Obesity is triggered by eating too much sugar, says US obesity expert.
In a fascinating new book, Robert Lustig, a professor of clinical paediatrics at the University of California, argues that the urge to overeat and lounge around doing nothing is not a sign of weakness, the Daily Mail reported.
He blamed the hormone leptin, which acts like an appetite thermostat.
As one of two 'hunger hormones' in the body, leptin works to decrease the appetite (its partner, ghrelin, increases appetite).
For many years scientists thought obesity could be caused by a shortage of leptin - thinking that without adequate levels, overweight people simply never received the message that they were full.
But more recent studies have shown that obese people have plenty of leptin (in fact, the fatter you are, the more of it you appear to have), but are more likely to be 'leptin-resistant'.
Scientists have been struggling to work out what causes leptin resistance.
But now Professor Lustig and his team have been able to show - in repeated studies on humans - that too much sugar in the diet is to blame.
High sugar diets lead to spikes in the hormone.
This is needed to clear sugar out of the blood and into storage as fat.
But repeated insulin spikes, due to a high sugar diet, can lead to a condition called 'insulin resistance' (when the cells have been so bombarded by insulin they no longer respond to it).
Professor Lustig believes insulin resistance triggers leptin resistance, and, crucially, he has discovered that by reducing insulin levels it is possible to improve 'leptin signalling' (the brain's ability to read leptin), stop cravings, put the brakes on food consumption - and trigger weight loss.
In his new book Fat Chance, Professor Lustig explains that leptin resistance - and sugar - is at the root of the obesity epidemic.
He believes 1.5billion overweight or obese people in the world suffer from this condition - and is convinced that the problem can be tackled by targeting insulin.
In his studies, many participants took insulin-lowering drugs, but the professor says similar results can be achieved by a few small lifestyle changes - notably reducing sugar in your diet.