Eating too much – not exercising too little – may be the biggest driver of weight gain

Exercising makes little difference to whether a child becomes fat, according to researchers. 

Instead, simply eating too many calories is thought to be the driving force behind obesity.

Scientists compared hunter-gatherer children living in the Amazon rainforest with youngsters from the UK and US. 

Youngsters from the Shuar tribe – in an isolated region of Ecuador – didn’t burn any more calories than their Western counterparts, results showed.  

Experts found their bodies simply changed what they used the energy for depending on the circumstances. 

For example, active forest-dwelling children used a lot for exercise and the immune system, while sedentary youngsters may have used it for growth.

The scientists said their work goes against theories of obesity causes but added it was still vital for people to exercise to stay healthy. 

Researchers from Baylor University in Waco, Texas, closely monitored the amount of energy used by 84 children aged between five and 12.

Forty lived in the US and UK and the other 44 lived were from the Shuar tribe, which relies on fishing, foraging and hunting to live.

Western children actually burned more calories – 1,811 compared to 1,738 in a day – despite being 25 per cent less active.

A breakdown of how their bodies used the energy showed the Shuar children used far more energy for their immune system and 20 per cent more while resting.

For those living in a clean, modern environment this was less necessary – the Western children conserved that immune energy, but its other use could not be explained.

About one in five children in the US are obese (18.5 per cent), along with a quarter of those in the UK. 

A Public Health England report last year found children, on average, consume 500 more calories each day than they need.

Children living in more physically demanding environments may have fewer calories to use for physical growth, researchers suggested (Pictured: A Shuar girl goes foraging with her father)

Baylor’s Dr Samuel Urlacher said: ‘Conventional wisdom suggests an increasingly sedentary and germ-free lifestyle, resulting in low daily energy expenditure, is a primary factor underlying rising rates of obesity.

‘We demonstrate that Amazonian children with physically active lifestyles and chronic immunological challenges don’t actually burn more calories than much more sedentary children living here in the US.

‘This similarity in energy expenditure suggests that the human body can flexibly balance energy budgets in different contexts.

‘Ultimately, eating too much, not moving too little, may be at the core of long-term weight gain and the global nutrition transition that often begins during childhood.’

The researchers said leading a less active life could mean children in the US and UK use more of their calories for growing, both taller and larger.

A study published in November revealed the average height of someone living in Ecuador is 5ft4ins (1.64m), while the average in the US and UK is 5ft8ins (1.76/7m). 

Eating significantly more could overload the body – which the scientists said is ‘constrained’ to using a certain number of calories – and lead to weight gain.

Exercise could not burn off calories beyond a certain limit, they suggested, and it simply burned up energy which would otherwise be used for something else.

‘We don’t know for sure [which activities are sacrificed first],’ said study author Professor Herman Pontzer, from Duke University in North Carolina.

The US study found that children living in indigenous communities like this one did not actually burn more calories than children living in modern homes in the West

‘But we know, for example, that chronic inflammation (which is immune activity) is lower among people who exercise regularly.

‘That fits the expectation that the body reduces certain inessential activity in response to regular physical exercise.’

The research was published in the journal Science Advances.