We are told that we need sunlight to make vitamin D, and that vitamin D is necessary to prevent osteoporosis and brittle bones. Is it possible to make vitamin D while sitting in a double-glazed conservatory? Or do I have to go out into the open air to get enough sunlight to make vitamin D?

Dr Fred Kavalier answers your health question:

Human skin makes vitamin D when it is exposed to sunlight, which contains ultraviolet B (UVB) radiation. There is less UVB in winter sunlight than in summer sunlight, and the further north you go, the less UVB you get from sunlight. People with dark skin and the elderly make less vitamin D from sunlight than fair skinned and young people. Air pollution and the ozone also cut down on the amount of UVB that gets through the atmosphere. Ordinary window glass cuts out about 95 per cent of UVB, so sitting in your conservatory is a very poor way of making sure you get enough vitamin D from sunlight. But you don't need all that much real sunlight to make vitamin D. It is estimated that about 30 to 60 minutes of sunlight each day on the exposed areas of the face, neck and hands is enough in the summer. In the winter, when sunlight is weaker, it takes longer. It is also possible to get vitamin D in your diet, or by taking supplements. Unfortunately, not many foods contain much vitamin D. Margarine and low-fat spreads are fortified with vitamin D, you can get a big dose from a spoonful of cod liver oil, and kippers and sardines are pretty good too.

Please mail your questions for Dr Fred to health@independent.co.uk. He regrets that he is unable to respond personally to questions.