Vitamin D: Deficiency, Testing and Supplements

Vitamin D helps the body absorb calcium and keep bones strong. The skin makes it in sunlight, and a few foods contain it. Even so, many people have low levels, including in sunny countries like India.

How much you need

The US National Institutes of Health sets the recommended amount at 600 IU (15 mcg) a day for people aged 1–70, and 800 IU (20 mcg) for those over 70. The upper limit for adults is 4,000 IU (100 mcg) a day [1].

Why low levels are common

• Spending most of the day indoors.
• Covering skin, or using sunscreen.
• Darker skin, which makes less vitamin D from the same sun.
• Older age and obesity.
• Few foods are naturally rich in vitamin D [1].

Testing and supplements

A blood test measures vitamin D status. Supplements help people with low levels, especially older adults at risk of bone loss. Too much can be harmful, so very high doses should only be taken with medical advice [1].

Large trials have not shown that vitamin D supplements prevent heart disease or cancer in people who are not deficient, so claims should be read with care [1].

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Why bones need vitamin D

Without enough vitamin D, bones can become thin, brittle or misshapen [2]. Enough vitamin D prevents rickets in children and osteomalacia (soft bones) in adults [2]. With calcium, it helps protect older adults from osteoporosis [2].

Vitamin D and falls

Many people take vitamin D to prevent falls. WHO says routine vitamin D should not be presented as a general fall-prevention step for all older adults [3]. Testing and treating a real deficiency may be right for some people. High-dose supplements may cause harm [3].

Exercise and home safety checks have stronger evidence for preventing falls [3].

Bones and joints worldwide

About 1.71 billion people live with musculoskeletal conditions, including 440 million with the effects of fractures [4]. Osteoporosis and fragility fractures are one group of these conditions [4]. Vitamin D, calcium, exercise and fall prevention all play a part in bone health.

Getting vitamin D

• Sun: the skin makes vitamin D in sunlight [2]. Season, latitude, skin colour, sunscreen and time indoors all affect how much.
• Food: few foods are natural sources. Fatty fish and egg yolks have some. Many milks and cereals are fortified [2].
• Supplements: D3 and D2 are both used [2].

Too much is harmful

Vitamin D is stored in the body, so very high doses can build up. Too much can raise blood calcium and harm the kidneys and heart. More is not better.

Key numbers at a glance

• Pregnant and breastfeeding women: 15 mcg (600 IU) a day.
• About 1.71 billion people live with musculoskeletal conditions.
• WHO advises against vitamin D as a general fall-prevention step.

Questions to ask your health worker

• Should my vitamin D level be tested?
• What dose is right for me?
• Do I need calcium too?
• Could it interact with my medicines?

Where to read more

All figures in this article come from public sources listed below. For more detail, start with the NIH Office of Dietary Supplements fact sheets on vitamin D and the WHO fact sheet on falls. Each is free to read online. If anything here raises a question about your own health, take this article to your doctor or pharmacist and talk it through together.

Common myths

• Myth: Everyone should take high-dose vitamin D. Fact: More is not better. High doses may cause harm [3].
• Myth: Vitamin D prevents falls for all older people. Fact: WHO advises against presenting it as a general fall-prevention step [3].
• Myth: You get enough vitamin D from any sun exposure. Fact: Season, latitude, skin colour and time outdoors all change how much the skin makes [2].

Reading the label

• The dose in micrograms (mcg) and international units (IU).
• Whether it is vitamin D3 or D2.
• Whether calcium is included, and how much.
• The total from all products you take, including multivitamins.

Who is more likely to be low

The NIH Office of Dietary Supplements lists groups at higher risk of low vitamin D [2]:

• Breastfed infants, since breast milk has little vitamin D.
• Older adults, whose skin makes less vitamin D.
• People who get little sun.
• People with darker skin.
• People with conditions that limit fat absorption, such as Crohn's disease.
• People with obesity or who have had weight-loss surgery.

Signs of deficiency

Many people with low vitamin D have no symptoms. Severe deficiency can cause bone pain and muscle weakness. In children it can cause rickets, with soft, bent bones [2]. A blood test of 25-hydroxyvitamin D is the usual way to check.

Vitamin D and calcium

Vitamin D helps the body absorb calcium from food [2]. The two work together for strong bones. Older adults with low intakes may be advised to take both. But calcium tablets are not right for everyone, and very high calcium intake can cause kidney stones. Getting calcium from food, such as milk, yoghurt, tofu and green leafy vegetables, is usually the first choice.

What this means

Vitamin D is a real need for many people, but not a cure-all. Testing, sensible doses and food fortification are the evidence-based path.

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Topics: Nutrition · Bones and joints

Sources

1. NIH Office of Dietary Supplements: Vitamin D fact sheet for consumers
2. NIH Office of Dietary Supplements: Vitamin D fact sheet for health professionals
3. WHO fact sheet: Falls
4. WHO fact sheet: Musculoskeletal health

Banner image: Vitamin D pills by Mx. Granger, CC0, via Wikimedia Commons.