Is vitamin D as important as everyone makes it out to be?
Vitamin D occupies a strange place in health culture. It is either dismissed as another supplement aisle gimmick or credited with preventing cancer, depression, dementia and everything in between. The honest answer sits between the two, and for anyone living in the UK the distinction matters more than it does almost anywhere else.
Why the UK is a special case
The UK sits between roughly 50 and 60 degrees north. From about October to March, the sun does not climb high enough for UVB radiation to reach the skin at the wavelength needed to make vitamin D. It is not a question of how long you stay outside or whether the day is bright.
During those months, skin synthesis is effectively switched off, and the body draws on stores built up over summer plus whatever comes from food.
Food does not fill the gap. The average adult in the UK gets around 3 to 4 micrograms a day from diet, against a recommended intake of 10 micrograms. Oily fish, egg yolks, fortified spreads and cereals all contribute, but you would need to eat oily fish several times a week to approach adequacy on diet alone.
The result is measurable. National survey data consistently shows that around one in six adults in the UK has a blood 25-hydroxyvitamin D level below 25 nmol/L, the threshold used here to define deficiency. In late winter that figure rises considerably, with roughly a third of adults falling below it in some age groups. Rates are substantially higher in people with darker skin, where more melanin means less efficient synthesis from limited sunlight, and in people who cover most of their skin, spend little time outdoors, or live in residential care.
What deficiency actually does
This is where the evidence is strongest and least disputed.
Vitamin D governs calcium and phosphate absorption from the gut. When levels fall low enough for long enough, the body cannot mineralise bone properly. In children this produces rickets, with bowed legs, delayed growth and skeletal deformity. Rickets never disappeared from the UK and hospitals still admit several hundred children a year with it. In adults the equivalent condition is osteomalacia, which typically presents as deep, aching bone pain, tenderness over the ribs, hips and shins, and weakness in the thigh and shoulder muscles that makes stairs and standing from a chair difficult. It is frequently mistaken for fibromyalgia or general fatigue and can persist for years before anyone measures a vitamin D level.
Before those extremes, low vitamin D drives up parathyroid hormone, which pulls calcium out of bone to maintain blood levels. Over time this accelerates bone loss. In older adults, correcting deficiency alongside adequate calcium reduces falls and hip fractures, an effect demonstrated most clearly in care home populations.
There is also reasonable evidence for a modest protective effect against acute respiratory infections. A large individual participant data meta-analysis found the benefit was concentrated in people who were deficient at the start, and only when supplements were taken daily or weekly rather than as occasional large doses.
What actually raises your level
Sunlight, seasonally. Between April and September, short exposures of forearms and lower legs around the middle of the day, several times a week, without burning, are enough for most people with lighter skin. People with darker skin need considerably longer for the same effect. Glass blocks UVB, so sitting by a window does nothing.
Food, as a supporting act. Salmon, mackerel, sardines, herring, egg yolks, fortified spreads and cereals, and mushrooms exposed to UV light. Useful, rarely sufficient on its own in this country.
Supplements, for the winter half of the year. UK guidance recommends 10 micrograms, or 400 IU, daily for everyone during autumn and winter, and year round for people at higher risk. Vitamin D3 raises blood levels more reliably than D2. Daily dosing works better than large intermittent doses.
So should everyone supplement?
For the UK, a 10 microgram daily supplement from October to March is a sensible default for almost everyone. It is inexpensive, extremely safe at that dose, and addresses a deficiency that is genuinely common here rather than a theoretical one.
Year round supplementation makes sense if you have darker skin, cover your skin for cultural or religious reasons, spend most daylight hours indoors, are pregnant or breastfeeding, are over 65, live in residential care, or have a condition affecting fat absorption such as coeliac disease, Crohn’s disease or previous bariatric surgery. Children from one to four years old should have a daily supplement throughout the year.
Higher doses are not better by default. The upper limit for routine use in adults is 100 micrograms, or 4,000 IU, a day. Sustained intakes well above that can cause hypercalcaemia, with nausea, excessive thirst, confusion, kidney stones and kidney damage. Toxicity is uncommon but it is real, and it comes almost entirely from people self prescribing very high doses over long periods.
Routine testing is not necessary for most people. It is worth measuring if you have unexplained bone or muscle pain, osteoporosis or a fracture with minimal trauma, a malabsorption condition, or if you sit in several risk categories at once and want to know where you are starting from before deciding on a dose.
if you are concerned about your vitamin D levels, you can speak to one of our clinical team experts here.