Control of Postprandial Glycemia: the Roles of Diet and Exercise

The two hypotheses in this study are:

1. A 24-h exposure to a 25%-carbohydrate diet will reduce postprandial glycemia to the same extent in the evening (19 h) as in the morning (7 h), and
2. One hour of moderate-intensity exercise (at 50% of maximal effort) will further reduce postprandial glycemia to the same extent in the evening (19 h) as in the morning (7 h).

Eight postmenopausal subjects (age 58.5 years, BMI 25.6 kg/m2) participated in 4 24-h long crossover trials, two terminating at 7h and the other 2 at 19h. At each circadian time one trial required 60 minutes of moderate-intensity exercise (50% of maximal effort), and the other two involved no exercise.

Three 25%-carbohydrate meals prepared by the Michigan Clinical Research Unit (MCRU) kitchen, were eaten at subjects' home, and the fourth was eaten 20 minutes after subjects' arrival at MCRU at either 19 h or 7 h. Blood was collected from antecubital catheter at 10-minute intervals until 23:20 h or 11:20 h, respectively.Plasma was treated with protease inhibitor to preserve GIP, frozen at -80o C until glucose measurements by glucose oxidase, hormone measurements by chemiluminescence, and ketone measurements by Abbott meter strips.

Exercise intensity was determined before the exercise trials on a treadmill from oxygen consumption (VO2) and carbon dioxide production (VCO2) by subjects walking on a treadmill at 3 mph with the treadmill slope increased by 2% every 3 minutes. Maximal effort was established when respiratory exchange ratio (VCO2/VO2) reached or exceeded 1. The treadmill speed and slope at half maximal effort was used during the one-hour exercise bout which was initiated 40 minutes after the start of the meal.