Abstract
In addition to the subcutaneous and the visceral fat tissue, lipids can also be stored in non-adipose tissue such as in hepatocytes (intrahepatocellular lipids; IHCL), skeletal (intramyocellular lipids; IMCL) or cardiac muscle cells (intracardiomyocellular lipids; ICCL). Ectopic lipids are flexible fuel stores that can be depleted by physical exercise and repleted by diet. They are related to obesity and insulin resistance. Quantification of IMCL was initially performed invasively, using muscle biopsies with biochemical and/or histological analysis. 1H-magnetic resonance spectroscopy (1H-MRS) is now a validated method that allows for not only quantifying IMCL non-invasively and repeatedly, but also assessing IHCL and ICCL. This review summarizes the current available knowledge on the flexibility of ectopic lipids. The available evidence suggests a complex interplay between quantitative and qualitative diet, fat availability (fat mass), insulin action, and physical exercise, all important factors that influence the flexibility of ectopic lipids. Furthermore, the time frame of the intervention on these parameters (short-term vs. long-term) appears to be critical. Consequently, standardization of physical activity and diet are critical when assessing ectopic lipids in predefined clinical situations.
Highlights
Obesity is related to the insulin resistance syndrome including type 2 diabetes mellitus, hypertension [1,2], non-alcoholic fatty liver disease (NAFLD) [3,4], and, increased risk for cardiovascular morbidity and mortality [5,6,7]
In addition to the subcutaneous and the visceral fat tissue, lipids can be stored in non-adipose tissues such as in hepatocytes, skeletal or cardiac muscle cells, and pancreatic beta cells [16]
This review summarizes the current knowledge on the flexibility of ectopic lipids (IMCL, IHCL ICCL) in humans
Summary
Obesity is related to the insulin resistance syndrome including type 2 diabetes mellitus, hypertension [1,2], non-alcoholic fatty liver disease (NAFLD) [3,4], and, increased risk for cardiovascular morbidity and mortality [5,6,7]. In addition to the subcutaneous and the visceral fat tissue, lipids can be stored in non-adipose tissues such as in hepatocytes (intrahepatocellular lipids; IHCL), skeletal (intramyocellular lipids; IMCL) or cardiac muscle cells (intracardiomyocellular lipids; ICCL), and pancreatic beta cells [16]. This fat is called ectopic fat [17]. The main focus is on the influencing factors of ectopic lipids, namely physical exercise and diet This narrative review summarizes the current knowledge on the flexibility of ectopic lipids (IMCL, IHCL ICCL) in humans. PubMed was used and the search terms were intramyocellular lipids, skeletal muscle lipids, intrahepatic fat, intrahepatocellular lipids, intracardiomycellular lipids, intramuscular triglycerides, ectopic fat, ectopic lipids, exercise, fat, diet, lipid infusion, MR-spectroscopy, and bariatric surgery
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