Quels sont les bienfaits de la vitamine D ?

What is vitamin D?

Vitamin D is a special kind of vitamin that can be found in small amounts in certain foods. Your body can make its own vitamin D when ultraviolet rays from sunlight hit your skin. For this reason, it's often called the 'sunshine vitamin'.

About 90% of our body's vitamin D is made in our skin from sunshine falling on it. That's all very well if you live in the Mediterranean, but in gloomy old Britain, the sun isn't strong enough between October and April to give us any vitamin D.

Benefits of vitamin D

Vitamin D helps your body absorb calcium - an important mineral that keeps your bones healthy and strong. It helps prevent conditions like rickets in children, and osteomalacia and osteoporosis in adults - which can cause weak bones. Vitamin D also plays a role in reducing inflammation, supporting cell growth, muscle function and immune health.

Is vitamin D deficiency common?

For the first 20 years I was a doctor, I hardly ever checked anyone's vitamin D levels. We knew people with very low levels had a higher chance of bone thinning (ostéoporose). So we did used to prescribe calcium and vitamin D supplements to reduce their chance of bone fractures.

Occasionally we'd check someone's vitamin D blood levels and discover they were very low - so we'd give them a single injection of very high-dose vitamin D.

But that was before our attitudes towards vitamin D changed. Once we started measuring it, I discovered at least half my patients were short of vitamin D - and 1 in 6 patients were severely deficient. So we started recommending vitamin D supplements routinely for anyone we thought might develop osteoporosis as well as other conditions.

Do vitamin D supplements work?

In the past, scientists have become very excited about other antioxidant vitamins - particularly vitamins A, C and E and their role in preventing heart disease. The thinking went something like this - people who eat lots of fruit and veg are less likely to have heart attacks. Fruit and veg are high in antioxidant vitamins.

Antioxidants are a good thing because they mop up unstable molecules (free radicals), which can damage DNA. So if people don't get their required amount of vitamins through fruit and veg, they thought giving them in vitamin supplement form would be just as good.

However, some studies actually show that taking these supplements doesn't cut your chances of getting heart disease at all¹.

One particular study shows a similar result for vitamin D - at least as far as fractures, falls and bone density are concerned². There is no evidence that taking vitamin D supplements reduces the chance of fractures or falls, or improves bone density. So doctors need to question whether they should be prescribing them to people with higher chances of developing osteoporosis.

Which foods contain calcium?

For strong bones, calcium is also important. Dairy products, seeds, pulses, tofu, tinned fish with bones, and some green leafy vegetables are all good sources. Interestingly, calcium supplements don't appear to help much, and may actually increase your chance of crise cardiaque. Doctors are much more wary about prescribing these too.

But there is one effective way to keep your bones strong - and it's never too early to start. Weight loss and weight-bearing exercise are key to preventing thinning bones. Anything except swimming counts - so get out your dancing shoes!

When to take vitamin D

It's important to put the above study into context. It didn't look at babies and children under 5, so does nothing to undermine current Public Health England (PHE) guidance that they - along with pregnant women - should take a daily vitamin D supplement. People who may experience vitamin D deficiency because they cover up for religious reasons - or those who don't get outside at all - should also continue to take supplementation.

In addition, this study was only looking at the impact of vitamin D on bone health. We've since discovered that vitamin D regulates at least 1,000 genes in the body - affecting blood vessels, immune system, muscle strength and more. Fatigue and muscle pains are the most common symptoms, but low levels have been linked to an increased risk of dépression, une maladie cardiaque et même sclérose en plaques.

As yet, there haven't been any large scale studies looking at whether taking a vitamin D supplement - if you're vulnerable to deficiency - protects against any of these. But vitamin D at the recommended levels doesn't appear to have any harmful side effects. So if vitamin D supplements do offer any protection at all, the pros of taking them will likely outweigh the cons.

How much vitamin D should you take daily?

If you do decide to take a vitamin D supplement, it can get a bit confusing. Vitamin D doses can either be measured in international units (IU) or in micrograms (mcg) and levels are very different. It's easiest to stick to one - I'd recommend you look out for the number of micrograms. For reference, 10 mcg is equivalent to 400 IU.

Public Health England recommends that everyone over one year should take a supplement of 10 micrograms of vitamin D from around the end of September to early April. If you don't get outside much - or if you cover up for religious reasons, or have darker skin - you should take it all year round.

Dr Louis Levy, Head of Nutrition Science at PHE, says: "A healthy, balanced diet and short bursts of sunshine will mean most people get all the vitamin D they need in spring and summer. However, everyone will need to consider taking a supplement in the autumn and winter if they don't eat enough foods that naturally contain vitamin D or are fortified with it. And those who don't get out in the sun - or always cover their skin when they do - should take a vitamin D supplement throughout the year."

Can you take too much vitamin D?

Many people don't realise that you can have too much vitamin D (called vitamin D toxicity), which can lead to a range of health problems. Which is why current guidelines say adults shouldn't take more than the equivalent of 100 micrograms a day.

Vitamin D is a fat-soluble vitamin, so your body can store it for months and you don't need it every day. That means you could safely take a vitamin D supplement of 20 micrograms a day or 500 micrograms once a month. You can divide the monthly dose by 30 to give you the daily equivalent, which is what counts.

Speaking of too much of a good thing - excessive sun exposure can seriously harm your health. Any redness or even tanning of your skin is a sign that your skin has been damaged. In spring and summer, you can get your vitamin D from 15 to 20 minutes a day with arms and face uncovered in full sun. But don't get burnt - this raises your chance of developing all kinds of cancer de la peau - but particularly of skin cancers such as melanoma.

Pour en savoir plus

1. Kris-Etheron et al: Antioxidant Vitamin Supplements and Cardiovascular Disease

2. Bolland et al: Effects of vitamin D supplementation on musculoskeletal health: a systematic review, meta-analysis, and trial sequential analysis