Vitamin D Explained: Benefits, Deficiency Symptoms, Food Sources and How Much You Need

Vitamin D is often called the "sunshine vitamin" because your skin makes it when exposed to sunlight. It is essential for strong bones, healthy muscles and a well-functioning immune system. Yet according to the US National Institutes of Health (NIH), roughly 1 in 4 Americans have vitamin D blood levels that are too low or inadequate, and low levels are common in many other countries too.

Welcome to Vitamins and Minerals Explained, a 16-part series based on official health sources. Each part explains what a nutrient does, how much you need, the best foods, who is at risk of deficiency, the symptoms, safety limits and what the research really says. We start with vitamin D.

What does vitamin D do?

Vitamin D is a fat-soluble vitamin with several important jobs:

• Bone health: it helps your body absorb calcium from food. Without enough vitamin D, bones can become thin, brittle or misshapen, and it helps protect older adults from osteoporosis.
• Muscles: your muscles need vitamin D to move properly.
• Nerves: nerves need it to carry messages between the brain and the body.
• Immune system: it helps your immune system fight off bacteria and viruses.

Technically, vitamin D acts more like a hormone than a typical vitamin. The body converts it, first in the liver and then in the kidneys, into an active form that controls how much calcium and phosphate are absorbed from the gut.

How much vitamin D do you need?

The NIH lists the following Recommended Dietary Allowances (RDAs). Vitamin D is measured in micrograms (mcg) or International Units (IU); 1 mcg equals 40 IU.

Other countries set slightly different targets. For example, UK guidance suggests 10 mcg (400 IU) a day for most people, especially in autumn and winter. Your doctor may recommend more if your levels are low.

Best food sources of vitamin D

Few foods naturally contain much vitamin D, which is why many people struggle to get enough from diet alone. Fatty fish are the best natural sources, and many countries fortify milk, plant milks and cereals. The table below shows amounts per serving from the NIH's food data (the Daily Value, or DV, used on US food labels is 20 mcg).

Mushrooms are an interesting case: those exposed to ultraviolet light during growing or after harvest can contain useful amounts, making them one of the few plant-based options.

Vitamin D from sunlight

Your skin makes vitamin D when exposed to ultraviolet B (UVB) rays. But how much you make depends on many factors:

• Season and latitude: in winter, or far from the equator, the sun may be too weak for much production.
• Time of day: UVB is strongest around midday.
• Cloud cover and smog reduce UVB.
• Skin colour: people with darker skin need more sun exposure to make the same amount.
• Age: older skin makes less vitamin D.
• Glass: sitting by a sunny window does not work, because glass blocks UVB.
• Sunscreen reduces vitamin D production.

Because UV exposure also raises skin cancer risk, health authorities generally recommend using sunscreen of SPF 15 or higher when outdoors for more than a few minutes and relying on food and supplements rather than deliberate sunbathing. Sun exposure itself cannot cause vitamin D toxicity, because the skin limits how much it makes.

Who is at risk of vitamin D deficiency?

• Breastfed infants: breast milk alone does not provide enough. The NIH notes that breastfed babies are generally advised to take a supplement of 10 mcg (400 IU) a day.
• Older adults: the skin makes less vitamin D with age.
• People who rarely go outdoors, such as those who are housebound, or who cover most of their skin.
• People with darker skin, especially in less sunny climates.
• People with conditions that affect fat absorption, such as Crohn's disease, coeliac disease or ulcerative colitis.
• People with obesity or who have had gastric bypass surgery, who may need more vitamin D.

Deficiency is also common in many sunny countries, often because of indoor lifestyles, clothing, pollution and diet. For a regional perspective, see our article on vitamin D deficiency symptoms, tests and food sources.

Symptoms of vitamin D deficiency

Mild deficiency often causes no clear symptoms, which is why many people do not realise they are low. Longer-term or severe deficiency can cause:

• In children: rickets, in which bones become soft, weak, deformed and painful.
• In teens and adults: osteomalacia, which causes bone pain and muscle weakness.
• Over time: low vitamin D, together with low calcium, can contribute to fragile bones and osteoporosis, raising the risk of fractures.

Some people report tiredness, aches or low mood, but these symptoms have many possible causes, so testing is needed rather than guessing.

The vitamin D blood test explained

Doctors check vitamin D status by measuring 25-hydroxyvitamin D in the blood. Results are given in nanomoles per litre (nmol/L) or nanograms per millilitre (ng/mL); 1 nmol/L equals 0.4 ng/mL. According to the NIH:

Some laboratories and organisations use slightly different cut-offs, so always discuss your result with your doctor.

Vitamin D supplements: D2 vs D3

Supplements come in two main forms:

• Vitamin D2 (ergocalciferol): made from plant and fungal sources.
• Vitamin D3 (cholecalciferol): the form the skin makes, usually from lanolin (sheep's wool), though vegan D3 from lichen is available.

Both raise vitamin D levels in the blood, but the NIH notes that D3 may raise levels higher and keep them up longer than D2. Because vitamin D is fat-soluble, it is better absorbed when taken with a meal or snack that contains some fat.

Can you have too much vitamin D?

Yes. Too much vitamin D, almost always from high-dose supplements rather than food or sun, can raise blood calcium to harmful levels. The NIH sets these daily upper limits (from all sources):

Symptoms of vitamin D toxicity can include 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. Doctors sometimes prescribe higher doses for a short time to treat deficiency, but this should only be done under medical supervision.

Medicine interactions

According to the NIH, vitamin D supplements can interact with some medicines, including:

• Orlistat (a weight-loss drug), which can reduce vitamin D absorption.
• Some statins, which may work less well with high-dose vitamin D.
• Steroids such as prednisone, which can lower vitamin D levels.
• Thiazide diuretics, which, combined with vitamin D supplements, may raise blood calcium too high.

What does research say about vitamin D and disease?

Vitamin D has been studied for many conditions. The NIH summarises the evidence as follows:

• Bones: vitamin D with calcium slightly increases bone strength in older adults, but effects on falls and fractures are unclear.
• Cancer: supplements do not clearly reduce the risk of breast, colorectal or lung cancer, though they may slightly reduce the risk of dying from cancer.
• Heart disease: clinical trials have not shown that supplements reduce heart disease risk.
• Depression: low levels are linked to depression in some studies, but supplements have not been shown to prevent or relieve it.
• Type 2 diabetes and weight loss: supplements have not been shown to help most people.

In short, getting enough vitamin D is important, but taking extra beyond what you need is not a cure-all.

Practical tips

1. Include oily fish such as salmon, trout or sardines in your diet once or twice a week.
2. Choose fortified milk, plant milks or cereals if available.
3. Spend some time outdoors, while protecting your skin from sunburn.
4. If you are in a high-risk group, ask your doctor whether testing or a supplement is right for you.
5. Do not exceed the upper limit without medical advice.

Frequently asked questions

Is it better to take vitamin D in the morning or at night?

Timing matters less than taking it consistently and with food containing some fat for better absorption.

Can I get enough vitamin D from sunlight alone?

Some people can in summer, but many cannot, especially in winter, at high latitudes, with darker skin or with limited time outdoors.

How long does it take to fix low vitamin D?

It depends on how low your levels are and the dose your doctor recommends. Levels typically rise over several weeks to a few months, and doctors often recheck the blood level afterwards.

In Part 2, we look at vitamin B12, a vitamin that vegetarians, older adults and people on certain medicines often lack.

Sources

• NIH Office of Dietary Supplements – Vitamin D (consumer fact sheet)
• NIH Office of Dietary Supplements – Vitamin D (health professional fact sheet)

Vitamins and Minerals Explained: the full series

• Part 1: Vitamin D (you are here)
• Part 2: Vitamin B12
• Part 3: Iron
• Part 4: Magnesium
• Part 5: Vitamin C
• Part 6: Calcium
• Part 7: Zinc
• Part 8: Omega-3 Fatty Acids
• Part 9: Folate and Folic Acid
• Part 10: Vitamin A
• Part 11: Vitamin E
• Part 12: Vitamin K
• Part 13: Potassium
• Part 14: Iodine
• Part 15: Biotin
• Part 16: Do You Need Supplements?

This article is for general information only and is not medical advice. Recommended amounts shown are US figures from the NIH Office of Dietary Supplements; guidance in other countries may differ slightly. Talk to a doctor or pharmacist before starting supplements, especially if you are pregnant, take medicines or have a health condition.