We all know that lifting weights can build up our muscles. But by changing the inner workings of cells, weight training may also shrink fat, according to an enlightening new study of the molecular underpinnings of resistance exercise. The study, which involved mice and people, found that after weight training, muscles create and release little bubbles of genetic material that can flow to fat cells, jump-starting processes there related to fat burning.
The results add to mounting scientific evidence that resistance exercise has unique benefits for fat loss. They also underscore how extensive and interconnected the internal effects of exercise can be.
Many of us pigeonhole resistance training as muscle building, and with good reason. Lifting weights — or working against our body weight as we bob through push-ups, squats or chair dips — will noticeably boost our muscles’ size and strength. But a growing number of studies suggest weight training also reshapes our metabolisms and waistlines. In recent experiments, weight workouts goosed energy expenditure and fat burning for at least 24 hours afterward in young women, overweight men and athletes. Likewise, in a study I covered earlier this month, people who occasionally lifted weights were far less likely to become obese than those who never lifted.
But how weight training revamps body fat remains murky. Part of the effect occurs because muscle is metabolically active and burns calories, so adding muscle mass by lifting should increase energy expenditure and resting metabolic rates. After six months of heavy lifting, for example, muscles will burn more calories just because they are larger. But that doesn’t fully explain the effect, because adding muscle mass requires time and repetition, while some of the metabolic effects of weight training on fat stores seem to occur immediately after exercise.