Before and after that process, the researchers drew blood, biopsied tissues, centrifuged fluids and microscopically searched for vesicles and other molecular changes in the tissues.
They noted plenty. 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. At the same time, the vesicles in their bloodstream now thronged with the stuff, as did nearby fat tissue. 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.
But what was the miR-1 doing to the fat once it arrived, the scientist wondered? To find out, they marked vesicles from weight-trained mice with a fluorescent dye, injected them into untrained animals, and tracked the glowing bubbles’ paths. The vesicles homed in on fat, the scientists saw, then dissolved and deposited their miR-1 cargo there.
Soon after, some of the genes in the fat cells went into overdrive. These genes help direct the breakdown of fat into fatty acids, which other cells then can use as fuel, reducing fat stores. 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.
“The process was just remarkable,” said John J. McCarthy, a professor of physiology at the University of Kentucky, who was an author of the study with his graduate student Ivan J. Vechetti Jr. and other colleagues.
Mice are not people, though. 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.
Of course, the study mostly involved mice and was not designed to tell us how often or intensely we should lift to maximize vesicle output and fat burn. But, even so, the results serve as a bracing reminder that “muscle mass is vitally important for metabolic health,” Dr. McCarthy said, and that we start building that mass and getting our tissues talking every time we hoist a weight.