Quick answer (TL;DR): Vitamin D is made in your skin from sunlight, and food alone rarely fills the gap. Most adults need 15 mcg (600 IU) a day by the US NIH guideline, and the UK NHS suggests considering 10 mcg (400 IU) in autumn and winter. Low levels are common: about one in four people in the United States, and many people in sunny countries too. The signs are vague, so a blood test, not guesswork, confirms it. Do not take high doses without medical advice.
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What does vitamin D do?
Vitamin D helps your body absorb calcium, one of the main building blocks of strong bones. Together with calcium, it helps protect against osteoporosis, a disease that thins and weakens bones and makes them more likely to break. Your muscles need vitamin D to move, your nerves need it to carry messages between the brain and the body, and your immune system needs it to fight bacteria and viruses. So a shortage can show up in more than one place.
Why does vitamin D fall in winter?
Your body makes vitamin D when bare skin is exposed to sunlight. In winter in northern countries the sun sits lower, days are shorter, and people spend more time indoors and covered up. Clouds, smog, older age and darker skin also reduce how much your skin makes, and skin does not make vitamin D from sunlight through a window.
The NHS puts it plainly for the UK. From late March or early April to the end of September, most people can make all the vitamin D they need from sunlight. Between October and early March, they cannot. The farther you live from the equator, the longer that low-sun window lasts. Countries closer to the equator, such as Bangladesh, have a much smaller winter effect. But as the next section shows, that does not protect people from low vitamin D.
Can you be low in vitamin D in a sunny country?
Yes. Studies in South Asia, including Bangladesh, and in the Middle East have found low vitamin D in many people despite plentiful sunshine. Common reasons are long hours indoors, clothing that covers most of the skin, sunscreen use, darker skin and few foods that naturally contain vitamin D. Obesity and older age also play a part. If you live in a sunny country and are at higher risk, ask your doctor about a blood test, and follow your doctor's advice rather than assuming that sunshine alone is enough.
What are the signs of vitamin D deficiency?
In adults, deficiency is not always obvious. Cleveland Clinic lists signs that might include:
- Fatigue: feeling tired without a clear reason
- Bone pain: including deep aching, often in the back
- Muscle weakness, aches or cramps
- Mood changes, such as depression
In children, a severe lack of vitamin D causes rickets, a disease in which bones become soft, weak, deformed and painful. In teens and adults, deficiency can cause osteomalacia, which causes bone pain and muscle weakness. Over time, low vitamin D and calcium can make bones fragile and more likely to break.
These signs are non-specific. Tiredness, aches and low mood have many other causes, so symptoms alone cannot confirm a deficiency. Do not start a high dose because of one symptom.
How much vitamin D do you need?
The amount depends on age. The US NIH Office of Dietary Supplements lists these daily recommended amounts:
| Who | Daily amount (NIH) |
|---|---|
| Birth to 12 months | 10 mcg (400 IU) |
| Children 1 to 13 years | 15 mcg (600 IU) |
| Teens 14 to 18 years | 15 mcg (600 IU) |
| Adults 19 to 70 years | 15 mcg (600 IU) |
| Adults 71 years and older | 20 mcg (800 IU) |
| Pregnant and breastfeeding women | 15 mcg (600 IU) |
Guidance differs a little by country. The NHS recommends 10 mcg (400 IU) a day for children from age 1 and for adults, including pregnant and breastfeeding women. It suggests that everyone consider a daily 10 mcg supplement in autumn and winter, and babies up to 1 year need 8.5 to 10 mcg a day. Follow your local guidance or your doctor's advice instead of mixing the two. One mcg equals 40 IU, so 10 mcg is 400 IU and 15 mcg is 600 IU.
Which foods provide vitamin D?
Very few foods naturally contain vitamin D. The best natural sources are fatty fish and fish liver oils.
- Fatty and oily fish: salmon, trout, tuna, mackerel, sardines and herring. I could not find reliable vitamin D values for local fish, so check with a dietitian.
- Egg yolks, beef liver and cheese: small amounts
- Red meat: small amounts, according to the NHS
- Mushrooms: a little, and more in mushrooms exposed to ultraviolet light
- Fortified foods: in the United States, almost all milk is fortified with about 3 mcg (120 IU) per cup, and vitamin D is added to many cereals and some juices, yogurts and spreads
Fortification varies by country. In the UK, for example, the NHS says cows' milk is generally not fortified. Always check the label where you live. For more everyday ideas, see our nutrition tips.
Sunlight or supplements: what works?
Where the sun is strong, sunlight on bare skin is the main source for most people. Where it is weak, the NHS says you need to get vitamin D from your diet, and since that is difficult, everyone should consider a daily supplement during autumn and winter. A sunny winter afternoon is not a reliable top-up in northern countries.
If you take a supplement, two practical details help. Vitamin D comes as D2 (ergocalciferol) and D3 (cholecalciferol), and D3 might raise blood levels higher and for longer. Vitamin D is fat-soluble, so it is better absorbed with a meal that includes some fat.
Sunscreen limits vitamin D production, but health experts still recommend SPF 15 or higher when you are out in the sun for more than a few minutes, because ultraviolet radiation can cause skin cancer. Do not skip sun protection to chase vitamin D.
Who is more likely to have low vitamin D?
- Older adults: the skin makes less vitamin D with age.
- People who seldom expose their skin to sunshine: because they are indoors or keep the body and head covered.
- People with dark skin: darker skin makes less vitamin D from sunlight.
- Breastfed infants: breast milk alone does not give enough, and breastfed infants need a 10 mcg (400 IU) supplement a day.
- People with conditions that limit fat absorption, such as Crohn's disease, celiac disease or ulcerative colitis.
- People with obesity or who have had gastric bypass surgery: they may need more.
The NHS adds that adults and children over 4 who are rarely outdoors, who live in a care home, or who usually wear clothing that covers most of their skin should consider 10 mcg a day throughout the year. The same applies to people with dark skin, for example African, African-Caribbean or South Asian backgrounds.
What beliefs do I hear in villages and neighbourhoods?
From my community health work, these are beliefs about vitamin D that I hear often. Here is what the evidence says about each.
"Food alone gives enough vitamin D."
For most people this is not true. Very few foods contain vitamin D naturally, and the NHS says it is difficult to get enough from food alone when sunlight is weak. Food helps, but it rarely closes the gap by itself.
"Sitting in the sun means you make plenty of vitamin D."
Not always. How much your skin makes depends on the season, the time of day, how much bare skin is exposed, your skin colour, clothing, sunscreen and air pollution. Sunlight through a window does not work. Your skin limits what it makes, so you cannot get too much from sun, but long sunbathing does raise the risk of sunburn and skin cancer. Regular, short, sensible time outdoors is the aim, not hours of sunbathing.
"Everyone should take vitamin D tablets regularly."
Not necessarily. Many people with normal levels do not need them, and too much is harmful. In the UK, the NHS suggests that everyone consider 10 mcg in autumn and winter, and that people at higher risk consider it all year. High-dose capsules are meant for treating a proven deficiency under medical advice. The upper limit for adults, from all sources, is 100 mcg (4,000 IU) a day.
"Vitamin D alone makes bones strong."
Vitamin D helps your body absorb calcium, but bones also need calcium, protein and regular weight-bearing activity such as walking. In older adults, vitamin D with calcium slightly increases bone strength, according to the NIH, but it is not clear that it prevents falls. Vitamin D is one part of bone health, not the whole story.
"Drinking milk meets your vitamin D needs."
Only if the milk is fortified. In the United States, a cup of fortified milk gives about 3 mcg, so you would need about five cups to reach 15 mcg. Plain milk that has not been fortified contains very little. In the UK, milk is generally not fortified. I could not verify whether milk sold in Bangladesh is fortified, so check the label. Milk is still a good source of calcium, which is a separate need.
Should you get tested?
A blood test that measures 25-hydroxyvitamin D shows your level, in nanomoles per litre (nmol/L) or nanograms per millilitre (ng/mL). One nmol/L is 0.4 ng/mL, so 50 nmol/L is 20 ng/mL. The NIH describes the range this way:
- 50 nmol/L (20 ng/mL) or above is adequate for most people.
- Below 30 nmol/L (12 ng/mL) is too low and might weaken your bones.
- Above 125 nmol/L (50 ng/mL) is too high and might cause health problems.
Labs and experts use slightly different cut-offs, so ask your doctor to explain your result. Not everyone needs a test. A doctor is more likely to suggest one if you have persistent bone pain or muscle weakness, a fracture after a minor fall, a condition that affects fat absorption, or several risk factors at once.
Can you take too much vitamin D?
Yes. Too much vitamin D is almost always caused by supplements, not sunshine. Very high blood levels (above 375 nmol/L or 150 ng/mL) can cause nausea, vomiting, muscle weakness, confusion, pain, loss of appetite, dehydration, excessive thirst and urination, and kidney stones. Extremely high levels can cause kidney failure, irregular heartbeat and even death. Long-term excess can also cause too much calcium to build up, which can harm the bones, kidneys and heart.
The upper limits include vitamin D from all sources. For adults, both the NIH and the NHS give 100 mcg (4,000 IU) a day. The NHS says that for most people 10 mcg a day is enough. For children aged 1 to 10 the NHS limit is 50 mcg (2,000 IU) a day, and for babies under 12 months it is 25 mcg (1,000 IU) a day. Your doctor may recommend a higher dose for a period to treat a deficiency. Follow that advice, not a dose you read online.
Tell your doctor and pharmacist about all the supplements and medicines you take. Orlistat can reduce vitamin D absorption, steroids can lower blood levels, and thiazide diuretics could raise your blood calcium too high if you take vitamin D. Get urgent medical help for persistent vomiting, confusion or severe weakness while taking high-dose vitamin D.
Which other vitamin D myths should you ignore?
- "More vitamin D is always better." No. There is a ceiling, and more than 100 mcg (4,000 IU) a day for adults over time can be harmful.
- "Vitamin D will fix low mood or prevent depression." Some studies link low levels with a higher risk of depression, but clinical trials show that supplements do not prevent or ease depression, according to the NIH.
- "It will help you lose weight or protect your heart." The NIH says supplements do not help you lose weight, and trials find that they do not reduce the risk of heart disease or of dying from it, even if you have low levels.
Frequently Asked Questions
Can I get enough vitamin D from food alone?
For most people it is difficult. Very few foods naturally contain vitamin D, and the NHS says it is hard to get enough from food alone when sunlight is weak.
Is 10 mcg the same as 400 IU?
Yes. One microgram of vitamin D equals 40 IU, so 10 mcg is 400 IU, 15 mcg is 600 IU and 20 mcg is 800 IU.
Should I take vitamin D3 or D2?
Both raise vitamin D in your blood, but D3 might raise it higher and for longer. Either can work. Ask your doctor or pharmacist if you have a preference or a medical reason to choose one.
Do I need a blood test before taking a supplement?
Not necessarily. A standard daily amount is a general amount, not a treatment dose. A test is more useful if you have symptoms, risk factors or a doctor has suggested it.
Can children take vitamin D?
Yes, in age-appropriate amounts. NHS advice is 10 mcg a day for children aged 1 to 4 throughout the year, and 8.5 to 10 mcg a day for babies up to 1 year who are breastfed or take less than 500 ml of formula a day. Ask a doctor or health worker before giving a child a supplement. See also our guide to healthy eating for babies and children.
Does sunscreen stop me making vitamin D?
Sunscreen limits vitamin D production, but experts still recommend it when you are in the sun for more than a few minutes. Use sunscreen, and get vitamin D from food or a supplement if sunlight is weak or you cover most of your skin.
The bottom line
Low vitamin D is common, including in sunny countries, and it is usually down to how we live, not anything we have done wrong. Eat vitamin D-rich foods when you can, get safe daylight when you can, and ask your doctor about a blood test if you are at higher risk. If you take a supplement, 10 mcg (400 IU) is the NHS autumn and winter amount, 15 mcg (600 IU) is the NIH daily amount for adults up to 70, and 100 mcg (4,000 IU) from all sources is the upper limit unless a doctor advises otherwise. If you have bone pain, muscle weakness or several risk factors, a blood test and a doctor's advice are the next step, not a mega-dose.
Sources
- National Institutes of Health, Office of Dietary Supplements: Vitamin D consumer fact sheet
- NHS: Vitamin D
- Cleveland Clinic: Vitamin D deficiency, causes, symptoms and treatment
- Bangladeshi study of vitamin D deficiency, with a review of earlier Bangladeshi research
- Bangladesh Journal of Medicine, 2026: vitamin D status of adults at a tertiary hospital in Dhaka
- Bangladesh Journal of Medicine, 2021: vitamin D deficiency among physicians in Bangladesh, with background on the Middle East and Asia
General information only, not medical, healthcare, or psychological advice. Check current medical guidelines with a certified physician or licensed healthcare provider before acting on anything published here. Never disregard professional medical advice or delay seeking it because of something you read on this site.
