Bright summer months may not be enough to bring vitamin D levels above the recommended threshold for everyone. A Newcastle University study found that more than 72 percent of younger adults with darker skin remained below the threshold.
The finding challenges the assumption that regular sunlight during the brightest part of the year reliably resolves low vitamin D levels. More than half of participants aged over 65 were also below the threshold.
The research followed 299 people from December 2024 to August 2025. It compared adults over 65 with adults over 18 who had darker skin.
| Participant group | Characteristics | Below vitamin D threshold |
|---|---|---|
| Older adults | Over 65 years old | More than half of participants |
| Younger adults | Over 18 with darker skin | More than 72 percent of participants |
Summer did not produce the expected recovery
Nutrition researcher Bernard Corfe said the results were unexpected because vitamin D levels did not improve as normally anticipated during summer. “What is surprising about these findings is that vitamin D levels did not improve, even in the summer months when we would normally expect recovery,” Corfe said, as quoted by Science Alert.
Vitamin D can come from food, but the body can also produce it in the skin after exposure to ultraviolet B, or UVB, sunlight. That ability makes sunlight an important part of the broader picture, but not a guaranteed solution for every individual.
Age is one factor that can affect this process. As people get older, their skin becomes less efficient at producing vitamin D, meaning equal sunlight exposure can produce different outcomes across age groups.
Skin melanin is another factor examined in the study. Melanin can act like sunscreen by blocking part of the ultraviolet radiation that reaches the skin, which may affect vitamin D production.
Location can add to the challenge
Geography also influences the strength of available sunlight. In northern parts of the world, weaker sunlight is one reason vitamin D deficiency is often discussed in countries with four distinct seasons, particularly during winter.
However, the Newcastle University findings indicate that the concern does not necessarily disappear after winter. Several months of sunlight may still be insufficient to lift vitamin D levels above the threshold for all participants.
The study does not present sunlight as irrelevant to vitamin D production. Instead, it highlights that sunlight exposure works alongside individual factors such as age, skin melanin, and the intensity of sunlight in a particular region.
Vitamin D has been associated with several health areas in previous research. A 2022 meta-analysis linked higher vitamin D levels with fewer depression symptoms among people already diagnosed with depression.
Scientists have also examined possible links between vitamin D and lower risks of cancer and diabetes. Research on those relationships is still ongoing, so the available findings do not provide a single answer about every potential health benefit of vitamin D.
The results underscore why sunny weather alone should not be treated as proof of adequate vitamin D status. The body’s response to sunlight can vary substantially between people and populations.
