Discussion
We reported previously that a continuous 6 min bout of ST-EX starting 90 min after a meal accelerated the decrease in postprandial BG levels in people with impaired glucose tolerance (IGT).9 We also demonstrated that a continuous 6.5 min bout of ST-EX starting 90 min after ingestion of a carbohydrate solution hastened the decrease in BG levels in people with T2D.11 However, we have realised that even the 6–6.5 min bout of ST-EX is too strenuous for some unfit people to perform regularly in daily life. Therefore, we conducted this study to clarify whether ST-EX for a shorter duration (∼3 min) would be sufficient to reduce the postprandial response in people with T2D. We employed two separate bouts of ST-EX after a meal, on the basis of our preliminary experiments, indicating that a single 3 min bout of ST-EX had a limited effect on BG levels (data not shown).
In the present study, we chose the timing for the first ST-EX (60 min after the meal) so as not to increase the BG levels again after exercise. Larsen et al14 ,15 demonstrated that moderate-intensity to high-intensity cycling exercise during the rapid rising phase of BG after a meal decreased the BG levels during exercise, but resulted in a rapid rebound after exercise. Similarly, we found in our preliminary experiments that ST-EX during the rapid rising phase of BG (<60 min) induced a rebound in BG levels after exercise (data not shown). As shown in figure 2, the first ST-EX was sufficient to hasten a decrease in the BG level after the meal. We added the second ST-EX (120 min after the meal) to boost this decrease during the declining phase in BG levels. As a result, our protocol clearly maintained a hypoglycemic effect up to 180 min (figure 2). However, it is possible that addition of ST-EX during the rapid rising phase (eg, at 30 min), after the first ST-EX (eg, at 90 min) and/or after the second ST-EX (eg, at 150 min), might improve postprandial hyperglycemia more significantly than the current protocol.
An acute bout of exercise is a physiologically relevant stimulus to promote glucose uptake in skeletal muscle. Exercise exerts an insulin-like effect on contracting skeletal muscle by stimulating translocation of the glucose transporter 4 (GLUT4) to cell surface membranes independently of insulin.16 ,17 The rate of glucose uptake increases rapidly (<5 min) after the initiation of exercise, and depends on the intensity of exercise performed.18 ,19 It has also been shown that the rate of glucose uptake reaches a near-maximal level (∼80% of the maximum) within 10 min after the start of exercise.20 Importantly, exercise-induced GLUT4 translocation and glucose uptake in skeletal muscle is intact in people with T2D.21–23 Thus, it seems reasonable to speculate that a 3 min bout of ST-EX is sufficient to substantially increase glucose uptake in working muscles. In addition, the postexercise period has been characterised by enhanced insulin sensitivity in skeletal muscle, which leads to a prolonged increase in insulin-stimulated glucose uptake.24 These insulin-dependent and insulin-independent mechanisms might contribute to the acute hypoglycemic effect of ST-EX in people with T2D.
C peptide and NEFA levels often reflect the intensity and/or duration of exercise. Larsen et al14 ,15 demonstrated that moderate-intensity to high-intensity cycling exercise for 45 min after a meal acutely blunted the postprandial increases in insulin and C peptide levels and postprandial decreases in NEFA levels in people with T2D. The reduction in insulin secretion might occur in response to the decreased BG level induced by exercise, as well as from the induction of a counter-regulatory response such as increased sympathetic nervous activity during exercise,25 which also causes NEFA release.26 In this study, our exercise protocol did not change NEFA levels (figure 1C), suggesting strongly that it does not elicit a clinically unfavorable counter-regulatory effect, and that insulin secretion might have been slightly reduced as a secondary response to the decreased BG levels induced by ST-EX (figure 1A, B).
It is notable that falls on stairs are common among older and/or obese people, particularly those with orthopedic disorders, and that stair descent seems to be more hazardous than stair ascent despite the low exercise intensity during descent. A survey of falls in community-living older people at least 75 years of age demonstrated that 80% (20/25) of falls on stairs occurred during descent.27 Aging is accompanied by deteriorations in musculoskeletal capacity for safe stair descent, such as decreased eccentric strength at the knee and ankle. In people with diabetes, visual dysfunction due to retinopathy may also increase the risk of accident. Furthermore, neuropathy is often accompanied by deterioration in sensory, motor, and autonomic nervous systems that may be critical to safe locomotion on stairs. Thus, the use of ST-EX in practice should be individualised with careful consideration of the risk of falling, particularly during descent.
There were some limitations to this study. First, the study participants were lean, maintained optimal HbA1c values, and already under exercise therapy, so the results might not be directly applicable to obese, poorly controlled, or sedentary patients. In particular, physical fitness and body mass index are closely linked in people with T2D: the heaviest people are the least fit,28 and thus the decrease in exercise intensity during the descending phase of ST-EX might not be generalised to severely obese patients. Second, no comparison of the exercise intensity between ST-EX and other exercise modalities such as level walking was made. In this regard, we reported previously that a single bout of ST-EX for 6–6.5 min was more effective than level walking for the same duration in decreasing postprandial BG in people with IGT9 and T2D.11 Third, an ST-EX protocol that could attenuate the increase in BG after a meal and blunt the peak postprandial BG, which should be lower than 10 mmol/L,29 remains to be determined. Thus, further studies are needed prior to clinical applications of this regimen to a variety of patient populations.