Vitamin D: What It Actually Does, and How Much You Need

Most people in the UK don't get enough vitamin D, and it has nothing to do with diet. It's the weather. Here's what vitamin D actually does for your body, how much you genuinely need, and why a UK winter makes it so easy to fall short.

Vitamin D helps your body absorb calcium and phosphate, which is what keeps bones, teeth and muscles working properly, and it also plays a role in normal immune function. Only a handful of foods contain much of it naturally, so for most people, around 90 percent of their vitamin D comes from sunlight on skin, not diet. The problem is that UK sunlight isn't strong enough for this from around October to March, whatever the weather looks like outside.

How Common Is Low Vitamin D, Really?

Estimates vary depending on how and when they're measured, but UK health data consistently shows a meaningful share of adults have low levels, with the proportion rising sharply over autumn and winter as the sunlight-making window closes. People with darker skin, anyone who covers most of their skin when outdoors, older adults, and people who spend little time outside are all at higher risk, because all of these factors reduce how much vitamin D the skin can make.

What Happens If You're Genuinely Deficient

A severe, long-term lack of vitamin D causes rickets, where bones become soft and can bend or fracture more easily. It's genuinely rare in the UK today, largely thanks to food fortification and widespread supplementation, but it hasn't disappeared entirely. More commonly, low vitamin D shows up as vaguer signs, tiredness, bone or muscle aches, or a general low mood, though these overlap with plenty of other things too. If you suspect a deficiency, a blood test from your GP is the only reliable way to know, and correcting a confirmed deficiency typically takes a course of a few months.

Two Claims Worth a Closer Look

You'll sometimes see vitamin D linked to mood and mental health. Researchers have genuinely investigated a possible connection between low vitamin D and depression, but the evidence is mixed and inconsistent: some trials show a modest benefit from supplementation, others show none at all. Vitamin D isn't a treatment for depression or anxiety, and if you're struggling with your mood, the right first step is speaking to your GP, not reaching for a supplement.

You'll also sometimes see a claim that too much vitamin D during pregnancy raises a child's allergy risk. The evidence actually points the other way, and even then it's mixed: some observational studies have linked low maternal vitamin D to a higher chance of allergic conditions like eczema in the child, but trials that actually tested giving pregnant women vitamin D supplements haven't consistently shown it prevents allergies. If you're pregnant or breastfeeding, ask your midwife or GP what's right for you rather than self-directing based on either claim.

Where to Get It

Beyond sunlight between March and October, useful dietary sources include oily fish like salmon and sardines, egg yolks, and foods fortified with vitamin D such as some cereals and dairy alternatives. For most people, food and safe summer sun exposure won't fully cover the autumn and winter gap, which is exactly why the standard UK advice is a modest daily supplement through the darker months.

Vitamin D won't fix your mood or melt away weight, whatever you might read elsewhere, but the basics are genuinely worth getting right: 10 micrograms a day is the target for most adults, and a UK winter is exactly when you're most likely to fall short.

Based on UK Department of Health and Social Care vitamin D guidance and published research on vitamin D, mood and pregnancy outcomes. Curious about immune support more broadly? Browse our Immune Support range.