Scientists in the United States have finally worked out why some people can pig out on sticky puddings and stay thin while others have to loosen their belts on a diet of lettuce and water crackers.
The answer: non-exercise thermogenic activity, or Neat for short. It is the stay-thin card dealt by evolution to people who breezily burn off calories just by sitting up - and fidgeting.
In a study praised as a "tour de force", James Levine, Michael Jensen and Norman Eberhardt of the Mayo Clinic in Rochester, Minnesota, fed 16 people with what they needed plus an extra thousand kilocalories every day (the equivalent of two Big Mac burgers). They also discouraged them from taking any vigorous exercise, and what little they took was monitored.
They kept this up for eight weeks, they report today in the US journal Science. The volunteers - 12 men, four women - were all healthy, aged from 25-36, and not obese to start with.
By the end of the experiment, each had put on an average of 10lb. There were, however, dramatic differences within the group. One person gained 16lb. Another put on only 2lb.
Those that showed the greatest increase in non-exercise thermogenic activity or Neat - the ones who burned calories just by sitting still, changing posture, or fidgeting a little - also disposed of the most calories: one radiated away 692 kilocalories a day. Others deposited most of their calories around the waistline.
The researchers allowed no cheating. They weighed their victims every day, and questioned them about how they spent their time. They even called up friends and families and colleagues to make sure that no one was leaving food on the side of the plate, or sneaking off to the gymnasium. They made random inspections of waste in the volunteers' dustbins.
They measured the energy spent by their volunteers with calorimeters, radioactive water, treadmills and a count of oxygen consumption. They examined samples of excreta to make sure that food was being digested properly.
One contributor to Science calls their study "a tour de force in the use of state-of-the-art modern techniques for assessing energy metabolism and body composition in humans".
It all went to confirm what most humans have observed anyway: some people can eat what they like and stay thin.
"When people overeat, Neat switches on in some people to 'waste' this excess energy. Conversely the failure to switch this on allows the calories to be stored as fat," said Dr Jensen. In theory, the knowledge could be useful in helping reduce obesity. But Neat is a form of exercise that people take without thinking about it, or even doing anything. So nobody knows how to make them take more of it.
Dr Levine said: "We know it's there, but we don't know what it is."
