Effect of Training on Muscle Triacylglycerol and Structural Lipids

Abstract

We studied whether endurance training impacts insulin sensitivity by affecting the structural and storage lipids in humans. Eight male subjects participated (age 25 ؎ 1 years, height 178 ؎ 3 cm, weight 76 ؎ 4 kg [mean ؎ SE]). Single-leg training was performed for 30 min/day for 4 weeks at ϳ70% of single-leg maximal oxygen uptake. After 8, 14, and 30 days, a two-step hyperinsulinemiceuglycemic glucose clamp, combined with catheterization of an artery and both femoral veins, was performed. In addition, a muscle biopsy was obtained from vastus lateralis of both legs. Maximal oxygen uptake increased by 7% in the trained leg (T), and training workload increased (P < 0.05) from 79 ؎ 12 to 160 ؎ 15 W. At day 8, glucose uptake was higher (P < 0.01) in the trained (0.8 ؎ 0.2, 6.0 ؎ 0.8, 13.4 ؎ 1.2 mg ⅐ min ؊1 ⅐ kg ؊1 leg wt) than the untrained leg (0.5 ؎ 0.2, 3.7 ؎ 0.6, 10.5 ؎ 1.5 mg ⅐ min ؊1 ⅐ kg ؊1 leg wt) at basal and the two succeeding clamp steps, respectively. After day 8, training did not further increase leg glucose uptake. Individual muscle triacylglycerol fatty acid composition and total triacylglycerol content were not significantly affected by training and thus showed no relation to leg glucose uptake. Individual muscle phospholipid fatty acids were not affected by training, but the content of phospholipid polyunsaturated fatty acids was higher (P < 0.06) after 30 than 8 days in T. Furthermore, after 30 days of training, the sum of phospholipid long-chain polyunsaturates was correlated to leg glucose uptake (r ‫؍‬ 0.574, P < 0.04). Endurance training did not influence muscle triacylglycerol content or total triacylglycerol fatty acid composition. In contrast, training induced a minor increase in the content of phospholipid fatty acid membrane polyunsaturates, which may indicate that membrane lipids may have a role in the training-induced increase in insulin sensitivity.

Human skeletal muscle ceramide content is not a major factor in muscle insulin sensitivity

“…Since the present study included an endurancetrained group, this is consistent with data from Goodpaster and colleagues [31], and is in line with previous studies that did not find coupling when muscle triacylglycerol was measured biochemically in samples of muscle tissue [3,32]. It is generally thought that excess intramyocellular lipid storage results in decreased insulin sensitivity because an increased fatty acid precursor supply leads to increased concentrations of one or more of the lipid intermediates: diacylglycerol, long-chain fatty acyl-CoA or ceramide [9,33,34].…”

“…In obesity and type 2 diabetes, decreased fat metabolism and an accumulation of triacylglycerol has been observed in skeletal muscle, the major site of insulin resistance [2]. In a number of studies, but not all [3], an inverse relationship between insulin sensitivity and intramyocellular triacylglycerol storage has been found [4][5][6][7]. Consistent with this notion, an increase in NEFA concen-trations by lipid infusion leads to decreased insulin sensitivity [8] and increased intramyocellular triacylglycerol stores [2].…”

Disassociation of muscle triglyceride content and insulin sensitivity after exercise training in patients with Type 2 diabetes

“…Here we report that exercise training in Type 2 diabetic individuals resulted in a "normalization" of TG m content, but did not completely reverse insulin resistance in these patients. It is well known that endurance training improves insulin action in healthy individuals [9], with an increase in insulin sensitivity being observed after only 7 days of training [18,19,20]. The mechanisms by which exercise training improves insulin action in healthy skeletal muscle have been attributed to increases in insulin-stimulated PI3-kinase activity [18], increased expression of GLUT4 [21], hexokinase [22] and glycogen synthase [22].…”

“…As the principal aim of the investigation was to examine the effects of exercise training per se, subjects were studied under free-living conditions and were instructed not to make any dietary changes throughout the study. This has been common practice in a number of other investigations [9,10]. The experimental protocol was approved by the Human Ethics Committee of RMIT University.…”

“…In this regard, several recent studies have reported that short-term (4-10 weeks) aerobic exercise training programs were effective in improving whole body aerobic capacity and insulin sensitivity in both young, healthy subjects [9] and overweight and obese men [10], independent of any changes in TG m content. Indeed, exercise training has been shown to increase TG m content in muscle from healthy individuals [11], suggesting that the presence of triacyglycerol in the myocyte is not always associated with insulin insensitivity.…”