D as individuals with lighter skin. At northern latitudes, where UVB intensity is already relatively low, this effect becomes even more pronounced.
Geography further contributes to the problem. Northern Britain receives relatively limited UVB radiation for much of the year, including parts of the summer. Frequent cloud cover, cooler weather, and lower sun angles can all reduce effective vitamin D production by limiting both UVB intensity and the amount of time people spend outdoors.
Research conducted in southern Europe illustrates how strongly vitamin D production depends on location and season. Scientists in Valencia, Spain, estimated that approximately seven minutes of midday summer sunlight may provide sufficient UVB exposure to support daily vitamin D production. During winter, however, the same location may require up to two hours of daily sun exposure—a duration that is often impractical and may not be advisable.
Why Vitamin D Matters
Vitamin D plays an essential role in maintaining bone health by supporting calcium absorption, helping reduce the risk of fractures and osteoporosis, particularly in older adults. It also contributes to normal muscle function and supports the immune system. Prolonged deficiency can lead to bone pain, muscle weakness, and, in severe cases, osteomalacia, a condition characterized by softening of the bones in adults.
Researchers continue to investigate whether vitamin D has broader health effects beyond the skeleton. Some observational studies have associated higher vitamin D levels with lower risks of depression, certain cancers, and type 2 diabetes. A 2022 meta-analysis published in Critical Reviews in Food Science and Nutrition reported that vitamin D supplementation was associated with reduced depressive symptoms among people diagnosed with depression.
However, scientists caution that many of these associations remain uncertain. Large randomized clinical trials have produced mixed findings regarding vitamin D’s ability to prevent chronic diseases in the general population. Consequently, most clinical guidelines continue to emphasize its established benefits for bone and muscle health while further research explores its potential effects in other areas.
Despite these uncertainties, experts generally agree that individuals at increased risk of vitamin D deficiency should not rely solely on sunlight. Older adults, people with darker skin, individuals who cover most of their skin for cultural or medical reasons, and those who spend little time outdoors remain at elevated risk, even during summer in northern regions.
Supplementation and Safety
The authors of the Newcastle study concluded that year-round vitamin D supplementation is likely to benefit many people at increased risk of deficiency. They noted, however, that their findings should not be generalized to the entire population because all participants had low vitamin D levels when the study began. Nevertheless, the persistence of vitamin D insufficiency across seasons supports existing public health recommendations.
In the United Kingdom, health authorities advise everyone aged four years and older to consider taking a daily supplement containing 10 micrograms (400 IU) of vitamin D during autumn and winter. Individuals with limited sun exposure, those who cover most of their skin, and people with darker skin are also advised to consider supplementation throughout the year. Similar recommendations are in place in several northern European countries.
In the United States, recommended daily vitamin D intake is 600 IU for most adults and 800 IU for adults aged 70 years and older. These recommendations include vitamin D obtained from both food and supplements. Fortified foods, including some dairy products and breakfast cereals, can also contribute to daily intake, although diet alone is often insufficient to correct a clinically significant deficiency.
Experts also emphasize that excessive vitamin D intake can be harmful. Because vitamin D is fat-soluble, excessive supplementation can lead to accumulation within the body. Very high doses taken over extended periods may cause hypercalcemia, a condition characterized by elevated blood calcium levels that can damage the kidneys, heart, and bones. Anyone considering high-dose supplementation should seek medical advice.
The Newcastle study was funded by BetterYou Ltd, a United Kingdom-based supplement company, and the authors acknowledged this industry support as a study limitation. Although commercial funding does not invalidate the findings, the researchers note that independent studies involving larger and more diverse populations will be important to confirm the results.
Overall, the findings suggest that many people living at northern latitudes—particularly older adults and those with darker skin—may not be able to rely on summer sunshine alone to maintain adequate vitamin D levels. Safe sun exposure remains beneficial, but appropriate supplementation, a balanced diet, and testing for people at increased risk are likely to remain important components of long-term vitamin D management.
