A combined insulin reduction and carbohydrate feeding strategy 30 min before running best preserves blood glucose concentration after exercise through improved fuel oxidation in type 1 diabetes mellitus

Journal Title (Medline/Pubmed accepted abbreviation): J. Sport. Sci.
Year: 2011
Volume: 29
Page numbers: 279-289
doi (if applicable): 10.1080/02640414.2010.531753

Summary of Background and Research Design

Background:Type 1 diabetic patients cannot produce insulin, the hormone that signals body cells to take up glucose from the blood stream. Consequently, patients need to administer exogenous insulin with meals. Exercise can be difficult for type 1 diabetic individuals because it changes glucose metabolism and they are at risk for hypoglycemia during and/or after exercise. Both the pre-workout carbohydrate (CHO) amount and type as well as insulin dose can be altered to prevent hypoglycemia.

Isomaltulose was the carbohydrate chosen in this experiment. It has a low glycemic index (32 where pure glucose is 100) and has been shown to yield peak blood glucose levels 120 min after consumption, which is much later than most carbohydrates.

Research Question: How does the timing of consumption of a low glycemic index carbohydrate (isomaltulose) with a rapid-acting insulin dose affect carbohydrate and lipid oxidation before, during, and after endurance exercise?

Subjects:7 male type 1 diabetic subjects who exercise regularly, age 31 ± 2 y

Research design: randomized, counter-balanced

Treatment/experimental protocol: Participants arrived at the laboratory after an overnight fast. All participants administered 25% of their normal dose of fast-acting insulin and consumed 75 g of isomaltulose in 750 mL water before the exercise session. Participants rested 30, 60, 90, or 120 min between consuming the beverage and exercising. Each participant underwent each different rest period once (4 trials per subject, 7 days apart). Fifteen min before exercising, resting heart rate and respiratory parameters were recorded. Then, subjects ran on a treadmill at 71% of their previously determined VO2max for 45 min. Blood samples were acquired before the insulin/CHO administration and 0, 5, 15, 30, 60, 120, and 180 min after exercise. If a participant experienced hypoglycemia (blood [glc] ≤ 3.5 mM), 20 g of carbohydrate was administered.

Summary of research findings:
  • Lipid oxidation was greater and carbohydrate oxidation was lower when the subjects waited 30 min opposed to 120 min between beverage consumption and exercise.
  • Pre-exercise blood glucose levels were lowest at the 30 min treatment and not significantly different between the other treatments.
  • Between pre- and post-exercise, the drop in blood glucose was greater with increased resting time between beverage consumption and exercise (-3.7 ± 0.4 mM with 30 min and -6.4 ± 0.3 mM with 120 min rest).
  • Five of the 7 subjects experienced hypoglycemia after exercise after the 120 min rest period. None experienced hypoglycemia with 30 min rest, 1 experienced hypoglycemia with 60 min rest, and 2 experienced hypoglycemia with 90 min rest.
  • The area under the curve of blood glucose for the 3 hrs post-exercise was lowest for the 120 min rest trial. Serum insulin levels and non-esterified fatty acid concentrations were similar for all trials. There were not any differences in adrenaline, noradrenaline, or cortisol between treatments.

Interpretation of findings/Key practice applications:

Consumption of 75 g of a low glycemic index carbohydrate in conjunction with a 75% reduction in rapid-acting insulin administration 30 min before a 45 min run were effective at increasing lipid oxidation, reducing carbohydrate oxidation, and preventing hypoglycemia. This is of clinical significance because this regimen is not consistent with current recommendations regarding insulin injection before exercise. Because diabetes affects every patient differently, individuals are encouraged to talk with a physician before beginning an exercise routine or before changing insulin administration or eating habits.
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