Lots of people with low vitamin D feel completely fine. There's often nothing to notice.

When there are signs, the usual ones are feeling tired, aching bones, and sore or weak muscles.¹ You can't tell from how you feel. A blood test is the only way to know your level.

What does low vitamin D actually mean?

When you get your vitamin D tested, the lab doesn't measure vitamin D itself. It measures a form your body makes from it, called 25-hydroxyvitamin D. That's the form your body stores, so it gives a better picture of how much you've got. The result comes back as a number, in units called nanomoles per litre. You'll see that written as nmol/L. The higher the number, the more you have.

One thing to bear in mind is that different labs use slightly different ranges. So go by the ranges printed on your own result, not by a table online.

Signs of low vitamin D

Often there aren't any. When there are, these are the ones UK guidance describes:

• Feeling tired¹

• Aching bones, often in your back, hips, or legs^1,2

• Sore muscles, weak muscles, or cramp¹

• In children, problems with the way bones grow. The name for this is rickets²

• In adults, being very low for a long time can soften the bones. The name for this is osteomalacia²

However, every one of those has dozens of other causes. Feeling tired doesn't mean your vitamin D is low. Only a blood test can tell you that.

Is it different for women?

No. The signs are the same for women and men. Pregnancy is the one place the advice changes. The NHS says that if you're pregnant or breastfeeding, you should take 10 micrograms of vitamin D a day.²

What causes it, and who's most at risk

Most of your vitamin D comes from sunlight landing on your skin.² So anything that means less sun on your skin makes low levels more likely. You're more likely to be low if you:

• Don't get outside much, for example if you're frail or housebound²

• Live in a care home²

• Cover most of your skin when you're outdoors²

• Have dark skin, for example if you have an African, African-Caribbean, or south Asian background²

• Are 65 or over¹

• Are pregnant or breastfeeding²

• Are living with obesity¹

• Have a condition that stops you absorbing food properly, like coeliac disease or inflammatory bowel disease¹

• Have kidney or liver disease¹

• Take certain medicines, like some epilepsy medicines or steroids taken long term¹

Why winter is the problem

From late March or early April through to the end of September, most people can make all the vitamin D they need from sunlight.² From October to early March, you can't.²

That surprises people. In the UK, winter sunlight isn't strong enough for your skin to make vitamin D. It doesn't matter how bright the day looks or how long you stay out. The sun is too low in the sky. That's why the NHS says everyone should think about taking 10 micrograms a day through autumn and winter.²

How to get your level checked

A vitamin D test measures 25-hydroxyvitamin D in your blood, which is the stored form.

Your GP may offer you one if you have symptoms, or if you're in one of the groups above.

You can also test at home. Numan's Nutritional Health Blood Test is a finger-prick kit. It checks 12 things in your blood, including vitamin D. Results come back in 3 to 5 days, with a written review from a clinician.

What to do if your level is low

This one's for your GP, or for the clinician who reviews your test. That’s because treating a real deficiency means prescribed doses, and those are far higher than anything you can buy in a shop. Your doctor works out the right amount and keeps an eye on it.

Don't try to fix a low result yourself with big doses. Taking too much vitamin D over a long time makes calcium build up in your body. That can weaken your bones and damage your kidneys and your heart.²

How much you need day to day

For most people, the number to remember is 10 micrograms a day.

The NHS says everyone should think about taking that through autumn and winter. If you're in one of the higher-risk groups above, take it all year round.²

Labels measure vitamin D two different ways, which gets confusing. Some say micrograms, some say IU. 1 microgram is the same as 40 IU.² So 10 micrograms is the same as 400 IU. 

Supplements come in all sorts of strengths, and stronger isn't better.

The NHS says not to take more than 100 micrograms (4,000 IU) a day. Some people have medical conditions that mean even that's too much for them.² If your doctor has told you to take a different amount, do what they've said.²

The numan take

So where does that leave you? For most people, 10 micrograms a day. Take it through autumn and winter, and all year round if you’re in one of the higher-risk groups above.² Supplements come in far bigger strengths. So don’t double up with another supplement containing Vitamin D. And check with a clinician first if you have a medical condition. 

Want to know your own number first? Take a look at the blood tests we offer. Or browse the full Numan supplement range and see what fits. Food supplements should not be used as a substitute for a varied, balanced diet and healthy lifestyle.

Frequently asked questions

What are the first signs of low vitamin D?

Often there are none. When there are, tiredness, aching bones, and muscle weakness are the most commonly described.¹

Can low vitamin D make you tired?

Tiredness can be a symptom of low vitamin D, but it has a great many other causes.¹ A blood test can show whether your level is low.

What vitamin D level is classed as deficient?

In the UK, a blood level below 25 nmol/L is linked to a higher risk of deficiency.¹

How can I test my vitamin D at home?

With a finger-prick blood test you post to a lab, such as Numan's Nutritional Health Blood Test, which includes vitamin D among its biomarkers.

How do I know if I need vitamin D in winter?

The NHS advises everyone to consider taking 10 micrograms a day from October to March, because there isn't enough sunlight for your skin to make it.²

References

1. ^{National Institute for Health and Care Excellence. Vitamin D deficiency in adults [Internet]. London: NICE Clinical Knowledge Summaries [cited 2026 Oct 23]. Available from: https://cks.nice.org.uk/topics/vitamin-d-deficiency-in-adults/}

2. ^{National Health Service. Vitamins and minerals: vitamin D [Internet]. London: NHS; 2020 [page last reviewed 2020 Aug 3; cited 2026 Oct 23]. Available from: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/}

3. ^{Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register [Internet]. London: DHSC [cited 2026 Oct 23]. Available from: https://www.gov.uk/government/publications/great-britain-nutrition-and-health-claims-nhc-register}

4. ^{Nutraceuticals Group. Full list of EFSA health claims [Internet]. Nutraceuticals Group [cited 2026 Oct 23]. Available from: https://nutraceuticalsgroup.com/uk/full-list-of-efsa-health-claims}