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.
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[edit] Diabetes
Ten years ago, it was believed that an increase in IMTGs was responsible for increased insulin resistance. However, the finding that athletes as well as obese individuals have high IMTG levels confounded these findings. It is now believed that it is the IMTG metabolites, such as diacylglycerol and ceramide, that are responsible for the insulin resistance. There are variable results about which of these metabolites is most important, as some studies have shown that neither has an effect while others have shown one has but the other has not. In studies that have demonstrated their effects, their mechanism has been shown to be via activation of protein kinase C theta. PKC theta promotes serine phosphorylation of IRS-1, thereby inhibiting the insulin signalling cascade.
[edit] 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.[1]
[edit] 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."[2]
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.[3]
Women have been shown to use more IMTG during exercise than men which directly correlates to the higher IMTG content in women over men.[4]
[edit] References
- ^ 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.
- ^ Roepstorff C, Vistisen B, Kiens B (2005). "Intramuscular triacylglycerol in energy metabolism during exercise in humans". Exerc Sport Sci Rev 33 (4): 182–8. doi:10.1097/00003677-200510000-00006. PMID 16239835.
- ^ 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. doi:10.1152/ajpendo.00543.2005. PMID 16449296.
- ^ 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. doi:10.1152/ajpendo.00097.2006. PMID 16822962.
