The most intriguing theories about why exercise isn’t great for weight loss describe changes in how our bodies regulate energy after exercise.
Researchers have discovered a phenomenon called “metabolic compensation.”
“The more you stress your body, we think there are changes physiologically — compensatory mechanisms that change given the level of exercise you’re pushing yourself at,” said Loyola University exercise physiologist Lara Dugas. In other words, our bodies may actively fight our efforts to lose weight.
This effect has been well documented, though it may not be the same for everyone.
For one fascinating study, published in the journal Obesity Research in 1994, researchers subjected seven pairs of sedentary young identical twins to a 93-day period of intense exercise. For two hours a day, nearly every day, they’d hit a stationary bike.
The twins were also housed as in-patients in a research lab under 24-hour supervision and fed by watchful nutritionists who measured their every calorie to make sure their energy intake remained constant.
Despite going from being mostly sedentary to spending a couple of hours exercising almost every day, the participants only lost about 11 pounds on average, ranging from as little as 2 pounds to just over 17 pounds, almost all due to fat loss. The participants also burned 22 percent fewer calories through exercise than the researchers calculated prior to the study starting.
By way of explanation, the researchers wrote that either subjects’ basal metabolic rates slowed down or they were expending less energy outside of their two-hour daily exercise block.
Dugas called this phenomenon “part of a survival mechanism”: The body could be conserving energy to try to hang on to stored fat for future energy needs. Again, researchers don’t yet know why this happens, and how long the effects persist in people.
“We know with confidence that some metabolic adaptions occur under some circumstances,” said David Allison, “and we know with confidence some behavioral compensations occur under some circumstances. We don’t know how much compensation occurs, under which circumstances, and for whom.”