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of adults have low levels of vitamin D between January and March every year and are consequently risking deficiency, according to the . For 11–18-year-olds, this jumps to 37 percent. Given the nutrient’s vital role in keeping muscles, bones and teeth healthy, what would deficiency mean for you and can you prevent it by taking supplements?

Known as the ‘sunshine vitamin’ because the body creates it from direct sunlight, vitamin D is only found in a small number of foods. It’s pretty much impossible to get the recommended 10 micrograms a day from the sun between October and March, so the recommends everyone over the age of five considers taking a daily supplement of 10 micrograms of vitamin D during these months. They add that children under five and people at risk of deficiency should take a daily supplement year-round.

Why do we need vitamin D?

Vitamin D helps control the amount of calcium and phosphate in the body. suggests a link between very low levels of vitamin D and rickets in children and the bone condition osteomalacia in adults. More recently scientists have started investigating its role in other areas of health. Research is limited, but studies suggest a link between adequate vitamin D levels and prevention of inflammatory diseases such as , a reduction in and a lower risk of some . And the British Nutrition Foundation is for young, healthy people in the UK to be aware of the risks associated with vitamin D deficiency.

Signs of deficiency

Signs of vitamin D deficiency are most commonly bone aches and pains and muscle weakness. Kay Niknam, from Bristol, explains what happened when she started to experience joint pain.

“I had been struggling increasingly with joint pain for several weeks, possibly months, when I went to see the GP. My daughter was a few months old and I was no longer able to carry her in a sling. Some days I struggled to walk because of the pain and was forced to cancel plans.

“The GP said I could self-refer for physiotherapy, but also booked me in for a blood test to check my levels. When I got the results, it turned out my vitamin D was low, and the GP said that could be the cause of my joint pain. I decided to take a vitamin D tablet daily when my daughter was born, as I was breastfeeding, but sleep deprivation made remembering things like taking vitamins much harder!”

Surprisingly, Kay was diagnosed with low vitamin D levels during the summer. “Because the summer was really hot and I had a young baby, I’d been avoiding going out in the sun”, she says.

The GP prescribed high-strength vitamin D capsules. “I noticed improvements in the first week, and after a few weeks was able to do all my normal activities again.”

To supplement or not to supplement?

The sun in the UK isn’t considered strong enough for our skin to produce adequate vitamin D from October to March, and it is very difficult to get 10 micrograms a day from diet alone. Oily fish is the main food source of vitamin D, while egg yolk, meat and offal contain small amounts, depending on the time of year. In the UK, only a handful of foods are fortified, including infant formula and some breakfast cereals, plant-based milks and fruit juices. For those following a plant-based diet, fortified drinks are a key dietary source: a 250ml serving of a fortified plant drink typically provides around 1.8 micrograms of vitamin D.

The NHS recommendation to consider taking a supplement is based on Public Health England’s advice, which followed a 2016 comprehensive into vitamin D and health by the Scientific Advisory Committee on Nutrition. The recommended daily intake of 10 micrograms is based on estimates of the amount required to keep blood levels of serum 25-hydroxyvitamin D above 25nmol/L. But some experts have reservations about the benefits of vitamin D supplements. A 2018 of existing research, published in The Lancet, states that “There is little justification to use vitamin D supplements to maintain or improve musculoskeletal health.”

Vitamin D is fat-soluble, which means it can build up in the body, and this can lead to overdose. Writing in in 2018, Professor of Genetic Epidemiology, King’s College London, Tim Spector, explains: “Patients with very high vitamin D blood levels (over 100nmol) are becoming routine in my clinic and elsewhere, and toxic overdoses are increasingly being reported. Several randomised trials have shown that patients with high blood levels or taking large doses of vitamin D (above 800IU/20 micrograms) had an unexpected increased risk of falls and fractures.”

In 2018 a published in The British Journal of Clinical Pharmacology concluded that cases of vitamin D toxicity are likely to rise due to an increased interest in the vitamin and widespread prescribing of supplements. It recommends legislation to ensure accurate dosage of all vitamin D products and limited or restricted use of very high dose formulations.

If you decide to supplement, be sure to check the dosage, as many vitamin D supplements contain more than the recommended daily intake of 10 micrograms. The NHS advises that taking more than 100 micrograms of vitamin D a day could be harmful. It’s difficult to know how much vitamin D you are getting from other sources in addition to a supplement.

Who is most at risk of vitamin D deficiency?

Some people are more at risk of vitamin D deficiency than others. Risk factors include:

• Breastfeeding
• Dark skin pigmentation
• Obesity
• Old age
• Multiple short interval pregnancies
• Overusing sunscreen
• Wearing clothes that cover every part of your body
• Malabsorption
• Lack of sun exposure.

The low-down on vitamin D supplements

Bahee Van de Bor, dietitian and (BDA) media spokesperson, has some tips on which supplements are best.

Is D2 or D3 a better supplement?

“ suggests that vitamin D3 is better absorbed, although both D2 and D3 are absorbed well by the body. Vitamin D3 (cholecalciferol) is obtained from animal sources such as oily fish. Our skin is also able to make D3.

“D2 (ergocalciferol) is the plant version of the vitamin. If you are following a plant-based diet, D2 is still a reliable form of vitamin D supplement.”

Is a spray supplement better than a tablet?

“If you struggle to take pills, the spray may be ideal. It’s down to personal preference. In terms of absorption, research suggests that vitamin D sprays are just as effective as tablets.”

Should we take supplements at a certain time of day, or with a certain food?

“Because vitamin D is fat-soluble, it is absorbed best when eaten with a meal consisting of fat. For this reason, it’s best to take it with meals.

“You are also more likely to remember to take your vitamins in the morning as opposed to at the end of the day, so it’s preferable to take it with breakfast, or lunch if that’s the first meal of your day. Having said that, if your mornings are rushed, the most important thing to remember is to take your supplement. For this reason, take it at a time that works best with your schedule.”

Can you get enough vitamin D by eating foods containing or fortified with it?

“As only around 28 percent of the population eat the recommended two portions of fish a week, it’s best to take a vitamin D supplement to meet requirements. Unless you live in a country with year-round sunshine, take a daily vitamin D supplement providing 10 micrograms during autumn and winter. If you have dark skin or tend to cover your face and hands throughout the day and/or are housebound, it’s best to take a supplement all year round.”

Keep an eye on your intake

The bottom line is that it’s difficult to know exactly how much vitamin D you are getting. Research suggests many of us may not be getting the recommended 10 micrograms during autumn and winter, and that taking a supplement of 10 micrograms could help prevent problems with our bones, teeth and muscles. But keep an eye on how much you take, because too much vitamin D could lead to toxicity.

All content within this article is provided for general information only, and should not be treated as a substitute for the medical advice of your doctor or any other health care professional. Always consult your GP if you’re concerned about your health.