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The basic idea behind is eventually similar to the notion of "localized fat loss", yet on the micro- not the macro-scale. In other words, instead of "do sit-ups to burn abdominal fat" the no-bro-science-variety of the "proximity hypothesis" says: "Train your legs to empty the huge intra-muscular fat stores and have them refilled from the abdominal fat."
Probably you will already have realized that this is "proximity" as in "right next to the skeletal muscle mitochondria" and not "proximity" as "a fat depot next to the muscle your train".
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"[...] are mechanistically linked to increases in muscle oxidative capacity, intramuscular triglyceride (IMTG) utilization during endurance exercise and increases in the content of the lipid droplet-associated perilipin-2 (PLIN2) and PLIN5." (Shepherd. 2014)These lipid droplet-associated proteins (or short PLINs) coat the lipid droplets in fat and other cells and protect them from lipases of which you, as a SuppVersity reader know that they are enzymes our body uses to break down and "free" stored body fat - a process scientists usually refer to as "lipolysis".
An increase in perilipin in the musculature will thus necessarily increase the storage of lipid droplets in the muscle; and since it does not affect fat storage in the adipose organ and in view of the fact that the fat must come from somewhere (nutrition, endogenous fatty acid production from glucose, or stored body fat) this is not a bad thing.
More fat in the muscle? That's bad, right?
It's thus not shocking to have a "high" amount of fat in the muscle, as long as it is deposited there as a fat reserves for the mitochondria and serves as an alternative, additional or auxiliary, astonishingly readily available energy source for the muscle. A "range extender" with profound beneficial effects on muscular endurance.
|Figure 1: Changes in heart rate VO2max, carbohydrate + fat oxidation, and respiratory exchange ratio (left); blood glucose + insulin levels after an oral glucose tolerance test before and after RT intervention (Shepherd. 2014)|
The guys had perfoemed a 6-weeks whole-body resistance training program (3 sessione per week) in the course of which Shepherd et al. observed not just the previously mentioned increases in IMTG, and PLIN2 and PLIN5 protein content, but also highly significant increases in intramuscular fat breakdown during "light" cardio training (65% VO2max; +43% in slow-twitch type I and +37% in fast-twitch type II fibers).
- Geirsdottir, Olof Gudny, et al. "Physical function predicts improvement in quality of life in elderly Icelanders after 12 weeks of resistance exercise." The journal of nutrition, health & aging 16.1 (2012): 62-66.
- O'Connell, Matthew DL, et al. "Do the effects of testosterone on muscle strength, physical function, body composition, and quality of life persist six months after treatment in intermediate-frail and frail elderly men?." Clinical Endocrinology and Metabolism 96.2 (2010): 454-458.
- Rizzoli, René, et al. "Quality of life in sarcopenia and frailty." Calcified tissue international 93.2 (2013): 101-120.
- Shepherd, Sam O., et al. "Resistance training increases skeletal muscle oxidative capacity and net intramuscular triglyceride breakdown in type I and II fibres of sedentary males." Experimental Physiology (2014).
- Silva, Neto LS, et al. "Association between sarcopenia, sarcopenic obesity, muscle strength and quality of life variables in elderly women." Revista brasileira de fisioterapia (Sao Carlos (Sao Paulo, Brazil)) 16.5 (2011): 360-367.