Naturally skinny people are not more active than us – they just eat 12% less

NATURALLY slim people may not be any more active than the rest of us – they just eat less.

It has long been assumed that those who never seem to put on any weight are athletic types who can eat whatever they like.

But researchers found the opposite after studying 150 naturally skinny people and comparing them to 173 people of a normal weight.

Over two weeks, naturally slim people did 23 per cent less physical activity than those of ordinary weight. Fitness trackers showed they spent more time sitting and lying around.

The study recruited slim people with an average BMI of 17 – classed as just below the underweight threshold – as well as those of normal weight with a BMI below 25 – the threshold for being overweight – and above 21.5.

To work out if naturally slim people really could eat what they liked, researchers calculated their calorie intake directly based on how much energy they burned off.

This energy expenditure suggested those who stay thin actually eat 12 per cent less than people of a normal weight. They were also found to have a faster resting metabolism, which may be a genetic quirk.

This was linked to higher levels of thyroid hormones, which can help to make people feel less hungry and keep them slim. Unfortunately, less than one in 50 people fall into the naturally slim category. Professor John Speakman, who led the study from the University of Aberdeen, said: ‘Often when people talk to super-lean individuals, they say that they can eat whatever they want. But our work showed that what they actually eat is much less than people in the normal BMI range.’

The study, published in the journal Cell Metabolism, looked at Chinese people only and other naturally thin people not involved in the study may be more active.