Vitamin D: The Complete Guide to Deficiency, Sources, and Supplementation

The journey of vitamin D from skin to active hormone involves three distinct organs and two chemical transformations. When UVB rays strike the skin, they convert 7-dehydrocholesterol into vitamin D3, also called cholecalciferol. This form is biologically inert; it must travel to the liver, where an enzyme called 25-hydroxylase adds a hydroxyl group to produce 25-hydroxyvitamin D, abbreviated as 25(OH)D and commonly known as calcidiol. This is the form measured in blood tests and the most reliable indicator of overall vitamin D status. From the liver, calcidiol circulates to the kidneys, where another enzyme — 1-alpha-hydroxylase — converts it into the fully active hormone 1,25-dihydroxyvitamin D, also called calcitriol. Calcitriol binds to vitamin D receptors found in nearly every tissue in the body, from the gut and bone to the immune cells and the brain. The kidneys regulate calcitriol production tightly based on calcium levels, parathyroid hormone, and fibroblast growth factor 23, which means blood levels of active calcitriol can appear normal even when overall vitamin D stores are depleted. This is why serum 25(OH)D is the diagnostic benchmark rather than the active hormone. Interestingly, many tissues — including immune cells and the colon — possess their own 1-alpha-hydroxylase and can produce local calcitriol independent of kidney output, which may explain the wide-ranging effects of vitamin D beyond calcium metabolism.

Vitamin D deficiency is notoriously insidious because its early symptoms are vague and easily attributed to other causes. Persistent fatigue that does not resolve with rest is one of the most commonly reported complaints, yet it is non-specific enough that most people never associate it with a nutrient shortage. Bone pain — particularly in the back, hips, and legs — and generalised muscle weakness are more suggestive warning signs, especially in middle-aged and older adults. Frequent colds, flu, and respiratory infections are another red flag: vitamin D is essential for triggering the production of antimicrobial peptides such as cathelicidin and defensins in immune cells. People with low levels are significantly more likely to suffer from upper respiratory tract infections than those with adequate status. Depression and mood changes, particularly in the autumn and winter months, are also closely associated with declining vitamin D levels. The brain contains vitamin D receptors in regions governing mood regulation, and supplementing deficient individuals has been shown in several trials to reduce depressive symptoms. Hair loss, impaired wound healing, and chronic pain syndromes including fibromyalgia have all been linked to deficiency in observational studies. In children, severe deficiency causes rickets — softening and deformity of bones — while in adults the equivalent condition, osteomalacia, manifests as bone pain and tenderness with an increased risk of stress fractures. Secondary hyperparathyroidism is a common laboratory finding when vitamin D is chronically low, as the parathyroid glands attempt to compensate for impaired calcium absorption by releasing more parathyroid hormone.

Certain groups face substantially elevated risk of vitamin D insufficiency, and understanding these risk factors helps prioritise who most urgently needs dietary attention or supplementation. Geography is the single biggest environmental determinant: anyone living above roughly 35 degrees north latitude (which includes most of the UK, Canada, northern Europe, and much of the northern United States) cannot synthesise meaningful vitamin D from sunlight between approximately October and March because the sun angle is too low for sufficient UVB to reach the Earth's surface. Skin pigmentation is equally important: melanin acts as a natural sunscreen, meaning people with darker skin require significantly longer sun exposure — estimates range from five to ten times longer — to produce the same amount of vitamin D as someone with very fair skin. Age compounds the problem because the skin's capacity to synthesise vitamin D decreases markedly with age, and older adults are also less likely to spend time outdoors. Obesity reduces effective vitamin D status because the fat-soluble vitamin becomes sequestered in adipose tissue, making it less bioavailable. People who cover most of their skin for religious or cultural reasons, those who are housebound, shift workers, and anyone using high-factor sunscreen consistently are also at elevated risk. Malabsorptive gut conditions including coeliac disease, Crohn's disease, cystic fibrosis, and gastric bypass surgery impair dietary and supplemental vitamin D absorption. Certain medications including anticonvulsants, glucocorticoids, antifungal drugs, and some cholesterol-lowering medications accelerate vitamin D catabolism.

Very few foods naturally contain meaningful amounts of vitamin D, which is one reason sunlight and supplementation are so important for most people. Oily fish are by far the richest natural dietary source: a 100g serving of wild-caught salmon provides around 600–1000 IU of vitamin D3, while farmed salmon delivers roughly 250–600 IU depending on feed composition. Mackerel, herring, sardines, and trout are similarly valuable, each providing 200–400 IU per typical serving. Cod liver oil, long used as a traditional remedy for rickets, contains approximately 450 IU per teaspoon and was historically a critical supplement in northern countries. Egg yolks contain modest amounts — roughly 40–50 IU each — though eggs from hens raised outdoors or given vitamin D-enriched feed can contain substantially more. Beef liver provides around 50 IU per 100g. Mushrooms are the only significant plant-based source and occupy a special niche: when exposed to ultraviolet light (either sunlight or artificial UV), they synthesise ergocalciferol (vitamin D2), and some commercially grown mushrooms are now UV-treated specifically to boost their vitamin D content to 200–800 IU per 100g. Fortified foods are an important practical consideration. In many countries, cow's milk, plant-based milks, breakfast cereals, and margarine are routinely fortified with 100–200 IU per serving. In the UK, mandatory fortification of certain foods has been the subject of ongoing policy debate. Reading food labels carefully is worthwhile, as fortification levels vary significantly between brands.

Sunlight remains the most powerful driver of vitamin D status for most people, and understanding how to use it effectively while managing skin cancer risk is a genuine balancing act. The critical variable is UVB radiation, specifically wavelengths between 290 and 315 nanometres, which is only present at the Earth's surface when the sun is at an angle greater than approximately 35 degrees above the horizon. Shadow length is a useful practical guide: if your shadow is shorter than your height, UVB is likely sufficient for vitamin D synthesis. In summer at temperate latitudes, as little as 10–30 minutes of midday sun exposure to the face, arms, and legs can generate between 1000 and 3000 IU of vitamin D3, though this varies widely with skin tone, age, body surface area exposed, and local UV index. The body has a natural ceiling on synthesis: once the skin's 7-dehydrocholesterol is depleted, further sun exposure does not produce additional vitamin D — it simply degrades the vitamin D already made, which is one reason sunburn without meaningful vitamin D production is a real risk if timing is poor. Glass, standard sunscreen above SPF 15, and clothing all substantially block UVB. The optimal strategy for most people at temperate latitudes is moderate, unprotected midday sun exposure during the warmer months to build up vitamin D stores, while avoiding sunburn and using sunscreen for prolonged outdoor activities. During autumn and winter, dietary sources and supplementation become the primary strategy.

For the many people who cannot meet their vitamin D needs through sunlight and diet alone, supplementation is a practical, safe, and effective solution. Vitamin D3 (cholecalciferol) is consistently shown to raise serum 25(OH)D levels more effectively than D2 (ergocalciferol) and is generally the preferred supplemental form. Standard maintenance doses for adults typically range from 1000 to 2000 IU daily, while those with confirmed deficiency or high-risk profiles may be recommended 3000–5000 IU daily under medical supervision. Public health bodies in the UK recommend 400 IU daily for the general population during autumn and winter, though many clinicians and researchers argue this is insufficient to meaningfully raise serum levels in people who start from a depleted baseline. Because vitamin D is fat-soluble, it is absorbed significantly better when taken with a meal containing fat — something as simple as taking the supplement with breakfast rather than on an empty stomach can meaningfully improve uptake. Toxicity from supplementation is possible but requires sustained high doses: the tolerable upper intake level is generally set at 4000 IU per day for adults, though short-term therapeutic doses of up to 10,000 IU are used clinically without toxicity in most people. Hypervitaminosis D causes hypercalcaemia — elevated blood calcium — with symptoms including nausea, weakness, frequent urination, kidney stones, and in severe cases cardiac arrhythmia. Combined vitamin D and K2 supplementation is often recommended because K2 (particularly MK-7) helps direct calcium to bones rather than soft tissues, which is particularly relevant when supplementing vitamin D at higher doses over the long term.