A primitive mechanism that keeps us alive may be the reason.
No matter how much you exercise, you’re going to lose only so much weight.
One study suggests that you may benefit as much from a two-mile walk per day as you will from running five miles. But, exercise is still an important part of keeping yourself healthy.
"Exercise is really important for your health," says the study’s author, Herman Pontzer, PhD, of City University of New York.
The first thing Pontzer says to anyone who asks about the implications of the study is “tons” of evidence shows exercise is important for keeping your body and mind healthy. His work does nothing to change that message.
He adds that the study affirms the common sense that you also need to focus on your diet to purposefully lose weight or prevent or reverse unhealthy weight gain.
The reason more intense exercise won’t increase weight loss is your body adapts to higher activity levels, so that you don't necessarily burn extra calories even if you exercise more.
The study suggests that humans have a primitive mechanism for survival that controls how many calories our bodies let us burn. Not that it’s as necessary today, but that allowance is still there and, if your body spends it, your calorie bank stops further withdrawals.
How the mechanism works is the next question that needs to be answered. "We think this is a really common evolutionary adaptation that all animals use to keep from outstripping their resources and to keep from starving. Your body is listening to your environment and setting an energy expenditure level it can maintain," Pontzer says.
The team of researchers say it's time to stop assuming that more physical activity always means more calories burned. There might be a "sweet spot" for physical activity — too little and we're unhealthy, but too much and the body makes big adjustments in order to adapt.
That phenomenon was made clear during Pontzer’s work with the Hadza, a population of traditional hunter-gatherers in northern Tanzania. The study was then carried out among 300 Westerners of various types.
"The Hadza are incredibly active, walking long distances each day and doing a lot of hard physical work as part of their everyday life," Pontzer says. "Despite these high activity levels, we found that they had similar daily energy expenditures to people living more sedentary, modernized lifestyles in the United States and Europe. That was a real surprise, and it got me thinking about the link between activity and energy expenditure."
What this means is that losing weight is much more than just burning calories.
"This study actually explains a phenomenon that I see quite commonly," Holly F. Lofton, MD, director of the Medical Weight Management Program at NYU Langone Medical Center, said to CNN. "I see patients training for a marathon and they ask me, 'Why am I not losing weight?'" even though they are exercising more and eating the same number of calories,” Lofton said.
That principle seems to work on the flipside of the equation as well. Experts presume that if patients take in fewer than 800 calories per day the body’s metabolism shuts down to make weight loss shut down.
If you have a higher level of activity, you may be able to “trick” the body into burning more calories by switching to biking or swimming if you run all the time or visa versa. The principle is that using different muscles during physical activity may increase energy expenditure again, Lofton said.
"It may also be possible to decrease and then increase your activity again and get an increase [in calorie burning]," Lofton added.
Pontzer and his colleagues now plan to study how the body responds to these changes in activity level. They'll start by looking for other changes — for example, in immune function or the reproductive system — that might explain how the body adapts to greater physical demands without consuming extra calories.
April 10, 2020
Janet O’Dell, RN