When the days shorten and the sun sinks low in the sky, that familiar winter slump sets in. There’s a fundamental reason for this seasonal fatigue: low levels of vitamin D – the “sunshine vitamin”.
People in the UK simply can’t make enough of this vital nutrient from sunlight during winter, which is why many are advised to take a supplement.
So, what is vitamin D, why does it matter, and how much do you really need?
Why is vitamin D important?
Vitamin D is a fat-soluble vitamin. This means it’s absorbed with other fats in our diet, and stored in the body’s liver and fatty tissue.
Vitamin D helps the body absorb calcium from the gut and regulates calcium and phosphate levels. These minerals are crucial for healthy bones, teeth and muscles.
Vitamin D plays a key role in bone development and in the continuous renewal of bone tissue. Without enough vitamin D, adults can develop osteomalacia (soft bones) and children can develop rickets (a bone deformity, often characterised by bowed legs).
Vitamin D supplementation, in conjunction with calcium, helps maintain bone mineral density and has been shown in trials to reduce fracture risk in people with osteoporosis.
Many tissues in the body have receptors that respond to vitamin D, as well. This means vitamin D can also affect neuromuscular function by helping us maintain muscle strength.
Low vitamin D levels have also been associated with a higher risk of several health problems, including cardiovascular conditions such as high blood pressure and heart failure. However, it’s uncertain whether low vitamin D directly contributes to cardiovascular disease or is simply a reflection of overall poor health in people with chronic conditions.
Although the evidence is mixed, some studies suggest vitamin D may help boost the immune system, too. Some studies suggest that the severity of respiratory infections (such as flu or COVID) may be reduced if you regularly take a vitamin D supplement.
Low vitamin D levels have also been linked to multiple sclerosis. In addition, research suggests that vitamin D may help regulate mood – particularly among people with depression or those who are vitamin D deficient. The studies aren’t definitive, though.
Where do we get vitamin D from?
Unlike other vitamins, which we obtain through diet, vitamin D is made when the skin is exposed to sunlight.
In the UK, around 80-90% of the vitamin D we need comes from the sunlight we’re exposed to. Between late March and September, the sun is usually strong enough that most people produce the vitamin D they need naturally.
But from October to early March, the UK’s sun isn’t strong enough for the skin to synthesise vitamin D – even on bright days. This is why it’s recommended everyone should consider taking a daily supplement during the colder months to maintain optimum levels. This becomes even more important as people spend more time indoors during winter.
Only a small proportion of our vitamin D is obtained through diet.
Tatjana Baibakova/ Shutterstock
Some food sources do in fact contain vitamin D – including oily fish (such as salmon or sardines), red meat, egg yolks and fortified cereals. However, only a small proportion (roughly 10–20%) of our vitamin D levels are obtained through diet.
How much vitamin D should I take?
Adults and children over four years old should take 10 micrograms (400IU) of vitamin D per day during the autumn and winter.
Babies under one should be given 8.5-10 micrograms of vitamin D throughout the year if they are breastfed or partly breastfed. Infants are born with limited vitamin D stores, which are primarily transferred from the mother. While infants can produce vitamin D through sunlight exposure, this is often minimal due to sun protection measures. This is why a supplement is recommended.
Because formula is already fortified with vitamin D, only babies who drink less than 500ml of formula a day should be given a supplement.
Although 10 microgram supplements are sufficient, products up to 25 micrograms (1,000IU) are considered safe for general use.
Vitamin D comes in two main forms: D₂ (ergocalciferol) and D₃ (cholecalciferol). Although they differ slightly in chemical structure, both are well absorbed in the small intestine. However, some research shows vitamin D₃ is generally more effective at increasing and sustaining vitamin D levels in the body.
Some people may need to take a vitamin D supplement all year round. This includes people with darker skin tones, such as those of African, African-Caribbean or south Asian heritage, whose higher melanin levels act as a natural sunscreen, reducing how efficiently they synthesise vitamin D from sunlight. People who cover their skin for cultural or medical reasons, or who are housebound or indoors for long periods, will also have less exposure to sunlight and may require a supplement.
Adults over 65 should also consider year-round supplementation. This is because the way the body produces and processes vitamin D changes as we age.
Can you take too much?
It’s important you do not take more than the recommended standard dose. Taking more than recommended is unnecessary and potentially harmful. Higher doses should only be taken under medical supervision for a confirmed deficiency.
Very high intakes (more than 100 micrograms daily) taken over a long time can lead to excessive calcium levels in the body – causing weakened bones and potential kidney and heart problems. Because vitamin D stays in your body for a long time (about two months), symptoms of an overdose can last for several weeks.
You can’t get too much vitamin D from sunlight alone, but it’s still important to protect your skin to reduce the risk of sun damage and skin cancer.
During a British winter, vitamin D is one of the simplest tools we have to support our overall health. With limited sunlight, a small daily supplement offers a reliable way to keep bones and muscles in good working order through the darker months.