Lifting Weights? Your Fat Cells Would Like to Have a Word - The New York Times

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.

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.

Once considered microscopic trash bags, stuffed with cellular debris, vesicles now are known to contain active, healthy genetic material and other substances.

Before their improvised weight training, the rodents’ leg muscles had teemed with a particular snippet of genetic material, known as miR-1, that modulates muscle growth.

In normal, untrained muscles, miR-1, one of a group of tiny strands of genetic material known as microRNA, keeps a brake on muscle building.

After the rodents’ resistance exercise, which consisted of walking around, though, the animals’ leg muscles appeared depleted of miR-1.

It seems, the scientists concluded, that the animals’ muscle cells somehow packed those bits of microRNA that retard hypertrophy into vesicles and posted them to neighboring fat cells, which then allowed the muscles immediately to grow.

In effect, weight training was shrinking fat in mice by creating vesicles in muscles that, through genetic signals, told the fat it was time to break itself apart.

So, as a final facet of the study, the scientists gathered blood and tissue from healthy men and women who had performed a single, fatiguing lower-body weight workout and confirmed that, as in mice, miR-1 levels in the volunteers’ muscles dropped after their lifting, while the quantity of miR-1-containing vesicles in their bloodstreams soared.