Obesity may be primarily caused by eating too much alone, and not due to poor physical activity, according to a study which challenges the conventional notion that an increasingly sedentary lifestyle may be contributing to the condition.

The researchers, including those from Baylor University in the US, collected the daily energy expenditure data from 44 children of ages 5 to 12 belonging to the forager-horticulturalists Shuar community in the Amazon rainforest.

To measure the children's energy expenditure, they tracked their consumption of energy using food with isotope markers, and with respirometry methods -- the first time these state-of-the-art approaches had been used among children in an indigenous, foraging community.

The team compared the data to those of industrialised children in the US and the UK.

According to the study, published in the journal Science Advances, the Amazonian children with physically active lifestyles, and chronic immunological challenges don't actually burn more calories than their much more sedentary counterparts in the US.

"Conventional wisdom suggests that an increasingly sedentary and germ-free lifestyle, resulting in low daily energy expenditure, is a primary factor underlying rising rates of obesity in the U.S. and elsewhere," said study co-author Samuel Urlacher, an assistant professor of anthropology at Baylor University.

The current study revealed that Amazonian children don't burn more calories than much more sedentary children living here in the US.

Shuar children were approximately 25 per cent more physically active than their industrialised counterparts, the findings revealed.

According to the researchers, the children also had a 20 per cent greater resting energy expenditure than those in the cities, which reflected elevated immune system activity in the Amazonian kids.

They said despite wide differences in lifestyle and energy allocation, the total number of calories that the Shuar children spent every day was indistinguishable from that of industrialised children.

"This similarity in energy expenditure suggests that the human body can flexibly balance energy budgets in different contexts," Urlacher said.

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

"These findings advance previous work among adults, showing that energy expenditure is also constrained during childhood," said co-author Herman Pontzer from Duke University in the US.

Since these energy expenditure tradeoffs may often limit physical growth, the study noted that such constraints can help scientists understand childhood growth faltering.

The findings may also help in understanding how these energy constraints are associated with increased risk for adult obesity, and metabolic diseases such as Type 2 diabetes and hypertension.

A rapid change in diet and increasing energy intake may most directly underlie the chronic weight gain driving the global rise of obesity, but not mainly contributed by decreasing physical activity, the researchers said.

However, they cautioned that exercise was critically important for health and for weight management since it affected appetite, muscle mass, and heart function.

"Our results don't suggest otherwise. Everyone should meet recommended daily physical activity levels," Urlacher said.