A pill for middle-age spread could be on the horizon potentially helping millions of Britons shed the pounds.

Scientists say they have discovered why people pile on the pounds as they get older, despite not eating more or exercising less.

Adipose tissue that stores energy becomes less effective at getting rid of fat – leading to dreaded pot bellies and muffin tops, according to new research.

The finding was based on 54 men and women in Sweden who were followed for up to 16 years.

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At the end they were 20 percent heavier on average – even if their calorie intake had remained the same.

This was because adipocytes were slower at removing fats, or lipids, consumed in food and drink, a process known as “lipid turnover”.

These potentially harmful cells form more than 90% of the white fat that gathers around internal organs, which can trigger disease.

The study published in Nature Medicine offers hope of developing a drug that mimics their role.

Lead author Professor Peter Arner said: “The results indicate for the first time that processes in our fat tissue regulate changes in body weight during ageing in a way that is independent of other factors.”

Two in three adults in the UK are overweight or obese raising the risk of type-two diabetes, heart disease and several cancers.

Mr Arner, of the Karolinska Institute, Stockholm, added: “This could open up new ways to treat obesity.”

The lipid turnover of all the participants reduced over time – regardless of whether they gained or lost weight.

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It can only be offset by consuming less calorie-dense food and drink and being more physically active, said the researchers.

The reason most people’s waistline expands as the years roll by has puzzled experts for decades.

By analysing the fat cells of the volunteers, Mr Arner and colleagues believe they have finally come up with the solution.

They also examined lipid turnover in 41 morbidly obese women who underwent bariatric surgery.

They discovered how it affected their ability to keep the weight off four to seven years later.

Only those who had a low-rate beforehand managed to increase it – and remain slimmer.

They may have had more leeway than those for whom it was already high, explained the researchers.

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Previous studies have shown one way to speed up the lipid turnover in fat tissue is to exercise more.

The latest findings support that idea and also indicate the long-term result of weight-loss surgery would improve if combined with increased physical activity.

Co-author Dr Kirsty Spalding, a cell biologist at the Karolinska Institute, said: “Obesity and obesity-related diseases have become a global problem.

“Understanding lipid dynamics and what regulates the size of the fat mass in humans has never been more relevant.”

The team is now planning to identify the genes involved in regulating the rate at which fat is processed.

Although the amount of weight people gain from the slowdown varies during adulthood, the principle remains the same.

Mr Arner added: “Together these findings identify adipose lipid turnover as an important factor for the long-term development of obesity and weight-loss maintenance in humans.”