What is the big secret of thin people? They eat less, study shows

Lean people do not exercise more than the rest of us – they just eat less, a study suggests.

It has long been believed that naturally slim people move more so that they can eat what they feel like.

But researchers have found that the opposite is true – they actually exercise and eat less.

They looked at the diets and energy levels of 150 ‘super lean’ people and compared them to 173 people of normal weight.

During the two-week study, naturally slim people did 23 percent less physical activity and spent more time sitting or lying down. They also ate 12 percent less food.

However, they were found to have a faster resting metabolism, which can help them burn more calories while inactive than the average person.

Lean people do not exercise more than the rest of us – they just eat less, a study shows (file)

Professor John Speakman, who led the study from the University of Aberdeen, said: ‘The results came as a real surprise.

‘Often when people talk to super-slim people, they say they can eat whatever they want.

“But our work showed that what they actually eat is much less than people in the normal body mass index (BMI) range.”

The study, published in the journal Cell Metabolism, recruited naturally slim people with an average BMI of 17.

A BMI below 18.5 is classified as underweight, and the people in the study appeared to be naturally slim, with researchers ensuring that they were not on a diet, had not lost weight due to illness and did not have an eating disorder.


The basic amount of calories that an average adult needs per day is 2,000 kcal for women or 2,500 kcal for men.

This is based on the amount of energy the body needs to perform basic functions and to walk and work during the day.

People who exercise a lot need to eat more calories to boost their efforts, and young people and children also burn more energy.

If you eat more calories than you burn in a day, you will gain weight.

Eating fewer calories than you burn will make you lose weight.

Foods that are processed and have high levels of carbohydrate, sugar and salt have higher calorie counts than fresh fruits and vegetables.

Examples of calorie counts include:

  • A McDonald’s Big Mac contains 508kcal
  • A KitKat with two fingers contains 106 kcal
  • A banana contains 95kcal
  • An apple contains 47kcal

Source: NHS

Surprisingly, it turned out that these naturally thin people spent 96 percent of their time being sedentary or doing only light physical activity.

However, they appeared to eat significantly less than normal-weight people whose BMI was below 25 – the threshold for overweight – and above 21.5.

To find out if naturally trimmed people could really eat what they liked, the researchers did not ask them to keep food diaries that people can remember incorrectly and make mistakes of, but calculated their calorie intake directly based on how much energy they burned.

People’s energy consumption was calculated by giving them a daily drink of water containing heavy isotopes of hydrogen and oxygen.

These are measured in the urine because they are eliminated in the body at a different rate based on how many calories a person burns and converts to carbon dioxide.

Using this energy consumption, measured in terms of physical activity, researchers suggest that naturally slim people in the study should have eaten, on average, 12 percent less than people of normal weight.

But they were also lucky enough to be shown to burn more energy while just sitting still and had a faster metabolism than normal-weight ones.

In fact, their metabolism was 22 percent higher than expected based on their body fat levels.

This was associated with higher levels of thyroid hormones, which can help make people feel less hungry and keep them slim.

Researchers are now looking at whether naturally thin people have a higher metabolism and enhanced thyroid hormones due to their genes, and have already found some genetic peculiarities that may help explain how people avoid gaining weight.

Evidence suggests that about 1.7 percent of people are underweight.

While some want eating disorders or have lost weight due to illness, many are perfectly healthy but simply remain thin.

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

However, the results suggest that naturally slim people can get away with not getting much exercise, based on their low body weight and eating less, as those in the study tended to have lower bad cholesterol than people with a normal weight.

Dr. Sumei Hu, co-author of the study from the University of Aberdeen, said: ‘It was a big shock to me that the super slim individuals were much less active than people in the normal BMI range.

‘I always thought that super slim individuals need to be really active to keep their body weight low, but our results showed the opposite.’

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