Vitamin D deficiency: sunlight, food sources and supplements

1. Introduction

Vitamin D is a nutrient that helps the body absorb calcium, the mineral that keeps bones strong. It also plays a part in how muscles, nerves and the immune system work ^{[1, 3]}. Deficiency means the body does not have enough of it. This article explains where vitamin D comes from, who is at risk of low levels, what problems low levels can cause, and when to talk to a doctor. It draws on information from the US National Institutes of Health, the NHS and MedlinePlus.

2. Where vitamin D comes from

There are three main sources ^{[1, 2]}.

• Sunlight. The skin makes vitamin D when it is exposed to sunlight. Clouds, smog, age, darker skin, sunscreen and glass windows all reduce how much the skin can make ^[1]. The NHS notes that in the United Kingdom, sunlight is not strong enough for this from about October to early March ^[2].
• Food. Very few foods contain much vitamin D naturally. Oily fish such as salmon, sardines and mackerel, egg yolks, red meat and liver contain some ^[2]. Some mushrooms exposed to ultraviolet light and cheese contain smaller amounts ^[1]. Many countries add vitamin D to certain foods, called fortified foods, such as some milks, plant-based drinks, cereals and spreads ^{[1, 2]}.
• Supplements. These come as tablets, liquids or drops ^{[1, 3]}. NIH explains that taking a supplement with a meal that contains some fat helps absorption ^[1].

The NHS points out that sun exposure can never cause an overdose of vitamin D, but skin should still be protected from sun damage ^[2]. MedlinePlus adds that sun exposure carries a risk of skin ageing and skin cancer, so many people rely on food and supplements instead ^[3].

3. Causes and risk factors

Low vitamin D can happen when too little reaches the body or when the body cannot use it well. MedlinePlus lists poor intake, poor absorption, limited sunlight and liver or kidney problems that stop the body converting vitamin D to its active form ^[3]. Some medicines can also interfere ^[3].

People who are more likely to have low levels include ^{[1, 2, 3]}:

• People who are rarely outdoors or who cover most of their skin, such as frail or housebound people and care home residents.
• People with dark skin, because darker skin makes less vitamin D from the same amount of sun.
• Older adults, whose skin makes less vitamin D.
• Breastfed babies, because breast milk alone does not provide enough.
• People with conditions that affect absorption, such as Crohn's disease, ulcerative colitis or coeliac disease.
• People with obesity or who have had gastric bypass surgery.
• People with long-term kidney or liver disease.

4. Signs and symptoms

Vitamin D deficiency may not cause obvious symptoms at first. When it lasts a long time it can affect the bones. In children it can cause rickets, a condition in which bones become soft, weak and bent ^{[1, 3]}. In teenagers and adults it can cause osteomalacia, which leads to bone pain and muscle weakness ^{[1, 3]}. Over the long term, low vitamin D can contribute to osteoporosis, a condition in which bones become fragile and break more easily ^{[1, 3]}.

5. Diagnosis and tests

A blood test measures a form of the vitamin called 25-hydroxyvitamin D. NIH explains that a level below a set cut-off is considered too low, while a level above a much higher cut-off may be too high ^[1]. Your doctor will interpret the result in the light of your health and decide whether testing is needed. MedlinePlus advises people who are at risk to talk to a health care provider about testing ^[3].

6. Treatment and prevention

If a blood test shows deficiency, MedlinePlus says treatment is with vitamin D supplements, and that a provider decides how much to take, how often and for how long ^[3]. Do not choose a strength yourself.

Everyday steps that help keep levels adequate include:

• Eating foods that contain or are fortified with vitamin D ^{[1, 2, 3]}.
• Spending some time outdoors with skin exposed in sunny months, while avoiding sunburn ^[2].
• Giving breastfed babies a vitamin D supplement, as advised by their doctor or health worker ^{[1, 3]}.
• Asking a doctor, pharmacist or dietitian whether a supplement suits you ^[1].

The NHS recommends that people at higher risk, such as those who are housebound or have dark skin, consider taking a supplement all year round, and that many other adults consider one in autumn and winter ^[2].

Vitamin D needs change with age and life stage. NIH and MedlinePlus give separate recommended daily intakes for babies, for people aged 1 to 70, for people over 70 and for pregnant or breastfeeding women, with older adults needing a little more ^{[1, 3]}. These are population guides rather than personal prescriptions, so ask your doctor what applies to you.

7. Taking too much

More is not better. Long-term excess, almost always from supplements rather than sunlight, can cause a high level of calcium in the blood. NIH and MedlinePlus list nausea, vomiting, weakness, confusion and weight loss as possible effects, and the high calcium can damage the kidneys and affect the heart rhythm ^{[1, 3]}. The NHS notes the same risk of high calcium weakening bones and harming the kidneys and heart ^[2]. Some medicines, such as steroids and some water tablets, can interact with vitamin D, so tell your doctor about everything you take ^[1].

8. When to see a doctor

Because low vitamin D often causes no clear symptoms, people in higher-risk groups are encouraged to talk to a health care provider about testing even when they feel well ^[3]. Speak to a doctor if you have bone pain, muscle weakness, or if you belong to a higher-risk group and are not sure whether you need testing or a supplement. Check with a doctor before giving supplements to a baby or child, and before taking them in pregnancy. Seek medical advice if you develop nausea, vomiting, great thirst or confusion while taking supplements ^{[1, 3]}.

In an emergency, call your local emergency number (112 in India, 911 in the United States).

9. Conclusion

Vitamin D keeps bones and muscles healthy, and it comes mainly from sunlight, a few foods and supplements. Some groups are more likely to run low, and a simple blood test can check. Because both too little and too much can cause harm, decisions about supplements are best made with your doctor.