According to the latest study, our bodies adapt to higher physical activity levels, which is why exercise alone is not sufficient to maintain long-term weight loss.
The results are published in the journal Current Biology.
Current obesity prevention approaches focus on increasing physical activity, with the assumption that increased activity will dovetail into increased energy expenditure and will, ultimately, lead to weight loss.
However, the researchers of this latest study, led by Herman Pontzer of City University of New York, note that while these models are supported by studies that report positive correlations between physical activity and energy expenditure, they are called into question by ecological studies showing that more active populations do not have a higher total energy expenditure.
Although the health benefits of exercise are clear, the researchers say the long-term effects of physical activity on total energy requirements are less so.
And this is observable in the world of fitness, typically labeled as a "plateau," whereby a person who starts an exercise program to lose weight sees immediate weight loss, only to have this taper off - or even reverse - after a few months.
When Pontzer was working among the Hadza, who are traditional hunter-gatherers in northern Tanzania, he noticed this relationship. "The Hadza are incredibly active," he says, "walking long distances each day and doing a lot of hard physical work as part of their everyday life."
"Despite these high activity levels, we found that they had similar daily energy expenditures to people living more sedentary, modernized lifestyles in the US and Europe," he adds, noting that it was a "real surprise."
There may be a 'sweet spot' for physical activity
As a result of his time spent among the Hadza, Pontzer and colleagues conducted a study in which they measured the daily energy expenditure and activity levels over 1 week in more than 300 men and women.
Although they did observe a weak effect of physical activity on daily energy expenditure, further assessment revealed that the pattern only applied to subjects who were in the lower half of the spectrum of physical activity.
Detailed results showed that study subjects with moderate activity levels had daily energy expenditures that were about 200 calories higher than the most sedentary group. However, subjects who had above moderate activity levels experienced no effect on energy expenditure.
Pontzer succinctly summarizes their findings when he notes that the "most physically active people expended the same amount of calories each day as people who were only moderately active."
What this all means is that a focus on a healthy diet, as well as physical activity, is important when it comes to weight loss. The team suggests there may be a "sweet spot" for physical activity: too little results in an unhealthy, sedentary lifestyle, but too much makes the body adjust, rendering the activity counterproductive for weight loss.
"Exercise is really important for your health. That's the first thing I mention to anyone asking about the implications of this work for exercise," says Pontzer, adding:
"There is tons of evidence that exercise is important for keeping our bodies and minds healthy, and this work does nothing to change that message. What our work adds is that we also need to focus on diet, particularly when it comes to managing our weight and preventing or reversing unhealthy weight gain."
For future study, the researchers plan to investigate how the body responds to activity level changes. They plan to look for changes in immune function or the reproductive system to make clear how the body adapts to more physical activity without burning extra calories.
Medical News Today recently reported on a study that suggested increasing intake of apples, pears, berries, onions and other fruits and vegetables rich in flavonoids could prevent weight gain.