Vitamin D: Benefits, Food Sources and Supplement Safety

Quick answer

Vitamin D helps the body absorb calcium and keeps bones and muscles healthy. Too little causes soft, weak bones (rickets in children, osteomalacia in adults) and contributes to osteoporosis. Correcting deficiency is clearly worthwhile.

But in people who already have enough vitamin D, large, well-run trials have found that extra supplements do not prevent heart disease, cancer, depression or most other conditions they are often marketed for. More is not better, and very high doses can be harmful.

How the body gets vitamin D

• Sunlight: the skin makes vitamin D when exposed to UVB light. This is the main source for most people.
• Food: naturally present in only a few foods: oily fish (salmon, sardines, mackerel), egg yolks, liver and some mushrooms exposed to UV light. Some milks, yoghurts, cereals and plant drinks are fortified.
• Supplements: vitamin D3 (cholecalciferol) or D2 (ergocalciferol). D3 tends to raise blood levels more effectively.

Why deficiency is common, even in sunny countries

It surprises many people that vitamin D deficiency is common in the Gulf region, including Oman, despite strong sunshine. Studies across Gulf countries have consistently found high rates of low vitamin D. Reasons include:

• Spending most of the day indoors or in cars, especially during hot months
• Covering most of the skin outdoors
• Darker skin, which needs more sun exposure to make the same amount of vitamin D
• Sunscreen use, which is important for skin cancer prevention but reduces vitamin D production
• Low intake of oily fish and fortified foods

Other groups at higher risk anywhere include older adults, people with obesity, people with conditions that affect fat absorption (such as coeliac or Crohn's disease, or after weight-loss surgery), and breastfed babies.

What the research says

Bones and muscles

Vitamin D prevents and treats rickets and osteomalacia. In older adults with low levels, vitamin D combined with calcium can reduce fracture risk. In people with normal levels, extra vitamin D does not seem to strengthen bones or prevent falls further.

Other conditions

The large US VITAL trial gave 2,000 IU of vitamin D a day or placebo to over 25,000 adults for about five years. Vitamin D did not reduce heart attacks, strokes or cancer diagnoses. Similar large trials elsewhere had similar results. There were hints of benefit in some subgroups (for example, a possible reduction in autoimmune disease), but these need confirmation.

For respiratory infections, some reviews found a small protective effect with daily doses in people with low levels, but results are mixed.

How much do you need?

US recommended daily intakes (the NHS gives similar advice):

• Babies up to 12 months: 400 IU (10 mcg). Breastfed babies usually need a supplement; formula is already fortified.
• Children and adults up to 70: 600 IU (15 mcg)
• Adults over 70: 800 IU (20 mcg)
• Upper limit for adults: 4,000 IU (100 mcg) a day, unless prescribed by a doctor

People with confirmed deficiency are often prescribed higher doses for a limited time, followed by a maintenance dose. This should be guided by a doctor.

Practical tips

1. Get some sensible sun. Short periods of sun on the arms and face, outside the hottest part of the day, help. Avoid burning.
2. Eat oily fish once or twice a week, and choose fortified foods if you don't.
3. Consider a daily supplement of 400 to 1,000 IU if you are in a high-risk group, especially if you rarely go outside or cover most of your skin.
4. Take it with a meal containing some fat, which improves absorption.
5. Ask for a blood test if you have risk factors or symptoms such as bone pain or muscle weakness, rather than guessing with high doses.

Side effects of too much vitamin D

Vitamin D toxicity almost always comes from supplements, never from sunlight. It raises calcium levels in the blood, which can cause:

• Nausea, vomiting, poor appetite and constipation
• Excessive thirst and frequent urination
• Confusion, weakness and tiredness
• Kidney stones and kidney damage
• Irregular heartbeat in severe cases

Interactions and who should be careful

• Kidney disease, sarcoidosis, some lymphomas, or a history of high calcium or kidney stones: take only under medical supervision
• Thiazide diuretics: combined with high vitamin D and calcium, can raise calcium levels
• Digoxin: high calcium increases the risk of heart rhythm problems
• Steroids, some epilepsy medicines, orlistat and cholestyramine can lower vitamin D levels; your doctor may recommend a supplement
• Pregnancy: a standard supplement (400 to 600 IU) is commonly recommended; avoid high doses unless prescribed

When to see a doctor

See a doctor if:

• You have bone pain, muscle weakness or aches, frequent fractures, or a child has bowed legs or delayed walking
• You are in a high-risk group and have never been tested
• You are taking high-dose vitamin D and develop thirst, frequent urination, nausea, confusion or weakness
• You have kidney disease or a condition affecting calcium levels

Frequently asked questions

Is it better to take vitamin D daily or weekly?

Daily or weekly doses can both correct low levels. Some research suggests daily doses work more consistently, and very large single doses (such as a big yearly dose) may be less effective or even increase falls in older adults.

Should I take vitamin D with calcium?

Only if your diet is low in calcium or your doctor advises it. Most people can get enough calcium from food.

Does vitamin D help with tiredness?

If tiredness is caused by vitamin D deficiency, correcting it can help. Tiredness has many causes, though, so it is worth seeing a doctor if it persists.

Does vitamin D help teeth?

Vitamin D supports healthy bones and teeth, and severe deficiency in childhood affects tooth development. It does not replace brushing with fluoride toothpaste and limiting sugar for preventing decay.

References

• NIH Office of Dietary Supplements. Vitamin D: fact sheet for health professionals. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
• Manson JE, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease (VITAL). New England Journal of Medicine, 2019.
• NHS. Vitamin D. https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/
• Bassil D, et al. Hypovitaminosis D in the Middle East and North Africa: prevalence, risk factors and impact on outcomes. Dermato-Endocrinology, 2013.