When I came back from India at the age of 19 I was thin – I have photographs to prove it - but ever since then I have been fat. In the Frontiers programme on Wednesday at 9 on Radio 4 we learn that obesity is about to become the biggest killer on the planet.

In the year I was born, 1943, the Metropolitan Life Insurance Company of New York published a set of height-weight tables showing for each height what weight corresponded to lowest mortality – that is the lowest chance of dying.

I am 6 ft 0 in, or 183 cm, and claim to be ‘large frame’. According to those tables my ‘ideal’ weight is 164 - 188 lbs (roughly 12 - 13 stone, or 74 - 85 kg). My actual weight is a closely guarded secret, but I weigh myself every evening on a digital balance, and I can reveal that I am above the upper limit. Actually quite a lot above.

Do I care? Yes, I do. For decades I have been trying to lose weight. I spend as much time as I can on my bike, I occasionally visit the gym, and I have seen diets come and go like the seasons. I particularly remember the grapefruit diet; I guess if you ate nothing but grapefruit you would get sick of the stuff and so would eat less, and lose weight.

In practice the only way to lose weight is to eat less and exercise more, but unfortunately human beings are by nature greedy and lazy. We are programmed by our genes to take in plenty of food in case there is none tomorrow, and shaking off that compulsion is just about impossible. Whatever diet you try, you are setting up a fight against your body, and your body will always win. Dieting can never work.

What is worrying is that children are getting obese, because they too are programmed to be greedy and lazy, and as a result they eat too much and get driven to school. In our rich culture with its bulging supermarkets there is fat chance of persuading anyone to eat less, but we could make people take more exercise, even if they don’t like it. The best hope would be to impose stiff congestion and parking charges in towns, and ban all cars from going within half a mile of schools.