One way I make it through my work out when I'm huffing and puffing on the treadmill is to tell myself that after it's all over, I can reward myself with a little extra food. That's because exercise revs up metabolism, right? Wrong. For years the common refrain from fitness gurus and diet books has been that if you exercise regularly you'll keep torching those Big Macs all day long--and even at night ("It's a miracle! Lose weight while you sleep!").
The "boost your metabolism" chorus makes many people scratch their heads when they start an exercise program and step on the scale a few weeks later to discover their weight hasn't budged, or maybe they have even gained a pound or two. To help understand the relationship between activity and weight loss, a team of researchers studied the Hazda, a hunter-gatherer tribe in Tanzania, and published the results in an article in the most recent volume of the journal PLoS One.
Walking miles a day to find food, the Hazda's lifestyle is far more active than the average American couch potato's. The researchers outfitted the tribespeople with GPS devices and meticulously measured their energy output and metabolic rate. Surprisingly, what they found was that the Hazda burned only about as many calories a day as a typical Westerner. The team concluded that the primary cause of the obesity epidemic that is plaguing countries like the United States is not our sedentary lifestyles. They also point out that more exercise without a major diet overhaul will not solve the problem.
A second study, also recently published, goes further in detailing the relationship between exercise, diet, and metabolism. "There's this expectation that if you exercise, your metabolism won't drop as you lose weight or will even speed up," Diana Thomas, a professor of mathematics at Montclair State University in New Jersey, tells the New York Times. Instead, she and her team measured that, if you are on a diet, your metabolism drops even if you are exercising every day.
The human body is incredibly efficient at conserving energy. Essentially, the Hazda don't eat much so their metabolism is slow to counterbalance their higher energy expenditure.
Thomas says that current weight loss calculators don't accurately factor in metabolic slow down and some even over predict based on exercise. Dr. Timothy Church of the Pennington Biomedical Research Center in Louisiana who oversaw the research adds, "It's been known for some time that, calorie for calorie, it's easier to lose weight by dieting than by exercise."
The bottom line: while exercise has many important health benefits, it won't significantly help you lose weight unless you cut calories.