Does Training Spare Insulin Secretion and Diminish Glucose Levels in Real Life?

Compared with untrained subjects, in trained subjects the increased insulin sensitivity and decreased glucose induced insulin secretion will tend to promote health by decreasing glucose levels and insulin secretion, whereas the increased food intake will tend to increase these variables. To evaluate the net effect of training, we administered oral glucose loads making up identical fractions of daily carbohydrate intake (i.e., same relative glucose loads) to 8 athletes and 7 sedentary subjects (age: 25 ± 1 vs. 24 ± 1 yr [mean ± SE] [NS]; body weight: 76.0 ± 1.3 vs. 79.3 ± 2.3 kg [NS]; maximal oxygen uptake: 76 ± 2 vs. 48 ± 1 ml O₂ · kg⁻¹ · min⁻¹ [2P < 0.05], respectively). Furthermore, 24 h plasma concentration profiles of glucose, C-peptide, and insulin were determined during ordinary living conditions. Daily carbohydrate intake was higher (2P < 0.05) in athletes compared with sedentary subjects (678 ± 34 vs. 294 ± 18 g · day⁻¹, respectively). In response to same relative oral glucose loads, glucose and C-peptide responses were similar in athletes compared to sedentary subjects. Twenty-four hour integrated glucose and C-peptide concentrations did not differ between athletes and sedentary subjects (7.4 ± 0.2 vs. 7.3 ± 0.6 mol · L⁻¹ · 1440 min [2P > 0.05] and 923 ± 99 vs. 1047 ± 175 pM · ml⁻¹ · 1440 min [2P > 0.05], respectively), and insulin concentrations tended to be lower in athletes compared with sedentary subjects (124 ± 13 vs. 175 ± 38 pM · ml⁻¹ · 1440 min [2P > 0.05]). It is concluded that, during training, adaptations in pancreas and insulin sensitive tissues allow the necessary increase in food intake without harmful hyperglycemia and overloading of (β-cells, but sparing of insulin secretion and reductions in glucose levels are only relative to food intake. However, training may be wholesome by increasing hepatic insulin extraction and thereby decreasing arterialinsulin levels.