Insulin-sensitive, trained people primarily distribute IMTGs in type I (endurance) muscle fibers, between myofibrils (rod-like units of a muscle fiber), and in the form of more numerous, smaller lipid droplets.

Insulin-resistant people with obesity primarily distribute IMTGs in type II (strength/rapid acting) muscle fibers, just beneath the surface membrane of the muscle fiber, and in the form of fewer, larger lipid droplets.

The greater surface area of more numerous, smaller lipid droplets, in greater proximity of more mitochondria, allows for more efficient IMTG turnover (greater rate of breakdown and synthesis) and greater insulin sensitivity in trained people.

Various nutritional strategies, including fasting, glycogen depletion, ketogenic dieting, weight loss/caloric restriction, and antilipolytic drug therapy, in combination with exercise were suggested to help stimulate enhanced IMTG turnover in insulin-resistant people with obesity.

Skeletal muscle anatomy and fiber type

Skeletal muscle is made up of three layers of connective tissue that encompass bundles of fascicles, which are made up of bundles of muscle fibers. Muscle fibers are long cylindrical cells extending from one end of the muscle to the other, consisting of bundles of myofibrils or rod-like contractile subunits of a muscle fiber.^[1] Other organelles and cell contents, such as mitochondria, lipid droplets, and glycogen, are distributed among the myofibrils to fuel the muscle.

Muscle fibers are also characterized based on how fast they contract and how they regenerate energy (i.e., how they regenerate ATP).^[2] Type I, or slow oxidative, fibers primarily depend on aerobic respiration, use fat as a fuel source more than other types, and are more active during activity of light to moderate intensity (i.e., endurance). Type II, or fast, fibers generally lie on a spectrum between aerobic/oxidative (Type IIa) and glycolytic (IIx) metabolism and power more explosive, strength based activity.^[3]

Basic muscle anatomy

Intramuscular fat distribution between insulin-sensitive and insulin-resistant individuals

Muscle, fat, and insulin resistance Original paper