A time for exercise: the exercise window

A TEAM of Canadian scientists reported in the American Journal of Physiology how exercise affected blood glucose levels (20).The exercise was a moderately paced treadmill walk that started 30 min postmeal and lasted for 45 min.Although the focus of the study lay elsewhere, the graphical results presented in the paper pointed to an intriguing possibility: could physical activity be used to drain off-in real time-the exogenous glucose entering the bloodstream from the food being digested along the alimentary canal?The authors noted that "with exercise, glycemia returned rapidly to fasting levels..." in healthy people and in those with type 1 diabetes.Since then, a large number of studies have come out featuring exercise timings that vary widely: premeal, 15 min postmeal, 30 min postmeal, 45 min postmeal, and 1 h to several hours postmeal (2, 7).Many others, however, have been inexplicably silent on the timing of the exercise in relation to meal.These studies have also used various exercise intensities, durations, frequencies, and combinations thereof.Now, 34 yr later, it is still not clear what exercise conditions would best improve the key metabolic parameters, HbA 1c and lipids.Translational efforts are exceedingly slow in this critical area.This report suggests a somewhat more refined framework than the binary premeal-postmeal categorization of the feeding cycle to help account for how meals and exercise influence blood glucose levels and other metabolic measures.Although high-intensity and resistance exercise (8, 25) offer many health benefits, this paper focuses on moderate aerobic exercise.The remainder of this paper reviews existing studies that point to the utility and rationale of this approach.Research over several decades now has shown that blood glucose levels are sensitive to various exercise conditions: timing, intensity, duration, frequency, and sequencing of exercise (2, 7).Light to moderate exercise premeal raised postprandial glucose, but a similar walk post-meal lowered glucose levels (4, 5).Remarkably, an hour-long, energy-intensive interval exercise gave the opposite results: it was the premeal exercise that improved glycemia (25).Exercising in the late postprandial period could lead to hypoglycemia (7).In people with type 1 diabetes, delayed nocturnal hypoglycemia, which usually follows high-intensity or resistance exercise, is a problem as well (17, 30).For many with type 1 diabetes, fear of hypoglycemia has been a potent deterrent against partaking in exercise activities.This and extra carbohydrate intake, meant to prop up falling blood glucose levels, lead to weight gain.Insulin pumps and continuous glucose monitoring have made the task of regulating the insulin