The December 15, 2021 issue of the New York Times ran an interesting article, “How Exercise Affects Metabolism and Weight Loss,” which reports on newly published research in the journal Obesity. It tracked the human energy metabolism and body composition before, during, and after Biggest Loser contestants participated in the show. Significantly,  the contestants also performed extreme amounts of exercise to aid their loss. Researchers longitudinally studied the results of 16 contestants and 16 individuals who lost drastic amounts of weight after gastric bypass surgery but who did not exercise. The comparison yielded some surprising results.

  • As expected, the gastric bypass group lost muscle as well as fat. The exercise performed by the Biggest Loser contestants spared their muscle mass, and they lost more fat.
  • Resting metabolic rate (RMR) dropped about the same in both groups. This is a surprising result as it was expected that RMR in the exercised group would not drop as much because muscle tissue is far more metabolically active than fat.
  • The RMRs of most dieters rise when they stop actively losing weight and tend to go up if they regain their former fat. This is thought to be associated with the greater bodily size of the individuals. However, in individuals who have lost extreme amounts of body fat quickly, such as the two groups studied here, their RMRs appear to remain permanently depressed, burning roughly 500 fewer daily calories than before their participation on the show.
  • Biggest Loser contestants who were very active or who exercised about 80 minutes a day tended to avoid the weight gain that most individuals experience after their respective losses. Interestingly, this is consistent with The National Weight Control Registry. This organization is a long-term registry and study of people who have lost at least 30 pounds and kept it off for a year. Individuals in this group report an average of one hour of exercise per day.
  • In the Biggest losers group, the individuals who exercised the most experienced the greatest RMR declines. They put on the least post-contest weight and had the lowest RMR years after the contest.

The researchers suspect that a metabolic mechanism might be limiting RMR in highly active people. This phenomenon is currently called the constrained total energy theory. It arose in 2012 when scientists measured the RMR of Hazda foragers in Tanzania. The physical activity levels of Hazda are much higher than individuals in the West when they are active. However, their total energy expenditure is about that of sedentary Westerners.

Comparison Distance Walked by Hazda and American Men and Women
  Hazda American
Men mean 11.4 km, std. dev. ±2.1 km/day 2.0-3.2 miles
(3.2-5.1 km)
Women mean 5.8 km, std. dev. ±1.7 km/day 2.1-2.7 miles
(3.4-3.4 km)
American distances: Pedometer-Measured Physical Activity and Health Behaviors in U.S. Adults, as reported in Medicine & Science in Sports & Medicine of the American College of Sports Medicine.

The researchers speculate that the reason the Hazda are lean while an obesity epidemic is gripping the developed world may be due to appetite suppression mediated by the periods of physical activity in which the Hazda engage. Interestingly, other research may report a mechanism. Exercise for Weight Loss: Further Evaluating Energy Compensation with Exercise, published in Medicine & Science in Sports & Exercise, reports that leptin is suppressed during and after exercise. This is significant because leptin acts in weight control as it reduces appetite.