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Marbled beef

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.

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. "The Influence of Endurance Exercise Training and Sex on Intramyocellular Lipid and Mitochondrial Ultrastructure, Substrate Use, and Mitochondrial Enzyme Activity". Am J Physiol Regul Integr Comp Physiol. PMID 17095651. 

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. Timmermans R, Saris W, van Loon L (2006). "[Insulin resistance: the role of intramuscular triglyceride and the importance of physical activity]". Ned Tijdschr Geneeskd 150 (3): 122-7. PMID 16463611. 

"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." Roepstorff C, Vistisen B, Kiens B (2005). "Intramuscular triacylglycerol in energy metabolism during exercise in humans". Exerc Sport Sci Rev 33 (4): 182-8. PMID 16239835. 

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. Nadeau K, Ehlers L, Aguirre L, Moore R, Jew K, Ortmeyer H, Hansen B, Reusch J, Draznin B (2006). "Exercise training and calorie restriction increase SREBP-1 expression and intramuscular triglyceride in skeletal muscle". Am J Physiol Endocrinol Metab 291 (1): E90-8. PMID 16449296. 

Women have been shown to use more IMTG during exercise than men which directly correlates to the higher IMTG content in women over men. Roepstorff C, Donsmark M, Thiele M, Vistisen B, Stewart G, Vissing K, Schjerling P, Hardie D, Galbo H, Kiens B (2006). "Sex differences in hormone-sensitive lipase expression, activity, and phosphorylation in skeletal muscle at rest and during exercise". Am J Physiol Endocrinol Metab 291 (5): E1106-14. PMID 16822962. 

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