Muscle glycogen content and glucose uptake during exercise in humans: influence of prior exercise and dietary manipulation

Abstract

There are many factors that can influence glucose uptake by contracting skeletal muscle during exercise and although one may be intramuscular glycogen content, this relationship is at present not fully elucidated. To test the hypothesis that muscle glycogen concentration influences glucose uptake during exercise, 13 healthy men were studied during two series of experiments. Seven men completed 4 h of two-legged knee extensor exercise 16 h after reducing of muscle glycogen by completing 60 min of single-legged cycling (Series 1). A further six men completed 3 h of two-legged knee extensor exercise on two occasions: one after 60 min of two-legged cycling (16 h prior to the experimental trial) followed by a high carbohydrate diet (HCHO) and the other after the same exercise followed by a low carbohydrate diet (LCHO) (Series 2). Muscle glycogen was decreased by 40 % when comparing the pre-exercised leg (EL) with the control leg (CL) prior to exercise in Series 1. In addition, muscle glycogen was decreased by the same magnitude when comparing LCHO with HCHO in Series 2. In Series 1, glucose uptake was 3-fold higher in the first 60 min of exercise, in the presence of unchanged pre-exercise GLUT4 protein in EL compared with CL, suggesting that the lower glycogen, and not the exercise the day before, might have provided the stimulus for increased glucose uptake. Despite the same magnitude of difference in pre-exercise glycogen concentration when comparing Series 1 with Series 2, neither direct-nor isotopic tracer-determined glucose uptake was higher in LCHO compared with HCHO in Series 2. However, arterial concentrations of insulin and glucose were lower, while free fatty acids and adrenaline were higher in LCHO compared with HCHO. These data suggest that pre-exercise glycogen content may influence glucose uptake during subsequent exercise. However, this is only the case when delivery of substrates and hormones remains constant. When delivery of substrates and hormones is altered, the potential effect of glycogen on glucose uptake is negated.

Figure 1

Skeletal muscle GLUT4 mRNA (A) and protein (B) in a leg exercised 16 h before experimentation (EL) and a control leg (CL) in Series 1. # denotes difference (P < 0.05) from CL. Data expressed as means ± s.e.m. (n = 7).

Figure 2

Arterial plasma free fatty acid (FFA; A), insulin (B), glucose (C) and net glucose uptake (D) in a leg exercised 16 h before experimentation (EL) and a control leg (CL) in Series 1. # denotes difference (P < 0.05) when comparing EL with CL. Data expressed as means ± s.e.m. (n = 7).

Figure 3

Arterial plasma free fatty acid (FFA; A), glucose (B) and net glucose uptake (C) during exercise after a low carbohydrate (LCHO) and high carbohydrate (HCHO) diet in Series 2. # denotes difference (P < 0.05) when comparing LCHO with HCHO. Data expressed as means ± s.e.m. (n = 6).

Figure 4

Rate of [6,6-²H]glucose appearance (R_a; A), disappearance (R_d; B) and metabolic clearance rate (MCR; C) during exercise after a low carbohydrate (LCHO) and high carbohydrate (HCHO) diet in Series 2. # denotes difference (P < 0.05) when comparing LCHO with HCHO. Data expressed as means ± s.e.m. (n = 6).

Figure 5

Arterial plasma adrenaline (A), noradrenaline (B), insulin (C) and cortisol (D) during exercise after a low carbohydrate (LCHO) and high carbohydrate (HCHO) diet in Series 2. # denotes difference (P < 0.05) when comparing LCHO with LCHO, § denotes main treatment effect (P < 0.05). Data expressed as means ± s.e.m. (n = 6).