The research by Dr Zane Andrews, a neuroendocrinologist with Monash University's Department of Physiology, has been published in Nature.

Dr Andrews found that appetite-suppressing cells are attacked by free radicals after eating and said the degeneration is more significant following meals rich in carbohydrates and sugars.

"The more carbs and sugars you eat, the more your appetite-control cells are damaged, and potentially you consume more," Dr Andrews said.

Dr Andrews said the attack on appetite suppressing cells creates a cellular imbalance between our need to eat and the message to the brain to stop eating.

"People in the age group of 25 to 50 are most at risk. The neurons that tell people in the crucial age range not to over-eat are being killed-off.

"When the stomach is empty, it triggers the ghrelin hormone that notifies the brain that we are hungry. When we are full, a set of neurons known as POMC's kick in.

"However, free radicals created naturally in the body attack the POMC neurons. This process causes the neurons to degenerate overtime, affecting our judgement as to when our hunger is satisfied," Dr Andrews said.

The free radicals also try to attack the hunger neurons, but these are protected by the uncoupling protein 2 (UCP2).

Dr Andrews said the reduction in the appetite-suppressing cells could be one explanation for the complex condition of adult-onset obesity.

"A diet rich in carbohydrate and sugar that has become more and more prevalent in modern societies over the last 20-30 years has placed so much strain on our bodies that it's leading to premature cell deterioration," Dr Andrews said.

Dr Andrews' next research project will focus on finding if a diet rich in carbohydrates and sugars has other impacts on the brain, such as the increased incidences of neurological conditions like Parkinson's disease.

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Lindley Owen, Consultant in Public Health at Cornwall and Isles of Scilly Primary Care Trust, said: "Staying a healthy weight can be a fun and relatively easy thing to do, even as people get older. People don't have to join a gym or take on complicated new diets. There are many everyday opportunities to stay active through regular walking, gardening or social groups, while eating fresh, nutritious food is enjoyable at any age.

"Our experience of running supported walking and cycling groups has shown that older people benefit in many ways from regular physical activity. Not only do they get fitter and physically stronger but the enjoyment of spending time with friends in the open air can give new confidence and a real zest for life.

"People are living longer but this study shows that excess weight can have a real impact on the quality of people's lives which can reduce the benefit of those extra years. We must do all we can to encourage older friends or relatives to build enjoyable exercise into their daily routine and develop good habits ourselves to take into our retirement years. People can visit www.strollbacktheyears or www.healthpromcornwall (01209 313419) for more advice."

Dr Gill Lewendon, Consultant in Public Health Medicine, Plymouth Teaching PCT, added: "This report highlights the problem of obesity in an increasingly ageing population. The PCT and City Council work closely with a wide range of voluntary and statutory agencies to provide increased opportunities for everyone to eat more healthily and to move around a bit more. For those who are already very overweight or obese, the PCT provides a comprehensive weight management service for people of all ages. "

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