Vitamin D deficiency: who is at risk, symptoms and treatment

Vitamin D regulates the amount of calcium and phosphate in the body, which keeps bones, teeth and muscles healthy. The British Dietetic Association adds that it assists the immune system, supports nerve function and helps maintain muscle strength to prevent falls, and points out that it is technically a hormone made in the skin in response to sunlight.

In the UK the sun is strong enough for the skin to make vitamin D from about late March or early April to the end of September. Between October and early March it is not. Food contributes little. The NHS lists oily fish, red meat, egg yolks, some fortified foods and liver (avoid in pregnancy), and notes that UK cows' milk is generally not fortified. The BDA says even a well-balanced diet is unlikely to supply enough.

The NHS says people who have little or no sunshine exposure are at risk, as are those with dark skin, such as African, African-Caribbean or south Asian backgrounds, who may not make enough even in summer. The BDA lists a wider set of risk factors.

• Being housebound, frail or living in a care home, or usually covering most of the skin outdoors.

• Being over 65, pregnant or breastfeeding.

• Having a darker skin tone.

• A high body mass index.

• Conditions affecting absorption, such as Crohn's disease, coeliac disease, previous weight-loss surgery or short bowel syndrome.

• Severe liver or chronic kidney disease, which affect how vitamin D is handled.

• Medicines such as anticonvulsants, certain antivirals, cholestyramine or rifampicin.

Many people with low vitamin D have no obvious symptoms. When deficiency is prolonged it can lead to soft bones. The NHS describes rickets in children and osteomalacia in adults. Rickets typically shows as bowed legs, swollen painful wrists, and children who are small for their age with late teeth. In adults and children, signs can include pain in the back, shoulder, ribs, pelvis or legs, changes in the way you walk, difficulty climbing stairs or rising from a chair because of weak muscles, muscle tingling, cramping or twitching, and breaking bones more often or after minor accidents.

The BDA adds that adult deficiency can contribute to osteoporosis and fractures. Symptoms like tiredness or low mood are common and have many causes, so they are not reliable on their own for diagnosing a vitamin D problem.

If a GP suspects rickets or osteomalacia, the NHS says they will check vitamin D and calcium levels, which may include blood tests and a urine sample, and may refer for X-rays and bone scans. For most people without symptoms or risk factors, government advice is to consider a supplement rather than to test. If you have a condition that affects absorption, kidney or liver disease, or bone symptoms, ask about testing.

Government advice, as set out by the NHS, is that everyone, including pregnant and breastfeeding women, should consider taking a daily supplement containing 10 micrograms of vitamin D during autumn and winter. People at high risk should take it throughout the year. Babies from birth to 1 year should have 8.5 to 10 micrograms daily if breastfed, or if formula-fed and taking less than 500ml of infant formula a day, and children aged 1 to 4 should have 10 micrograms daily all year. One microgram equals 40 IU, so 10 micrograms is 400 IU.

The BDA says 10 micrograms daily meets the needs of most people, that larger amounts are unnecessary for most and can be harmful, that vitamin D3 is the most effective form, and that vitamin D2 is a plant-based alternative suitable for vegetarians and vegans. Avoid doubling up with cod liver oil or multivitamins that also contain vitamin D. Check with a doctor or pharmacist if you have high calcium, take steroids, or take medicines for liver or kidney disease, cancer, epilepsy, fungal infections or heart disease.

The NHS pages do not set out a treatment dose for confirmed deficiency, and they say that if a doctor recommends a different amount you should follow that advice. Treatment doses for diagnosed deficiency are set individually, so take what your prescriber advises and ask how long to continue and when levels will be rechecked.

You cannot overdose on vitamin D from sunlight, but sunburn raises skin cancer risk. The NHS advises covering up or protecting your skin during long periods in the sun, and for rickets prevention suggests 15 minutes of sun without sunscreen 2 or 3 times a week between late March and September, perhaps more often with brown or black skin. The BDA recommends shade between 11am and 3pm, at least SPF 30, and avoiding sunbeds.

Supplements are widely sold in pharmacies and supermarkets, and women and children eligible for Healthy Start can get free vitamins that include vitamin D. Testing and treatment of confirmed deficiency usually go through a GP, and referral for bone disease goes to endocrinology or rheumatology.

Private health checks sometimes include vitamin D testing. A result in isolation is less useful than one interpreted alongside calcium, kidney function and symptoms, so ask who will explain it and whether action is actually needed.