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. [link]