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Many are familiar with the fatty pockets of adipose tissue that can accumulate along the outside of muscle tissue, with just the skin to contain it. The intramuscular fat in small pockets throughout the inside of muscle groups, however, has gone largely ignored until recent decades. Commonly referred to as intramuscular triglycerides (IMTG), these help to provide energy during periods of exertion but, in excess, are closely linked to medical conditions like diabetes.
Intramuscular fat is one of several sources of fuel for the body. The body's hormonal, carbohydrate, amino acid and fat stores all contribute to the body's caloric needs. Generally, the duration of exertion is an important factor in which stores are being used, with fat stores taking over as the predominant fuel source once the body's amino acid and carbohydrate stores have run dry. All of these sources continually feed the body's needs, though.
Some competitive bodybuilders point to how a high level of intramuscular fat — especially in relation to extramuscular fat — can make muscles look fuller to help earn trophies. Reportedly, this is caused by the exertion of intramuscular fat, which signals the body to make more muscle tissue, instead of the unsightly adipose. Endurance athletes also are interested in IMTG for the long-term energy they provide, especially since many of these athletes do not have much extramuscular fat to utilize.
Agricultural scientists have studied ways to increase levels of intramuscular fat on bovine populations. This resulted from a higher price being attached to meat with the highest level of marbling. Since the amount of intramuscular fat that accumulates in the muscles is mostly a genetic trait, genetic engineering is employed to bring together bulls and cows with the highest levels of marbling in their meat.
Early studies of IMTG linked high levels of the lipid with resistance to insulin — a problem that confounded type 2 diabetes sufferers on insulin therapy. Today, however, it is largely believed that two metabolites of intramuscular fat — ceramide and diacylglycerol — are responsible for insulin resistance. The body will increase its stores of IMTG at two extremes: when triggered by exertion during regular exercise as well as when the body experiences a heavy increase in extramuscular adipose tissue.