Living in the UK makes vitamin D hard to come by for half the year. According to the NHS, most people can make all the vitamin D they need from sunlight on their skin from about late March or early April to the end of September. For the rest of the year we depend on what the body stored over the summer, on food, and on supplements.
You’ll often hear that almost everyone in the UK is deficient. The national figures tell a more measured story, though still one worth taking seriously, and they show a strong winter pattern.
Why latitude matters
Your skin makes vitamin D when ultraviolet B (UVB) light from the sun reaches it. Watford sits at roughly 51.7 degrees north. Through autumn and winter the sun stays low in the sky, its UVB has to travel through more of the atmosphere, and too little reaches the ground for your skin to make much vitamin D, even on a bright day.
Several things make the problem worse:
- spending most of the day indoors
- covering most of your skin when you go out
- having dark skin (the NHS specifically mentions people from African, African-Caribbean and south Asian backgrounds, who may not make enough vitamin D from sunlight)
- being frail, housebound or living in a care home
Levels fall steadily through the winter as the body uses up its summer stores, which is why the lowest readings come between January and March.
How many people are actually low?
The best UK figures come from the National Diet and Nutrition Survey, which measures vitamin D in blood samples from a representative sample of people across all four UK nations. Its latest report, published in June 2025, covers fieldwork from October 2019 to July 2023. It counts someone as having low vitamin D status if their blood level of 25-hydroxyvitamin D falls below 25 nanomoles per litre, the level below which the risk of poor bone and muscle health rises.
Over the year as a whole, the proportion below that level was:
- 18% of adults aged 19 to 64
- 12% of adults aged 65 and over
- 23% of children aged 11 to 18
- 10% of children aged 4 to 10
The season makes a large difference. Between January and March, the figure ranged from 21% to 38% across the age and sex groups aged 11 and over. Between July and September, it fell to between 0% and 10%.
Low vitamin D, then, isn’t universal. In the first three months of the year, though, somewhere between one in five and nearly two in five people over the age of 11 have levels below that threshold, depending on their age and sex.
What vitamin D does, and claims to treat with caution
According to the NHS, vitamin D helps regulate the amount of calcium and phosphate in the body, which you need for healthy bones, teeth and muscles. A lack of vitamin D can cause rickets in children and a painful bone condition called osteomalacia in adults.
You’ll also read that vitamin D lifts mood, prevents colds and protects the heart. Here the evidence is much less certain. When the government’s Scientific Advisory Committee on Nutrition (SACN) reviewed vitamin D in 2016, it based its recommendations on bone and muscle health, because it judged the evidence on other outcomes insufficient to set recommendations. A later SACN review, in 2020, found that the evidence did not support recommending vitamin D to prevent acute respiratory infections in the general UK population.
Vitamin D still matters for bones and muscles, which is reason enough to take it through the winter. Tiredness or low mood, though, deserve a proper look from your GP rather than an assumption that a supplement will fix them.
What the NHS advises
The NHS advice is short and simple:
- Everyone, including pregnant and breastfeeding women, should consider a daily supplement of 10 micrograms (400 IU) of vitamin D during autumn and winter.
- Adults and older children who are rarely outdoors, live in a care home or usually cover most of their skin outdoors should take 10 micrograms a day all year round. People with dark skin should consider doing the same.
- All children aged 1 to 4 should have 10 micrograms a day all year. Breastfed babies under one, and babies who have less than 500ml of formula a day, should have 8.5 to 10 micrograms a day.
Food helps, but the NHS notes that it’s difficult to get enough vitamin D from food alone. The main sources are oily fish, red meat, egg yolks, liver (which you should avoid if pregnant) and fortified foods such as some fat spreads and breakfast cereals. UK dairies generally don’t fortify cows’ milk.
Taking more than you need brings its own risks. The NHS advises adults not to take more than 100 micrograms (4,000 IU) a day, children aged 1 to 10 no more than 50 micrograms, and babies under 12 months no more than 25 micrograms. Taking too much over a long period can cause a build-up of calcium in the body, which can weaken bones and damage the kidneys and heart. You can’t overdose on vitamin D from sunlight.
The advice costs little, yet the winter figures suggest that many people still don’t take it up.
When to see your GP
See your GP if you have bone pain, if you’ve felt tired for a few weeks without an obvious reason, or if you belong to one of the higher-risk groups above and feel worried. They can arrange a blood test if they think you need one. If a test confirms a deficiency, higher-dose treatment may follow, guided by a clinician who has seen your results. Please don’t treat yourself with high doses on the strength of a hunch.
A habit for the darker months
Chinese medicine has always paid close attention to the seasons. In the Five-Element tradition, winter belongs to Water, a time for stillness, rest and conserving energy. A daily vitamin D supplement from October to March fits that rhythm well: one small, practical way to look after yourself through the darker months, alongside getting outside in daylight in spring and summer while taking care not to burn.