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