Vitamin D is one of the few nutrients your body can make for itself, which is exactly why it is so easy to run short on. Make it from sunlight in July and you may barely need to think about it. Spend the winter commuting in the dark and working under office lights, and the picture can be very different.
This guide covers the basics: who is more likely to be low, how deficiency is actually checked, what the official reference intakes say, and why the right amount for you is a conversation with a clinician rather than a number on a bottle.
What vitamin D does, briefly
Vitamin D helps the body absorb calcium and keep bones and teeth healthy. The skin makes it when it is exposed to sunlight, and smaller amounts come from food: oily fish, egg yolks, and fortified products such as some cereals, spreads and plant milks.
Because sunlight is the main source for most people, how much you make depends heavily on where you live, the season, your skin, and how much of it the sun actually reaches.
Who is more likely to be low
No single factor guarantees deficiency, but several raise the odds. The NHS and MedlinePlus both point to a similar set of groups:
- People who are rarely outdoors. Anyone who is housebound, lives in a care home, or works long indoor hours gets little sun on their skin.
- People who cover most of their skin outside, for cultural, religious or medical reasons, or who always wear high-factor sunscreen.
- People with darker skin, such as those of African, African-Caribbean or South Asian background. More melanin means the skin makes less vitamin D from the same sunlight.
- Babies who are breastfed. Breast milk is naturally low in vitamin D, which is why infant guidance exists separately.
- People with conditions that affect absorption, such as Crohn's disease or coeliac disease, and people who have had weight-loss surgery.
- People with kidney or liver disease, since both organs are involved in turning vitamin D into its active form.
- People taking certain medications that affect how vitamin D is handled. Your pharmacist can tell you whether any of yours are among them.
Latitude matters too. The NHS notes that in the UK, between October and early March, people do not make enough vitamin D from sunlight. The same effect applies anywhere far enough from the equator.
What deficiency can look like
Mild deficiency often causes no obvious symptoms at all, which is part of the reason it goes unnoticed. When it is more pronounced, MedlinePlus lists signs such as bone pain, muscle weakness, and in more serious cases soft bones and fractures. In children, severe deficiency can cause rickets.
None of these are specific to vitamin D. Tiredness and aches have dozens of causes, which is why guessing from symptoms is a poor way to decide whether you need a supplement.
How deficiency is tested
The standard test is a blood test for 25-hydroxyvitamin D, often written 25(OH)D. MedlinePlus describes it as the most accurate way to see whether you have enough vitamin D. It reflects vitamin D from sunlight, food and supplements combined.
A clinician might order it if you have a bone condition such as osteoporosis, unexplained bone pain or muscle weakness, a condition that affects absorption, or several of the risk factors above.
A few practical points about the result:
- Labs and countries use different cut-offs. What counts as "insufficient" versus "deficient" is not identical everywhere, so read your result against the reference range on your own report, and ask what it means for you.
- Units differ. Results may be reported in nmol/L or ng/mL. The numbers are not interchangeable, so do not compare a figure from one lab with a threshold you read online in the other unit.
- One result is a snapshot. Levels change with the seasons. A reading in September can look quite different from one in March.
Routine testing for everyone is not generally recommended. Whether a test makes sense for you is a decision for your doctor.
The official reference intakes, and what they are for
Reference intakes are population figures. They describe roughly how much most healthy people need to meet their requirement, not how much any particular person should take.
In the US, the NIH Office of Dietary Supplements gives a Recommended Dietary Allowance of 15 mcg (600 IU) a day for most adults, and 20 mcg (800 IU) a day for adults over 70. It lists a Tolerable Upper Intake Level of 100 mcg (4,000 IU) a day for adults.
In the UK, the NHS advises that adults and children over 1 consider a daily supplement of 10 micrograms during autumn and winter, and year-round for people in the higher-risk groups above. It gives the same adult upper figure of 100 micrograms a day, with lower limits for children.
Those numbers do not contradict each other as much as they seem to. They are written for different populations and different purposes: one is a total intake target from all sources, the other is a supplement recommendation for a country with long, dark winters.
Why your amount should come from a clinician
It is tempting to treat vitamin D as harmless and pick whatever strength the shop has on offer. A few reasons not to:
- Treating a measured deficiency is different from maintenance. If a blood test shows you are genuinely deficient, the plan a clinician gives you may be different from the everyday maintenance figures above, and it is usually followed by a re-test. That plan is specific to you and your result.
- More is not better. The NHS warns that taking too much vitamin D over a long period can cause too much calcium to build up in the body, which can weaken bones and damage the kidneys and heart. Because vitamin D is stored in the body, excess accumulates rather than simply passing through.
- Other medications and conditions change the picture. Some conditions, and some medicines, make extra calcium or vitamin D a problem rather than a help.
- Products vary. Strengths on shelves range widely, and "one a day" on two different bottles can mean very different amounts. Check whether the label says mcg or IU. 1 mcg is 40 IU, so 10 mcg is 400 IU.
So the sensible order is: talk to your doctor or pharmacist, get tested if they think it is warranted, and take the amount they give you.
Keeping a supplement routine that actually sticks
If your clinician does recommend vitamin D, the hard part is often not the decision but remembering it every day for months. Supplements are the first thing to drop out of a routine, because missing one feels like it doesn't matter.
A few things help:
- Tie it to a meal you never skip. Vitamin D is fat-soluble, and many people find it easiest to take with a regular meal. Anchor it to breakfast or dinner rather than a clock time.
- Keep it with your other daily medicines, so one routine covers everything. If you take several things at different times of day, our guide on managing reminders for multiple medications covers how to keep them straight.
- Watch the bottle, not the calendar. A seasonal supplement that runs out in January tends to stay run out. The days supply calculator works out how long a bottle lasts at the amount you have been told to take.
Doz can hold these alongside your prescriptions on iPhone: reminders tied to real mealtimes, a follow-up nudge if a dose stays unlogged, and refill reminders while you still have some left.
Frequently Asked Questions
Who is most at risk of vitamin D deficiency?
People who rarely go outdoors, people who cover most of their skin outside, people with darker skin, breastfed babies, and people with conditions that affect absorption such as Crohn's or coeliac disease. Kidney or liver disease, weight-loss surgery and some medications also raise the risk. Living far from the equator adds to all of these in winter.
How do I know if I am deficient in vitamin D?
Only a blood test can tell you. The standard test measures 25-hydroxyvitamin D. Symptoms such as tiredness, aches or muscle weakness are too common and too general to diagnose it on their own. Ask your doctor whether a test makes sense for you.
How much vitamin D should I take a day?
That depends on your age, your health, your test result if you have one, and what your clinician advises. For reference, the NIH Office of Dietary Supplements gives 15 mcg (600 IU) a day for most adults and 20 mcg (800 IU) for adults over 70, and the NHS suggests 10 micrograms a day in autumn and winter. Those are population figures, not a personal prescription.
Can you take too much vitamin D?
Yes. The NIH Office of Dietary Supplements lists 100 mcg (4,000 IU) a day as the Tolerable Upper Intake Level for adults. The NHS warns that too much over a long period can cause calcium to build up in the body and damage bones, kidneys and the heart.
Is it better to take vitamin D in the morning or at night?
There is no single right time for everyone. What matters most is taking it consistently, and many people find it easiest to tie it to a regular meal. If your clinician or pharmacist gives you a specific timing, follow that.
Where to start
If you think you might be in one of the higher-risk groups, bring it up at your next appointment or ask a pharmacist. Mention any other medicines and supplements you take. Let them decide whether a test is worth doing and what amount suits you, then build that amount into the routine you already have.
Sources
Download Doz on the App Store · Read the FAQ · More from the blog