Dieting is hard because weight loss sets off an evolutionary "red flag" in the brain which triggers hunger cravings, a Cambridge University geneticist has warned.
Giles Yeo told the Hay Festival in Wales that losing weight was meant to be difficult because humans were hard-wired to maintain a constant size for the best chance of survival.
Some people find it harder to diet because their genes cause their brain to underestimate body weight from signals released by fat cells, causing the body to make hunger hormones.
"Losing weight ain't easy, and it isn't meant to be," said Mr Yeo. "It doesn't matter how skinny you are, your brain perceives weight-loss as a big red flag, a decrease in your chances of survival.
"The moment you lose weight your brain... drags you kicking and screaming back up to the weight you were."
Mr Yeo said the only way to lose weight was to follow the first law of thermodynamics: energy cannot be created or destroyed. People must eat more than they burn to gain weight, and burn more than they eat to lose weight.
"It's physics," he said. "Anyone who is selling you an effortless way to lose weight is lying."
No foods should be off limits, Mr Yeo said, unless there was a clinical reason not to eat them. Gluten or milk were fine for 95pc of white Europeans. Humans developed a mutation after the birth of agriculture and animal domestication about 7,500 years ago that allowed the digestion of lactose in adulthood.
Instead of cutting out food groups, he suggested eating a little of everything.
He advised dieters to stop counting calories because with many foods all the calories were not absorbed. People absorb only about 70pc of the calories in meat, which was why diets that cut out carbohydrates were effective. And vegan diets work because plants were bulky, he said.
"The caloric availability from vegan food is lower than everything else. In 29 days on a vegan diet I lost 10lb while eating as much as I wanted and my blood cholesterol dropped 12 per cent.
"Except let's look at why. A plant-based diet is very bulky. So what actually happened is I ended up eating less calories. My cholesterol level dropped because I lost weight but largely because I gave up saturated fats."
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