However, we remain unclear about how weight training modifies body fat. Part of the effect is because muscle is metabolically active and burns calories, so adding muscle mass through weight lifting should increase energy expenditure and metabolic rates at rest. After six months of lifting weights, for example, your muscles will burn more calories just because they are bigger. But that doesn’t fully explain the effect, because adding muscle mass takes time and repetition, while some of the metabolic effects of weight training on fat stores appear to occur right after exercise.
So something is likely to happen at the molecular level immediately after fat cell-oriented resistance training, a hypothesis that a group of scientists from the University of Kentucky at Lexington, the University of Nebraska-Lincoln, and other institutions decided to investigate in recent dates. Researchers had been studying muscle health for years, but they became increasingly interested in other tissues, especially fat. Perhaps, they speculated, muscle and fat had a friendly exchange after a workout.
In the last decade, the idea that cells and tissues communicate throughout our bodies has been widely accepted, although the complexity of these interactions remains puzzling. Sophisticated experiments show that muscles, for example, release a cascade of hormones and other proteins after exercise that enter the bloodstream, target various organs, and trigger biochemical reactions there, in a process known as cellular crosstalk.
Our tissues can also pump out tiny bubbles, known as vesicles, during crosstalk. Vesicles, once thought of as microscopic garbage bags filled with cellular debris, are now known to contain healthy, active genetic material as well as other substances. When these substances are released into the bloodstream, they transmit this biological material from one tissue to another, as tiny messages.
Curiously, some experiments indicate that aerobic exercise makes muscles release those vesicles, which carry a series of messages, but few studies have looked at whether endurance exercise could also lead to vesicle formation and message exchange between tissues.