By Jonathan Amos BBC News Online science staff in Leicester

Professor Andrew Prentice told the British Association's science festival in Leicester that people were now undergoing changes similar to those that occurred two centuries ago when Europeans shot up in height by 30 centimetres or more.

But whereas that shift in shape is generally regarded to have been beneficial, the bulging waistlines now seen in many countries around the world would bring nothing but disease and misery, he said.

"I'm talking about the remarkable change that has occurred in man's evolution in just the twinkling of an eyelid," he told the BBC.

Not only has man adopted a more sedentary lifestyle, he has been given unprecedented ready access to high-energy foods.

"It's now quite a normal biological response for people to become obese and it means a massive increase in obesity in a way we had a big change in height 200 years ago."

TV and computers

Many nations now record more than 20% of their population as clinically obese and well over half the population as overweight. The situation is now so dire that in some cases, as an American obesity expert recently predicted, parents are likely to outlive their grossly fat children.

"What usually happens with evolution is that it's an imperceptibly slow process, an organism can change to meet small changes in its niche.

"What's happening now is that we've changed the environment that we live in in an incredibly short time - one generation or perhaps two generations at most, and this has challenged our ancient metabolism, which for thousands of generations has been geared to fighting famine."

Professor Prentice, an expert on international nutrition from the London School of Hygiene and Tropical Medicine, said the obesity pandemic was compounded by sedentary lifestyles induced by TV, computing and mechanisation.

Expensive option

He said the health outcomes for the current obese generation were bleak; they will suffer a wide range of chronic diseases ranging from mild ailments such as breathlessness and varicose veins at one extreme to serious conditions such as diabetes and cancer at the other.

He said the emergence of type 2 diabetes (formally known as "adult-onset" diabetes) in paediatric clinics was a particular worry.

Professor Prentice said concerted effort was needed to alter the "obesogenic" aspects of the modern human environment. He rejected the idea that there was a technical fix in the form of drugs which could stop people from getting fat.

"The obesity pandemic is gathering pace rather than slowing down, and current interventions are only marginally effective and very expensive.

Reliance on medical interventions to combat obesity would require highly costly lifelong treatments administered to over half of the adult population. There is a strong imperative to find other preventative measures."

Average weights have increased significantly over the last 20 years.

In 1980, the average man weighed 73.7 kg and a woman 62.2. By 2000, that had increased to 81.6 and 68.8 kg respectively.

"We are at a fascinating time in human evolution. The increase in height 200 years ago stayed with us and now we're increasing in girth and that is going to stay with us as well," Professor Prentice said.