Scientists have worked out why losing weight is such a battle.

They believe that the fatter someone is, the bigger the fight their body puts up when they diet.

This biological battle of the bulge takes the form of a protein that stops flab from being burnt off.

Oxford University researcher Andrew Whittle said: "Our discovery may help explain why overweight individuals find it incredibly hard to lose weight. Their stored fat is fighting actively against efforts to burn it off."

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Dr Whittle and colleagues identified a protein called sLR11 which prevents excess fat being turned into heat.

Learning more about the process could lead to new obesity drugs. Close to two-thirds of British adults are overweight or obese but there is just one weight-loss pill available on prescription.

Lead researcher Professor Toni Vidal-Puig said: "We have found an important mechanism that could be targeted not just to help increase people's ability to burn fat but also help people with conditions where saving energy is important, such as anorexia."

The British Heart Foundation, which helped fund the study, cautioned any new drug is still some way off. In the meantime, people can take simple steps to improve their weight and heart health, including exercising more and reading food labels.

The research is published in the journal Nature Communications.

- Daily Mail