Every autumn, the same advice gets repeated: take a vitamin D supplement because the British sky simply cannot deliver enough sunlight between October and March. Most of us comply. We pick up a bottle from the chemist, take one daily, and assume the job is done. Yet Public Health England data has consistently shown that around one in five UK adults has low vitamin D levels, many of whom are already supplementing. Something is going wrong between the capsule and the bloodstream. I’d argue the problem isn’t a lack of awareness about vitamin D, it’s a lack of awareness about what stops it actually working.
Why vitamin D is genuinely hard to absorb
Vitamin D is fat-soluble. Unlike vitamin C, which dissolves in water and is absorbed fairly readily, vitamin D needs dietary fat to hitch a ride through your gut wall. If you’re taking your supplement first thing in the morning on an empty stomach, which a lot of people do, your body may be absorbing a fraction of what’s on the label. Research published in the Journal of the Academy of Nutrition and Dietetics found that taking vitamin D with a meal containing fat increased absorption by roughly 32%. That’s not a minor footnote; that’s a third of the dose potentially lost just because of timing.
The form of the supplement also matters more than most people realise. Vitamin D3 (cholecalciferol) is the form your skin naturally produces and is consistently shown to raise serum levels more effectively than D2 (ergocalciferol), which is derived from plants and fungi and often used in cheaper or vegan-labelled products. If your supplement says “vitamin D” without specifying D3, check the ingredients panel. You may be taking the less bioavailable version without knowing it.
How body weight affects how much you actually need
The standard NHS recommendation is 10 micrograms (400 IU) per day for most adults through winter. That figure is designed as a population baseline, not a personalised dose. One of the most overlooked factors in vitamin D status is body size. Vitamin D distributes into fatty tissue, and a larger body has more tissue for it to spread through. Studies consistently show that people with a higher BMI tend to have lower circulating vitamin D even when taking the same supplement dose as someone lighter. This doesn’t mean anything is wrong, it’s simply physics. But it does mean that 400 IU might be adequate for a slight person and largely insufficient for someone heavier.
I’ve spoken to people who’ve been supplementing conscientiously for two winters running and still come back with blood test results showing deficiency. In most of those cases, their dose hadn’t been adjusted to account for body weight. A GP or pharmacist can advise on a more appropriate dose if standard supplementation isn’t shifting your levels, so it’s worth having a conversation rather than just doubling up on your own.
The gut health connection most people miss
Your gut has to absorb the vitamin D before it can do anything at all. If your gut lining is compromised, through conditions like coeliac disease, Crohn’s disease, or irritable bowel syndrome, absorption will be reduced regardless of how high the dose is. Coeliac disease in particular affects the small intestine, which is precisely where fat-soluble vitamins are taken up. Many people with undiagnosed or poorly managed gut conditions end up with vitamin D deficiency as one of the downstream effects, without ever connecting the two.
Even without a formal gut diagnosis, chronic low-grade inflammation, dysbiosis, or years of a low-fat diet can dull absorption. The microbiome appears to play a role in vitamin D metabolism too, though research is still developing in this area. What’s clear is that supplementing into a poorly functioning gut is a bit like trying to fill a leaking bucket, the answer isn’t always a bigger bottle.
If you’re also experiencing persistent fatigue and low energy alongside suspected vitamin D issues, it’s worth asking your GP for a broader blood panel rather than attributing everything to one deficiency. Several overlapping issues often present together.
Magnesium, the missing co-factor
Vitamin D doesn’t work in isolation. It relies on magnesium to be converted from its supplemented or sun-derived form into the active hormone your body actually uses (calcitriol). If your magnesium levels are low, and a significant proportion of British adults don’t reach the recommended intake through diet, then vitamin D can accumulate in its inactive form without being properly utilised. Taking vitamin D without adequate magnesium is a bit like having the right key but the wrong lock mechanism.
This is a commonly missed piece of the puzzle. Some people find that adding a magnesium supplement, or increasing dietary sources like dark leafy greens, nuts, seeds and wholegrains, is what finally moves their vitamin D levels. If you want to understand more about how to get magnesium right, our piece on magnesium and the mistakes most British people make with it covers the different forms and what to look for.
Liquid drops vs softgels vs tablets, does the format matter?
Short answer: yes, a little. Tablets are generally the least well absorbed of the three common formats because they need to be broken down before any absorption can happen. Softgel capsules dissolve more readily and come suspended in an oil, which provides built-in fat for absorption. Liquid drops in an oil base are similarly efficient. If you’ve been using a standard tablet and haven’t seen results, switching to an oil-based softgel is a low-effort change that may genuinely help.
Some supplements now come as vitamin D3 combined with K2. The pairing makes sense physiologically, K2 helps direct calcium (which vitamin D helps absorb) to bones rather than soft tissue. Whether you need both depends on your diet and health profile, and a pharmacist can advise without a GP appointment if you’re unsure.
The only way to know if it’s actually working
Blood testing is the only reliable way to confirm your vitamin D status. A serum 25-hydroxyvitamin D test measures what’s actually circulating, and your GP can request this on the NHS if you have symptoms or risk factors. Private finger-prick tests are also widely available for around £30-£40 from services like Thriva or Medichecks, and give you a result within a few days.
Optimal levels are generally considered to be between 75 and 200 nmol/L. Anything below 50 nmol/L is considered deficient, and below 25 nmol/L is classified as severe deficiency. Supplementing blind, taking a dose without knowing your baseline, means you can’t tell whether you’re making progress, and some people stay deficient for years while believing their supplement is doing the job.
If you’re already managing a winter-related health issue like seasonal low mood or energy dips, testing your vitamin D alongside getting appropriate support makes sense rather than treating each thing in isolation.
A few practical changes worth making now
Take your supplement with your largest meal of the day, especially if it contains some fat. Choose D3 over D2, and an oil-based format over a plain tablet. If you have any gut symptoms, speak to your GP about whether an underlying condition might be affecting absorption. Check your magnesium intake. And if you’ve been supplementing for more than three months without certainty that it’s working, get tested.
None of this is complicated. The problem is that “take a vitamin D supplement” is such simple advice that most people assume there’s nothing more to know. There is. A small amount of attention to how and what you’re taking can make the difference between a supplement that genuinely supports your health through winter and one that essentially goes straight through you.
Frequently Asked Questions
Why am I still vitamin D deficient if I'm taking supplements every day?
Several factors can reduce how much vitamin D your body actually absorbs: taking it on an empty stomach, using a D2 formulation instead of D3, having a gut condition like coeliac disease, or low magnesium levels that prevent conversion to the active form. A blood test will confirm whether your levels are improving.
How much vitamin D should I take in winter if I'm overweight?
The NHS recommends 10 micrograms (400 IU) for most adults, but this is a population-level baseline. People with a higher body weight often need a higher dose because vitamin D distributes into fatty tissue, diluting circulating levels. Speak to your GP or pharmacist about an appropriate dose based on your individual situation.
Is vitamin D3 better than D2 for raising levels?
Yes, consistently. Vitamin D3 (cholecalciferol) is the form your skin naturally synthesises and is significantly more effective at raising and maintaining serum vitamin D levels compared to D2 (ergocalciferol). Many standard supplements and own-brand products use D2, so always check the label.
Should I take vitamin D with food?
Yes, ideally with a meal that contains some fat. Vitamin D is fat-soluble, meaning it needs dietary fat to be absorbed properly through the gut. Taking it on an empty stomach can reduce absorption by up to a third. The largest meal of the day is typically the best time.
Can I get a vitamin D blood test on the NHS?
Your GP can request a serum 25-hydroxyvitamin D test if you have symptoms of deficiency or are in a higher-risk group. If your GP doesn’t feel a test is clinically indicated, private finger-prick home tests are available from services like Medichecks or Thriva for around £30-£40 and provide results within a few days.