Before and after that process, the researchers took blood, biopsy tissues, centrifuged fluids, and looked microscopically for vesicles and other molecular changes in the tissues.
They noticed enough. Before their impromptu strength training, the rodents’ leg muscles were packed with a particular piece of genetic material known as miR-1, which modulates muscle growth. In normal, untrained muscles, miR-1, one of a group of small strands of genetic material known as microRNA, acts as a brake on muscle building.
However, after the rodents’ resistance exercise, which consisted of walking around, the animals’ leg muscles appeared depleted of miR-1. At the same time, the vesicles in their bloodstream now crowded with the stuff, as did the nearby fatty tissue. It appears, the scientists concluded, that the animals’ muscle cells somehow stuffed those bits of microRNA that slow hypertrophy into vesicles and placed them on neighboring fat cells, allowing the muscle to grow immediately.
But what did the miR-1 do 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 followed the paths of the glowing bubbles. The vesicles settled in fat, the scientists saw, then dissolved and deposited their miR-1 charge there.
Soon after, some genes in the fat cells went into overdrive. These genes help break down fat into fatty acids, which other cells can then use as fuel, reducing fat storage. In fact, strength training was to reduce fat in mice by creating vesicles in muscle that, through genetic signals, told the fat it was time to disassemble itself.
“The process was just remarkable,” said John J. McCarthy, a professor of physiology at the University of Kentucky who, along with graduate student Ivan J. Vechetti Jr. and other colleagues wrote the study.
However, mice are not people. So, as the final facet of the study, the scientists collected blood and tissue from healthy men and women who had performed a single, strenuous exercise at a lower body weight and confirmed that, as in mice, miR-1 levels in the muscles of the volunteers decreased after lifting, while the amount of miR-1-containing vesicles in their bloodstream increased.
Of course, the study mainly involved mice and was not intended to tell us how often or intensely we should lift to maximize vesicle production and fat burning. Still, the results serve as an invigorating reminder that “muscle mass is vital for metabolic health,” said Dr. McCarthy, and that we start to build that mass and get our tissues talking every time we lift a weight.