The other day, I hailed a taxi and asked the driver to help me put my bag in his boot. He apologised, and said he was unable to get out without help himself. 'It's all this', he said, indicating his substantial corporation.

This man was not just overweight. He was what we doctors call morbidly obese. The problem, of course, was that my taxi driver took in more energy than he expended. He had gained weight gradually but inexorably, putting on about an ounce a day for the past 10 years. That added up to two pounds a month - almost two stone every year. But given that his energy intake-output imbalance was so small, and that he was (he claimed) motivated to lose weight, why could this apparently intelligent man not control himself?

On track to obesity

An article by Boyd Swinburn and Garry Egger in a recent issue of the British Medical Journal (25 September, see www.bmj.com) offers a new perspective on runaway weight gain. Rather than focus on individual decisions about food and exercise, they look mainly at the environmental context in which those decisions are made. The various personal influences on our body weight - the genes we inherit, our metabolism, our personality (the famous 'willpower' factor), our preference (or not) to exercise, and so on - can be thought of as parts of a train. The wider environment - the food on offer in shops and restaurants, advertising, opportunities to take exercise - is the track on which our personal train runs.

Today's world is what Swinburn and Egger call an 'obesogenic environment'.

Everywhere you look, there are 'labour saving' devices from the TV remote to electronic walkways (on which most people, of course, stand still).

Fast-food chains encourage us all to 'go large' - and use the drive-through.

As the picture shows, all these environmental pressures have the collective effect of putting your personal weight train on a downward (that is, weight-gaining) slope. Of course, there are brakes that can be applied - of which the most powerful is probably our own self-image (generally, we like to be slim and feel uncomfortable when our clothes don't fit), together with the social stigma of being overweight. Unlike many diseases of modern society (such as high blood pressure or high cholesterol), obesity is clearly visible and carries overtones of gluttony and sloth. In our society, it's considered unattractive. This provides an effective brake on the train for most of us.

But in the US, where 20% of the population is now obese, there is a strong support movement for 'big people' and powerful political lobbies campaigning against stigma and discrimination. All well and good - except that the messages coming from such movements weaken the brakes on the runaway weight gain train.

Movement inertia

As the train gains momentum down this runaway slope, several vicious cycles come into operation. One is movement inertia - the fatter you are, the more you want to stay sitting on the sofa. Another is mechanical dysfunction - the heavier you are, the more likely you are going to do yourself damage (knees, hips, back) by walking or jogging, so you stop exercising. And another is psychological dysfunction - the fatter you are, the more miserable you are, so the greater the tendency to comfort eat.

Finally, especially in women from lower socio-economic classes, obesity leads to reduced opportunities for jobs, education, marriage and friendship; and the chronic stress associated with low income may lead to comfort eating and high alcohol consumption. Because of all these vicious cycles, fat people are getting fatter even though most thin people aren't.

What's to be done? Apart from legislating about junk food advertising and encouraging the industry to voluntarily restrict its 'go large' message, we need to think long-term. 'Retro fitting' walking, cycling and public transport into a car-oriented built-environment is an expensive project that will take half a century or more. Are we prepared to invest in such an environment for the sake of our grandchildren?

Thanks to the BMJ Publishing Group for permission to reproduce the picture.