The nature of work and the working environment in the last 50 years means that the physical activity of our occupation no longer protects us from an unhealthy diet.

We live in a society where calorie dense, salt enriched food, high in fat and carbohydrate is easily and cheaply available; where subtle advertising and sheer convenience invites us to eat for pleasure, for recreation rather than from hunger and necessity.

Plainly optimal health requires a certain amount of physical exercise but what is the dose of the "drug" exercise and how does it vary with age, sex and race. Is overdose as dangerous as overdose?