Why vitamin D matters all year round

Vitamin D is one of the few nutrients your body is designed to make for itself. When ultraviolet B light from the sun reaches your skin, a cholesterol-like compound is converted into a precursor that your liver and kidneys then switch into its active form. In a sunny July this process is remarkably efficient. In a grey British January it is close to useless.

Once active, vitamin D behaves more like a hormone than a vitamin. Its best-known job is helping you absorb calcium and phosphate from your food, which is why it matters so much for bone strength and teeth. But it also supports normal muscle function, and immune cells throughout your body carry receptors that respond to it. Low levels are linked with aching bones, muscle weakness and a greater susceptibility to infections. Getting enough will not make you invincible, but it does help your body work as it should.

What the darker months actually do to your levels

From roughly October to late March, the sun sits too low in the sky over the UK for the right wavelength of UVB to reach the ground effectively, even on a bright day. Add in cloud cover, working indoors, long sleeves, sunscreen and the fact that older skin synthesises vitamin D less efficiently, and it becomes clear why deficiency is so common here in winter.

Melanin also affects how much UVB you need. People with darker skin tones require considerably longer sun exposure to make the same amount of vitamin D, which is one reason UK guidance suggests they consider a daily supplement all year round rather than just in winter. The same applies if you cover your skin for cultural or religious reasons, spend most of your day indoors, or live in the far north of the country.

Your summer reserves do carry you for a while, but generally only a few weeks to a couple of months. By midwinter, many people are running on empty without realising it.

Signs worth noticing — and why they are easy to miss

Low vitamin D is often silent. When symptoms do appear, they tend to be vague and easy to blame on the season:

• Persistent tiredness that sleep does not fix
• Aching bones or muscles, particularly in the lower back, hips and legs
• Muscle weakness or slower recovery after exercise
• Low mood that creeps in as the days shorten
• Catching every cold that goes round the office or classroom

Because none of these point uniquely to vitamin D, symptoms alone are not a diagnosis. If several of them feel familiar, it is worth asking your GP about a blood test rather than guessing.

How much is sensible, and what to avoid

For most adults, UK public health advice is a daily supplement of 10 micrograms (400 IU) during autumn and winter. Some people are advised to take it year-round: those with darker skin, those who cover up or stay indoors, and anyone living in a care home. If you are pregnant, breastfeeding, or have a condition that affects how you absorb fat, ask your GP or pharmacist whether a higher dose is appropriate for you.

More is not better here. The safe upper limit for adults is around 100 micrograms (4000 IU) a day, and taking far more than that over months can push calcium levels up, causing nausea, kidney problems and, in serious cases, damage to the heart and bones. Be wary of "megadose" products offering thousands of micrograms in a single weekly or monthly tablet unless a clinician has recommended one. Consistency matters far more than size — a steady daily dose is what keeps your blood levels stable. If you forget a day, simply take the next one; do not double up.

Choosing a supplement you will actually take

• Look for vitamin D3 (cholecalciferol) rather than D2, as it tends to raise blood levels more effectively.
• Check the label carefully. Some products list only IU, others only micrograms. Remember that 1 microgram equals 40 IU.
• Take it with a meal containing some fat — vitamin D is fat-soluble, so absorption improves alongside eggs, oily fish, olive oil, nuts or full-fat yoghurt.
• Tablets, capsules, sprays and drops all work. Choose the format you will remember to use daily.
• Add up your sources. Multivitamins, cod liver oil and fortified products may already contain vitamin D, so check you are not stacking doses.

Food sources such as oily fish, egg yolks, fortified cereals and spreads, and UV-exposed mushrooms contribute something, but it is difficult to reach 10 micrograms a day from diet alone. That is exactly why supplementation is such a practical option in the darker months.

Gut health, absorption and when to seek advice

Vitamin D is absorbed in the small intestine along with dietary fat, so anything that disrupts fat absorption can undermine your levels. Coeliac disease, Crohn's disease, ulcerative colitis, chronic pancreatitis and weight-loss surgery are all recognised risk factors. If you have ongoing digestive symptoms alongside tiredness or aching bones, mention both to your GP — the two may well be connected, and vitamin D status is a straightforward thing to check.

At the same time, be cautious about self-diagnosing food sensitivities or cutting out major food groups in the hope of feeling better. Unnecessary restriction can narrow your diet and reduce your intake of the very nutrients, including vitamin D and calcium, that keep bones and muscles strong. A blood test for 25-hydroxyvitamin D gives you real information, and a GP or pharmacist can then advise on an appropriate dose and how long to take it for.

For the vast majority of healthy adults, a small daily dose of vitamin D through the darker months is a simple, low-cost habit with genuine benefits. Pair it with regular daylight walks, a varied diet and decent sleep, and you are giving your body sensible support right through to spring.