Caloric restriction is the most efficient way to lose body fat. However, typically reported with weight loss is the undesirable loss of muscle primarily due to the increase in catabolic degradation of muscle protein during muscle protein turnover. Protein turnover within the muscle cell is an intricate process that involves protein synthesis as well as the breakdown of existing muscle protein. Simply put, increasing muscle mass requires a greater amount of muscle protein synthesis than muscle protein breakdown. Consuming the correct amount and type of protein, especially while cutting calories, preserves muscle protein synthesis while reducing muscle protein degradation— ultimately promoting greater levels of fat-free muscle mass. While much of the scientific literature states that high-quality protein intake increases muscle protein levels by activating the enzyme mTOR, which directly activates the buildup of muscle protein, more recent studies illustrate mTOR’s potent ability to prevent muscle protein degradation particularly while dieting— thus increasing or maintaining muscle while cutting body fat.
Protein Consumption Prevents Muscle Cell Breakdown
Autophagy is a process by which components of the cell are degraded and utilized as energy sources to maintain essential cellular function in response to nutrient limitation during caloric restriction. Extensive studies have shown that autophagy is promoted by AMPK, which is a key energy sensor that maintains energy homeostasis. Conversely, autophagy is inhibited by mTOR, a central cell-growth regulator that assimilates nutrient signals. A study by Kim et al. demonstrated that low caloric consumption turns on AMPK, directly promoting autophagic degradation of muscle cells in order to reestablish energy levels while sufficient nutrient intake triggers mTOR activity that thwarts autophagic muscle loss. Moreover, an additional study by Fontana et al. confirmed that low protein intake during caloric restriction reduces mTOR signaling, most likely promoting autophagy and muscle cell breakdown. Two other studies demonstrated that high protein intake increases mTOR signaling, presumably inhibiting autophagy and preventing the loss of muscle mass.
Boosting Protein Synthesis While Decreasing Catabolism
Several scientific studies highlight mTOR activation by protein intake, especially the essential amino acid leucine. One study by Walker et al. showed that essential amino acid consumption shortly after working out increased mTOR activity, leading to greater post-workout muscle protein synthesis by an additional two hours, as compared to an exercised group that was not fed essential amino acids. Another scientific inquiry by Pasiakos et al. demonstrated that consumption of leucine immediately after exercise specifically enhances muscle protein synthesis by as much as 33 percent.
In addition to protein consumption activating mTOR during normal caloric consumption, a review by Layman et al. confirmed that leucine ingestion during caloric restriction also triggers mTOR-activated muscle protein synthesis. Leucine consumption while restricting calories also increased blood sugar levels by triggering gluconeogeneis, which is the metabolic process that generates glucose from non-carbohydrate sources such as leucine. This increase in blood sugar should augment muscle protein amounts by preventing protein degradation, as a previous scientific study by Roy et al. showed that increased blood sugar levels improved protein balance by decreasing protein degradation. Roy et al. observed that consuming 1 gram of carbohydrate per kilogram of bodyweight during and one hour after resistance exercise slightly decreased muscle protein breakdown. The rationale for carbohydrates solely influencing protein degradation most likely comes from the increased cellular energy from carbohydrate consumption, which turns off the protein degradation triggered by the enzyme AMPK. As previously mentioned, AMPK is the muscle cell’s energy gauge that promotes the breakdown of protein into amino acids when cellular energy is low, so the amino acids can be oxidized to restore the cell’s energy status. Consequently, protein consumption while limiting calories should trigger mTOR-driven muscle protein synthesis while mitigating the AMPK-driven breakdown of lean body mass.
In conclusion, sufficient protein supplementation during caloric restriction promotes the most efficient adaptation within the muscle cell by modulating the activity of the all-important nutrients-sensing mTOR and AMPK. For instance, consuming ample protein while reducing caloric intake will activate mTOR, thus promoting muscle-building protein synthesis. Yet more notably, amino acid consumption while dieting will also prevent AMPK-stimulated muscle protein catabolism as well as other destructive cellular mechanisms that deplete muscle size and strength.