Vitamin D is often called the sunshine vitamin because the body can produce it when skin is exposed to sunlight. But a new study suggests that summer weather may not be enough to correct low levels for everyone.
Researchers at Newcastle University in the United Kingdom analyzed vitamin D levels from 299 people screened for a supplementation trial between December 2024 and August 2025. The study focused on adults age 65 and older, along with adults with darker skin. The project was wholly funded by BetterYou Ltd., a U.K.-based company that sells nutritional supplements. The researchers reported that the company was not involved in the study design, delivery or interpretation of the data.
The findings, published in the European Journal of Clinical Nutrition, showed that vitamin D insufficiency or deficiency remained common in both groups. Among older adults, 54.8% had levels below 50 nanomoles per liter. Among participants with darker skin, the rate was 72.1%.
The percentage of participants with low vitamin D levels did not meaningfully decline during the summer months.
“What’s striking about these findings is that vitamin D levels didn’t improve, even in the summer months when we would usually expect them to recover,” said Bernard Corfe, a professor of human nutrition and health at Newcastle University who co-led the research.
The study adds useful context, but it does not show that summer sunlight failed to raise vitamin D levels in every participant. Researchers did not follow the same people from winter into summer. Instead, they screened different people at different points during the year.
The study also grouped vitamin D insufficiency and deficiency together. Those terms are related, but they are not interchangeable.
The U.S. National Institutes of Health considers blood levels below 30 nanomoles per liter to be associated with vitamin D deficiency. Levels between 30 and less than 50 nanomoles per liter are generally considered inadequate for bone and overall health in healthy people. Levels of at least 50 nanomoles per liter are generally considered adequate for most people.
Vitamin D plays an important role in bone health because it helps the body absorb calcium. A severe deficiency can contribute to rickets in children and osteomalacia, a condition that can cause weak bones and bone pain, in adults.
Researchers have also studied whether vitamin D may affect a much broader range of health outcomes. However, the evidence does not support treating vitamin D supplements as a catchall solution for preventing chronic disease. This study did not examine whether supplements improved health outcomes. It only looked at how common low vitamin D levels were among the people screened.
Some groups are more likely to have inadequate vitamin D levels. The skin’s ability to produce vitamin D declines with age. Melanin, the pigment that gives skin its color, also reduces the skin’s ability to make vitamin D from sunlight.
Season, cloud cover, time spent outdoors and the amount of skin exposed to sunlight can also make a difference. The new findings apply most directly to higher-risk groups living in northern Britain, where climate and latitude affect sun exposure. They should not be interpreted to mean that summer sunlight is inadequate for every person in every location.
Food can help contribute to vitamin D intake, although relatively few foods naturally contain much of it. Fatty fish such as trout and salmon are among the stronger natural sources. Egg yolks, cheese and some mushrooms also contain smaller amounts.
Fortified foods provide much of the vitamin D in American diets. Milk is commonly fortified in the United States. Many soy, almond and oat beverages also contain added vitamin D, although amounts vary by brand. Some breakfast cereals, orange juices and yogurts are fortified as well.
The recommended dietary allowance for vitamin D is 600 international units per day for most adults through age 70 and 800 international units per day for adults older than 70. Those recommendations assume minimal sun exposure.
Some people may need a supplement to meet their needs, but the right approach depends on individual circumstances. Taking more is not always better. Excessive vitamin D from supplements can be harmful, especially at high doses taken over time.