Intramuscular fat

Intramuscular fat or Intramuscular triglycerides (IMTG) is located throughout skeletal muscle and is responsible for the marbling seen in certain cuts of beef. In humans, excess accumulation of intramuscular fat is associated with insulin resistance and type 2 diabetes. The human immunodeficiency virus (HIV)-lipodystrophy syndrome is associated with over-accumulation of intramuscular fat which may contribute to AIDS wasting syndrome.

Diabetes
IMTG is known to be a cause of type 2 diabetes though this is only true for those individuals with a high level of adipose tissue as well. Athletes also have a high level of IMTG which is favorable for energy uptake via mitochondria.

Insulin Resistance
Increased supply of adipose tissue correlates with the increased accumulation of IMTG which both can lead to insulin resistance in the muscles. Athletes often do not exhibit this correlation as they are typically insulin sensitive while having high levels of IMTG. It is thought that the improved efficiency of trained skeletal muscles prevents the development of insulin resistance.

Exercise
"Intramuscular triacylglycerol (IMTG) represents an energy store that can be used during exercise, when it may contribute up to 20% of total energy turnover depending on diet, gender, and exercise type."

It is thought that a low calorie diet and exercise induced proteins (Sterol regulatory element-binding proteins) are what cause high levels of IMTG in the skeletal muscle of trained athletes. This is in contrast to the build-up of IMTG in the obese which correlates to high levels of adipose tissue.

Women have been shown to use more IMTG during exercise than men which directly correlates to the higher IMTG content in women over men.