The days of blaming long hours sitting stagnantly at office desks for rising obesity levels could be over.
US researchers have found that office workers burn roughly the same amount of calories as prehistoric hunter-gatherers would have.
It's long been assumed that our hunter-gatherer forebears would have burnt more calories than we do today thanks to their higher exercise levels.
However when researchers measured the number of daily calories burnt by the Hadza tribe in the open savannah of Tanzania, they found it was very similar to that of average Americans and Europeans.
Herman Pontzer from Hunter College in New York collaborated with colleagues from Stanford and Arizona universities in the study that's the first to measure the energy expenditure of hunter-gatherers.
"The vast majority of what we spend our calories on is things you will never see, like keeping our organs and immune system going. Physical activity is just the tip of the iceberg," Pontzer told the UK's Telegraph.
"If you spend a bit more energy on something like physical activity, you spend a bit less on something else but you do not notice it. This study shows that you can have a very different lifestyle, but energy use all adds up to the same level no matter what."
So this would suggest obesity comes down to our high food consumption, not our lower levels of activity.
"But even if we had a lifestyle like our ancestors did ... we would not burn more calories than we do today," Pontzer said.
"That has not changed a lot, but over the last 50 years we are eating a lot more than we need to be, so that gets to the heart of this issue."
Pontzer was quick to point out that exercise still has important health benefits.
"We are not saying that physical activity is not important for health — clearly it is — but it does not appear to be the main cause of obesity," he said.
But John Hawley, a professor of exercise metabolism and nutrition at RMIT University, told ninemsn more research needs to be done.
"It's a bit like comparing apples and oranges — the Tanzanians eat a very good diet that's totally natural and high in fruit and fibre — it's a massive difference [to Westerners]," he said.
"Studies show that we are eating more than we were 50 years ago, however the increased energy intake is not reflective of either the prevalence or the degree of obesity that's present in all Western societies. It's a little bit more complicated than to say that our energy expenditure is the same, therefore it's all calories."
Professor Hawley said the study should have measured the participants' basal (resting) metabolic rate.
"I would guess it was far higher in the Tanzanians," he said.
He said we can all benefit from plenty of exercise.
"We spend far too long in prolonged sitting and we've removed incidental physical activity so our basic metabolic rate is so low now," he said.
Hawley said exercise helps us build a higher muscle mass so that our metabolic rate increases even when we're not moving.
"We need a high muscle mass because that's the tissue that expends more calories when you're just sitting," he said.
"Too many people are concerned with measuring weight when the whole problem is the ratio of muscle mass to fat mass. The scales don't discriminate between fat and muscle so they are pretty useless at predicting health outcomes."
More than 60 percent of Australian adults are classified as overweight or obese.
The study was published in the PLoS ONE journal.