Vitamin D — The Sunshine Hormone the UK Is Running Short Of

The body makes vitamin D from sunlight — which makes it unlike almost any other nutrient on this list. In the UK, that manufacturing window is open for roughly six months. Food and supplementation are supposed to cover the rest. For most people, they are not covering the rest.

Not Quite a Vitamin

Vitamin D is technically a misnomer. Unlike true vitamins — compounds the body cannot make and must obtain entirely from food — vitamin D is manufactured by the body itself, in the skin, when UVB radiation from sunlight converts a cholesterol derivative into a precursor that the liver and kidneys then activate into the hormone form: calcitriol.

It is a steroid hormone. It circulates in the blood, binds to receptors in virtually every cell in the body, and regulates the expression of hundreds of genes. The fact that it was classified as a vitamin when first discovered — because it could prevent rickets when added to food — has shaped how people think about it ever since. Most people reach for a dietary source when the real leverage is the relationship between skin, sunlight, and geography.

In the UK, that relationship is complicated. Above roughly 51.5° north — which is the latitude of London — UVB radiation is too weak to produce vitamin D in the skin for approximately six months of the year, from October to March, regardless of how much time is spent outdoors. North of Birmingham, that window is shorter still. Food and supplementation are not optional additions to sunlight in the UK winter. They are the only options.

What Vitamin D Actually Does

Calcium absorption and bone health

The best-known role of vitamin D — and the one that has been understood longest — is its control over calcium absorption from the gut. Without adequate vitamin D, the intestine can absorb only around 10–15% of dietary calcium. With sufficient levels, that rises to 30–40%. This is why the calcium and vitamin D pages in this series need to be read together: eating more calcium while vitamin D status is poor is largely ineffective. The gatekeeper must be open for the mineral to get through.

Vitamin D also regulates calcium reabsorption in the kidneys and, alongside parathyroid hormone, controls the release of calcium from bone into the bloodstream when circulating levels fall. Chronic vitamin D deficiency in children produces rickets — soft, deformed bones. In adults, it produces osteomalacia — painful, softening of existing bone that is often misdiagnosed as fibromyalgia or unexplained musculoskeletal pain. Both conditions resolve on repletion.

Immune system regulation

Vitamin D receptors are found on virtually every type of immune cell — T cells, B cells, macrophages, dendritic cells. Calcitriol modulates both the innate immune response (the rapid, non-specific first line of defence) and the adaptive immune response (the targeted, antigen-specific response that produces antibodies). It simultaneously promotes the antimicrobial activity that fights infection and dampens the inflammatory overreaction that causes autoimmune damage.

This dual role explains two patterns that have been consistently observed in population data: higher rates of respiratory infections in winter (when vitamin D levels are lowest in the Northern Hemisphere) and higher rates of autoimmune conditions — multiple sclerosis, type 1 diabetes, rheumatoid arthritis, inflammatory bowel disease — at higher latitudes where sun exposure is chronically lower.⁴ Causality in both directions is still debated, but the biological mechanism is established and plausible.

Muscle function and fall prevention

Vitamin D receptors are present in skeletal muscle tissue, and deficiency produces a characteristic myopathy — muscle weakness, particularly in the proximal muscles of the thighs and upper arms, that makes climbing stairs, rising from a chair, and carrying objects progressively harder. In older adults, this muscle weakness is one of the most significant modifiable contributors to falls and the fractures that follow them.

Meta-analyses of supplementation trials in elderly people show consistent reductions in fall risk with vitamin D repletion — particularly when combined with calcium.⁵ Given that a hip fracture in an 80-year-old carries a one-year mortality rate of around 30%, the public health significance of this relatively simple intervention is considerable and chronically underappreciated.

Mood, mental health, and seasonal patterns

Vitamin D receptors are found in areas of the brain involved in mood regulation, including the prefrontal cortex and the limbic system. Vitamin D appears to influence the synthesis and metabolism of serotonin — the neurotransmitter most closely associated with mood stability and wellbeing. The association between low vitamin D levels and depression is consistent across population studies, and seasonal affective disorder maps closely onto the geography and seasonality of vitamin D deficiency.

Whether supplementation reliably treats established depression is less certain — the intervention trials are mixed — but the association between chronic insufficiency and low mood is strong enough to treat it as a contributing factor rather than a coincidence, particularly in people whose mood follows a seasonal pattern.

Cardiovascular and metabolic health

Vitamin D receptors are found in the heart and in blood vessel walls. Low vitamin D levels are consistently associated in epidemiological studies with higher rates of hypertension, heart disease, and type 2 diabetes. The biological mechanisms are plausible — vitamin D modulates renin (a key regulator of blood pressure), influences insulin secretion, and has anti-inflammatory effects on blood vessels. Whether supplementation reduces cardiovascular events in people with existing deficiency is still being studied in large-scale trials.

What's Robbing You

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  Living and working indoors. The most impactful single factor for most people in the UK. Office work, commuting by car or public transport, and spending leisure time indoors means that for many people — even in summer — meaningful skin exposure to UVB is rare. The skin needs to be exposed (not covered by clothing or glass) to direct midday sun for the reaction to occur. Brief, regular outdoor exposure at lunchtime in summer is more effective than longer exposure at low sun angles.
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  SPF sunscreen applied before going outside. SPF15 blocks approximately 93% of UVB radiation; SPF30 blocks around 97%. This is not an argument against sunscreen — the balance between vitamin D synthesis and skin cancer risk is a real one, and sunscreen prevents skin cancer. But brief unprotected sun exposure of the arms and face for 10–15 minutes around midday, before applying sunscreen for the day, is the recommended approach for vitamin D synthesis during the summer months.
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  Skin tone. Melanin — the pigment that gives darker skin its colour — absorbs UVB radiation and reduces the amount available for vitamin D synthesis. People with darker skin produce significantly less vitamin D from the same sun exposure as people with lighter skin. At UK latitudes, this is clinically significant: people of South Asian, Black African, or Black Caribbean heritage living in the UK are at considerably higher risk of vitamin D deficiency than the general population, and may need higher supplemental doses to reach adequate blood levels.
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  Glass. UVB radiation does not penetrate glass. Sitting beside a sunny window — in a car, at home, in an office — provides light and warmth but no vitamin D synthesis whatsoever. Exposure must be direct — skin to sky, unmediated.
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  Age. The skin's capacity to synthesise vitamin D from sunlight declines with age — a 70-year-old produces roughly 25% of the vitamin D from the same sun exposure as a 20-year-old. Simultaneously, older adults are more likely to spend time indoors, cover more skin, and eat less. Vitamin D deficiency is particularly prevalent in the elderly and directly contributes to bone fractures, muscle weakness, and falls — the very conditions most likely to end independent living.
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  Obesity. Vitamin D is fat-soluble and is sequestered in adipose tissue. In people with high body fat, a significant proportion of any vitamin D produced or consumed is absorbed into fat stores rather than circulating in the blood where it is active. Studies suggest obese individuals may need two to three times the supplemental dose of a person of healthy weight to achieve the same blood levels.

Food Sources — and Their Limits

This is the page in the Eat to Heal series where food alone genuinely cannot do the job — and saying otherwise would be dishonest. The number of foods that contain meaningful vitamin D is small, and for a plant-based wholefood diet the options are narrower still. The sources below are real and worth using. But they are contributors to a strategy that must also include sunlight in summer and supplementation in winter — not a solution in themselves.

Supplementation — The Part the Food Cannot Cover

Daily Target — Quick Reference

Smart Pairings

Vitamin D works most directly alongside calcium — the two are inseparable in the body. It also works in partnership with vitamin K2, which helps direct calcium into bones and teeth rather than into arteries and soft tissue.