Vitamin D: Why So Many Adults Are Deficient and What to Do About It

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Vitamin D has a reputation as the “sunshine vitamin” — and for good reason. Your skin produces it when exposed to ultraviolet B rays from sunlight. But in countries with limited sunshine for much of the year, and with many adults spending most of their day indoors, vitamin D deficiency is surprisingly common and often goes unnoticed.

What Does Vitamin D Actually Do?

Vitamin D plays a key role in calcium absorption, making it essential for maintaining healthy bones and teeth. It also supports immune function, muscle health, and there is ongoing research into its role in mood regulation. Severe deficiency can cause bone pain and muscle weakness; milder deficiency may cause fatigue, low mood, and an increased susceptibility to illness.

Who Is Most at Risk of Deficiency?

• People who spend little time outdoors
• People who cover most of their skin when outside
• People with darker skin tones, who require more sun exposure to produce the same amount of vitamin D
• Older adults, whose skin is less efficient at producing vitamin D
• People living in northern latitudes (including most of the UK and northern Europe) during autumn and winter

Sunlight: The Primary Source

In summer, most light-skinned adults can get enough vitamin D from around 15–20 minutes of sun exposure on the face, arms, and legs during midday hours. However, from October to March in the UK, the sun is not strong enough to trigger sufficient production regardless of time spent outside.

Food Sources

Relatively few foods naturally contain significant amounts of vitamin D. The best dietary sources include:

• Oily fish such as salmon, mackerel, and sardines
• Egg yolks
• Liver (though not recommended in large amounts)
• Some fortified foods including certain cereals, plant-based milks, and spreads

Supplementation

UK public health guidance recommends that adults consider a daily 10 microgram (400 IU) vitamin D supplement during autumn and winter. People in higher-risk groups may benefit from taking it year-round. Do not take very high doses without medical supervision, as excessive vitamin D can cause toxicity. Consult your GP if you think you may be deficient — a simple blood test can confirm your levels and guide appropriate supplementation.

For a nutrient that your body makes from sunlight, vitamin D is remarkably easy to fall short on in modern life. A simple supplement habit during the colder months is one of the lowest-effort, highest-impact things many adults can do for their health.