How long can we live? Is that ordained from birth, or are we all going to live longer than ever before?

It seems that lifespan is predictable from birth and there's nothing we can do about it - except change our genetic make-up, and that is a long way off.

The crucial point is how often our cells can renew themselves. In each organ our cells are constantly renewing themselves - some faster than others.

Critical to this programme is the telomere - the structure at the end of each chromosome. Each time a cell is renewed, the chromosomes line up and divide to produce two more cells just like their parent. The problem is that, in order to divide, the chromosomes lose a little piece of the telomere. As we age, our telomeres become shorter, until there is no telomere left. Without a telomere, the cells can't be renewed, and they die - and we die, too.

So we could find out precisely how long we are likely to live by measuring our telomeres at birth. The longer they are, the longer we live. Do you want to know about yours? No, I guessed you don't. As far as I know, no one is routinely measuring them - but I'd be surprised if it didn't happen in the future.

Why do people in health shops look so unhealthy?

Why do you think they went there in the first place? Healthy people don't need to go into health shops. If you are already healthy, there's nothing that you can take to make you more healthy.

· Taken from Dr Tom Smith's Doctor, Have You Got A Minute?, published by Shortbooks at £9.99. Do you have a question for Dr Smith? Email doctordoctor@theguardian.com