Researchers at Newcastle University measured vitamin D in nearly 300 people living across northern Britain and tracked their blood levels from winter through summer.
In adults aged 65 and older, and in participants from minoritised ethnic backgrounds, the expected summer rise did not appear. More than half of the older adults, and almost three quarters of the minoritised ethnic participants, fell below the threshold for adequate vitamin D, and those rates barely shifted once the sunny months arrived.
The finding cuts against a common assumption that time outdoors in warm weather is enough to keep vitamin D topped up. It also matches something dermatologists and nutrition researchers have known for years.
As people age, the skin becomes less efficient at making vitamin D from ultraviolet light, so the same amount of sun produces less of it.
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Melanin, the pigment that gives skin its color, also blocks some UVB rays, meaning people with darker skin generally need more sun exposure to reach the same blood level.
What the Newcastle team measured
The study, published in the European Journal of Clinical Nutrition, tested 299 people between December 2024 and August 2025 using a finger-prick blood test.
Almost 55 percent of adults aged 65 and over had insufficient levels of 25-hydroxyvitamin D, the standard marker doctors use to judge vitamin D status. Among participants with darker skin, more than 72 percent fell short. Those numbers barely moved between the cold months and the summer months.
Bernard Corfe, Professor of Human Nutrition and Health at Newcastle University, who co-led the research, said in the university's announcement that the pattern was striking.
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"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," he said. He added that for people living in places like the North of England, sunlight alone may not be enough, particularly for older adults and those from minoritised ethnic backgrounds.
Why the skin loses efficiency with age
Vitamin D synthesis starts when UVB light hits the skin and converts a cholesterol-like molecule into the vitamin's active precursor.
The concentration of that starter molecule falls with age, so an older person exposed to the same summer sun as a younger person produces far less vitamin D from it. Sunscreen, clothing, cloud cover and the low winter sun at northern latitudes further reduce how much reaches the skin.
That is why national guidelines set a higher intake target after age 70. In the United States, the recommended dietary allowance is 600 IU (15 micrograms) of vitamin D per day for adults up to age 70, and 800 IU (20 micrograms) per day for adults 71 and older. Diet alone rarely delivers that much, so oily fish, fortified milk and cereal, and a supplement when a clinician advises it, are the practical ways to close the gap.
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What to do about testing and supplements
Routine blood testing is not currently advised for otherwise healthy adults. In its 2024 clinical practice guideline, the Endocrine Society recommended against routine 25-hydroxyvitamin D screening in the general population, saying outcome-specific benefits have not been established.
The same guideline pointed to adults aged 75 and older as one group who may benefit from vitamin D intake above the standard recommended amount.
Anyone with bone pain, muscle weakness, frequent falls, or a diet with little fish or fortified dairy has reason to raise it with their doctor. A short conversation at a routine appointment can settle whether a test or a supplement is worth the trouble, and previous coverage has explored how a vitamin shortfall can also show up in the mouth before it shows up anywhere else.
For adults 71 and older in the US, that conversation usually starts at the 800 IU daily target, delivered through fortified foods, oily fish, or a low-dose daily supplement rather than occasional high-dose pills, the Endocrine Society specifically recommends daily lower-dose vitamin D over non-daily higher-dose regimens for adults 50 and older who need supplementation.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.