Vitamin D: 9 questions to Prof. Heike Bischoff-Ferrari, DrPH
How does the body produce vitamin D?
Vitamin D is produced when the skin is exposed to the sun. The rule of thumb is that a young adult will produce enough vitamin D of their own if they fully expose their forearms and face to the sun for 20 minutes a day. But this only applies from May to October. In the winter and early spring the sun isn’t intense enough for the body to produce enough vitamin D by itself. So you can do a lot to actively prevent vitamin D deficiency by being outdoors in the summer and autumn. But the reality is that around 50% of people don’t get enough vitamin D, especially during the winter. And as many as 40% of people don’t get enough even during the summer months.
Is food also a source of vitamin D?
Yes. In theory you can also get vitamin D from food. But in practice, the best source is fatty fish such as smoked salmon, and you’d have to eat 200 grams a day to get enough! Or you’d have to consume 20 eggs a day to get 600 to 800 IU of vitamin D, which is not a diet we’d recommend.
Why is vitamin D especially important for older people and children?
Aged skin produces vitamin D more slowly, or in smaller quantities. This means senior citizens are particularly likely to be deficient. This even applies to older people in Mediterranean countries, who tend to spend the whole day indoors because of the heat and get too little sunlight as a result.
And because sunblock reduces vitamin D, children who always go out wearing cream and clothing to protect them from the sun are also more likely to be deficient. This is why the Federal Department of Health has recently increased its recommended daily intake of vitamin D from 200 to 600 IU (international units). Vitamin D supplements are a good option if you want to be sure of preventing a vitamin D deficiency, especially since unprotected exposure or overexposure to the sun is problematic from the point of view of preventing skin cancer.
Does going to the solarium also help prevent vitamin D deficiency?
We don’t recommend going to the solarium to top up on vitamin D because there you’re usually exposed to a mixture of UVB (which produces vitamin D) and UVA (which doesn’t, and is only harmful). Not only this, but going to the solarium can increase the risk of skin aging and tumours.
What are the recommended doses for vitamin D?
In the last four years authorities in the United States, large parts of Europe and Switzerland (the Federal Office of Public Health, FOPH/BAG) have revised their recommended daily intakes for vitamin D. The new recommendations are as follows:
Can too much vitamin D be harmful?
Sure. But you’d have to consume more than ten times the recommended amount. The current recommended daily intake of 600-800 IU per day has a large safety margin built in, but still ensures that more than 97% of the population are not deficient in vitamin D. These safety margins are also reflected in our own physiology: if a young man or women lies in the blazing sun in their swimsuit (without sun protection) for ten minutes, their skin produces 10,000 to 14,000 units of vitamin D.
Can the body store vitamin D?
Yes, but only for three to six weeks, which is the half-life of vitamin D. This means you don’t have to worry if you don’t get enough sunlight for a day at a time or even for a whole week. Your body will soon make up the difference. On the other hand it does mean that vitamin D levels, which are usually at their highest in September (after the summer), will fall off sharply again by November if you don’t take a supplement. So a nice summer won’t keep you going through the winter – and this applies to everyone, whether they’re young or old.
Does a vitamin D supplement have to be prescribed by a doctor?
Not necessarily. You can also buy vitamin D over the counter from a chemist. Even so, we recommend talking to your family doctor, who’ll be able to tell you what the risks and implications of having too little vitamin D are, and do a blood test if they suspect a serious deficiency. The new vitamin D recommendations only advise a blood test if a serious deficiency is suspected.
Are there certain factors that can point to a vitamin D deficiency?
Vitamin D deficiency is most likely to affect people who:
Each of these factors is associated with an elevated risk of vitamin D deficiency.
Heike Bischoff-Ferrari is Chair of the Department of Geriatrics and Aging Research at the University of Zurich; Director of the Centre on Aging and Mobility at the University of Zurich and City Hospital Waid; and Principal Investigator, Sponsor, and Coordinator of DO-HEALTH.