Vitamin D: Deficiency Signs, Best Food Sources, Sun Exposure and Supplement Dosing Guide

The scale of vitamin D insufficiency across developed and developing nations is striking. A landmark 2007 New England Journal of Medicine review by Michael Holick — one of the world's foremost vitamin D researchers — estimated that one billion people globally had deficient or insufficient vitamin D levels, a figure that has not meaningfully improved in the intervening decades. A 2022 position statement by Lips et al. in the Journal of Internal Medicine found that across Europe, between 40 and 70 percent of adults have serum 25-OHD levels below 50 nmol/L, with the highest rates in northern latitudes, among institutionalised elderly populations, and in dark-skinned individuals living far from the equator. In the United Kingdom, Public Health England has recommended routine supplementation for the entire population from October to March since 2016 — a remarkable public health acknowledgment of how little sunlight British winters provide. In the United States, NHANES data consistently shows that approximately 41 percent of adults are deficient (below 50 nmol/L), with Black Americans disproportionately affected: 82 percent show deficiency, partly explained by higher melanin content reducing cutaneous vitamin D synthesis. The clinical consequences are substantial — vitamin D deficiency is a significant contributor to osteoporotic fractures (estimated global cost: US$17 billion annually), impaired immune function and higher infection rates, and emerging associations with depression and cognitive decline in older adults.

Vitamin D is produced in the skin when ultraviolet B radiation (UVB, wavelength 290–315 nm) converts 7-dehydrocholesterol to pre-vitamin D3, which is then converted to vitamin D3 (cholecalciferol). Dietary or supplemental vitamin D undergoes two hydroxylation steps: first in the liver to 25-OHD (the main circulating and measured form), then in the kidney and peripheral tissues to the active hormone 1,25-dihydroxyvitamin D (calcitriol). A 2019 Endocrine Reviews paper by Bouillon et al. comprehensively catalogued the genomic and non-genomic actions of vitamin D: it regulates calcium absorption in the gut (upregulating TRPV6 and calbindin channels), modulates immune cell differentiation including T-regulatory cells and macrophage function, regulates insulin secretion, and suppresses renin expression with implications for blood pressure. A landmark 2017 BMJ meta-analysis by Martineau et al. analysed individual participant data from 25 randomised controlled trials (11,321 participants) examining vitamin D supplementation and acute respiratory tract infections. They found that supplementation reduced the risk of at least one acute respiratory infection by 12 percent overall — and by 70 percent in individuals who were severely deficient at baseline (25-OHD below 25 nmol/L), demonstrating that the population most deficient benefits most. The evidence for bone health is strong — multiple meta-analyses confirm that vitamin D (combined with adequate calcium) reduces fracture risk in older adults. Evidence for cancer prevention, cardiovascular disease reduction and mortality is rated as moderate to preliminary, with large recent trials like VITAL producing mixed results depending on the outcome measure and baseline vitamin D status.

Vitamin D deficiency follows predictable patterns. Older adults are doubly affected: aging skin produces vitamin D 25–40 percent less efficiently than young skin under identical UV conditions, and older adults spend less time outdoors, often with more clothing coverage. People with darker skin require significantly longer sun exposure to produce equivalent vitamin D — a person with Fitzpatrick skin type VI may need 10–50 times more sun exposure than someone with type I skin to produce the same amount. Individuals who cover most of their skin for religious or cultural reasons are at very high risk of deficiency, as are those who live in northern latitudes (above approximately 37 degrees north or below 37 degrees south) during winter months when the solar zenith angle prevents adequate UVB transmission. People with obesity have lower circulating 25-OHD levels partly because vitamin D is fat-soluble and sequestered in adipose tissue. Conditions causing fat malabsorption — Crohn's disease, coeliac disease, cystic fibrosis and bariatric surgery — impair dietary vitamin D absorption. Exclusively breastfed infants are at high risk unless supplemented, as human breast milk is a poor vitamin D source regardless of maternal status.

The dietary sources of vitamin D are limited but meaningful. Oily fish lead by a substantial margin: wild-caught salmon provides 600–1,000 IU per 100 g serving, farmed salmon 100–250 IU, canned tuna approximately 150 IU, mackerel 360 IU and sardines 270 IU. Egg yolks from pasture-raised chickens exposed to sunlight can provide 150–300 IU each, significantly more than conventionally raised eggs (approximately 30–40 IU). UV-exposed mushrooms are the only meaningful plant source — portobello mushrooms placed gill-side up in direct sunlight for 15–20 minutes can generate 400–800 IU per 100 g serving; commercially available UV-treated mushrooms are increasingly available. Fortified foods form the backbone of vitamin D intake for many people: fortified milk (typically 100 IU per 240 ml), fortified plant milks, fortified breakfast cereals and fortified orange juice. These fortification levels are set relatively conservatively and are unlikely to correct significant deficiency on their own. Sun exposure is the most efficient source for most people, but is highly variable. In summer at temperate latitudes, 10–20 minutes of midday sun (solar index 3 or above) on the arms and legs (approximately 25 percent of body surface area) produces around 10,000 IU of vitamin D3 in fair-skinned individuals. Sunscreen SPF 30 reduces synthesis by approximately 95 percent when applied correctly, though most people apply it inconsistently. From October to March in the UK and similar latitudes, UVB intensity is insufficient for meaningful cutaneous synthesis regardless of time spent outdoors.

Monday: Breakfast — fortified oat milk porridge with UV-treated mushrooms on toast; Midday — 15 minutes of outdoor activity during peak sun hours when UV index permits; Dinner — baked salmon. Tuesday: Breakfast — two pasture-raised eggs; Supplement — take vitamin D3 with breakfast if indicated; Dinner — sardine pasta with olive oil and garlic. Wednesday: Lunch — smoked mackerel salad; Outdoor activity — lunchtime walk without sunscreen on arms for 10–15 minutes. Thursday: Breakfast — fortified cereal with fortified milk; Dinner — trout fillet. Friday: Breakfast — eggs and smoked salmon on wholegrain toast; Dinner — vitamin D-rich mushroom stir-fry with tofu and egg noodles. Saturday: Brunch — outdoor dining where possible; Dinner — pan-fried herring. Sunday: Batch cook UV-treated mushrooms for the week; Supplement audit — confirm you have adequate supply of vitamin D3 for autumn and winter. Year-round habits: If between October and March (in northern Europe or Canada), take 400–1,000 IU vitamin D3 daily with a fat-containing meal, as recommended by Public Health England and Health Canada. In summer, aim for regular brief midday sun exposure while avoiding burning. Retest 25-OHD after 3–4 months of supplementation to assess response.

Myth 1: You can get enough vitamin D from diet alone without sun or supplements. For most people in northern climates, this is not achievable. Even an excellent diet rich in oily fish provides only a fraction of what the body needs. The dietary baseline is important but insufficient as a sole strategy. Myth 2: A daily supplement of 400 IU is enough for everyone. Current UK guidelines recommend 400 IU as a minimum for the general population, but individuals who are deficient typically require 1,500–2,000 IU daily to restore and maintain adequate levels, and some — particularly those with significant deficiency, obesity or malabsorption — need higher therapeutic doses under medical supervision. Myth 3: Sunscreen makes it impossible to make vitamin D. Most people apply sunscreen inconsistently and at insufficient quantities to achieve the SPF stated on the label. Brief unprotected sun exposure before applying sunscreen is a common real-world pattern that does produce some vitamin D, though this should never be relied upon as a primary strategy. Myth 4: Taking vitamin D supplements in summer is wasteful. UV index, cloud cover, time outdoors, skin area exposed and skin tone all determine whether any useful vitamin D is being made. Urban office workers in London in June may still produce very little. Supplementation year-round at modest doses is appropriate for many people regardless of season. Myth 5: Very high-dose vitamin D supplementation is always safe. Vitamin D toxicity (hypercalcaemia) can occur at sustained doses above 10,000 IU per day without medical supervision. While the therapeutic window is wide, casual megadosing — 5,000–10,000 IU per day from supplements found online — without monitoring is inadvisable and can cause kidney damage.

Vitamin D3 (cholecalciferol) is consistently shown to raise serum 25-OHD more effectively than vitamin D2 (ergocalciferol) and is the preferred form for supplementation in humans. Vitamin D3 is derived from lanolin (wool grease) or lichen; the lichen-derived form is vegan. Dosing should be guided by baseline serum 25-OHD where possible. For maintenance in adults with normal status: 400–1,000 IU daily. For correction of mild to moderate deficiency (25–50 nmol/L): 1,500–2,000 IU daily for at least three months, then recheck. For severe deficiency (below 25 nmol/L): a clinician may recommend a loading protocol — for example, 3,000–4,000 IU daily or supervised loading doses — before transitioning to maintenance. Vitamin D works synergistically with vitamin K2 (specifically menaquinone-7, MK-7) in directing calcium to bones rather than soft tissues; a combined D3/K2 supplement is a reasonable choice for those over 50 or with osteoporosis risk. Magnesium is required as a cofactor for vitamin D activation; chronic magnesium insufficiency can impair the conversion of vitamin D to its active form. Take vitamin D with a fat-containing meal for best absorption — studies show absorption can be 50 percent higher with a fatty meal compared to a fasted state. Third-party tested supplements (NSF, USP, Informed Sport) provide assurance of label accuracy.

The gold-standard test for vitamin D status is serum 25-hydroxyvitamin D (25-OHD), measured in nmol/L or ng/mL. Interpretation varies slightly between guidelines: most endocrinology societies define deficiency as below 50 nmol/L (20 ng/mL), insufficiency as 50–75 nmol/L (20–30 ng/mL), and sufficiency as above 75 nmol/L (30 ng/mL); some experts recommend a target of 100–125 nmol/L (40–50 ng/mL) for optimal bone and immune health, though evidence at the higher end is less definitive. Testing is recommended for: all patients with osteoporosis or fracture risk, those with fat malabsorption conditions, chronic kidney disease, institutionalised elderly patients, pregnant women, exclusively breastfed infants, and symptomatic individuals. In the UK, GPs may not routinely test asymptomatic adults; private testing is inexpensive (typically £25–£50 from a home finger-prick kit). Reassess 25-OHD after three months of supplementation. Calcium levels should be checked if very high doses are being used, as vitamin D toxicity manifests as hypercalcaemia. Report any symptoms of toxicity — nausea, weakness, frequent urination, kidney pain — immediately to your doctor.