Exercise prior to a freely requested meal modifies pre and postprandial glucose profile, substrate oxidation and sympathovagal balance

Journal Title (Medline/Pubmed accepted abbreviation): Nutr. & Metab.
Year: 2011
Volume: 8
First page: 66
Website: http://www.nutritionandmetabolism.com/content/8/1/66

Summary of background and research design:

Background: Exercise affects glucose and insulin concentrations in the blood because of increased energy demands.  In addition, exercise affects neural signals from the sympathetic and/or parasympathetic nervous system, which can affect hormone release and response. 
              A relatively new subcutaneous (under the skin) device called the continuous glucose monitoring system has been developed that can continuously monitor blood glucose for long periods of time (in this experiment, 3 days).  Also in this study, the investigators continuously monitored heart rate during the day and were able to examine the cardiogram for activity of the autonomic nervous system.

Hypothesis: Exercise will alter activity of the autonomic nervous system, altering communication between the gut and the brain and thereby delaying the hunger response after exercise if exercise is done before the meal.

Subjects: Nine healthy males, age 21.9 ± 1.8 yrs old

Experimental design: randomized, cross-over

Conditions: mid-morning exercise or rest

Protocol : On day 0, the glucose monitoring system was inserted into the participant’s lateral abdominal wall.  Test days were days 1 and 3.  On these days, they reported to the laboratory after an overnight fast.  They ate a standard breakfast at 8:30am.  At 10:15 am, they rested or exercised (stationary bicycle at 70% of their previously determined VO 2max). Lunch (the same amount on day 3 as was consumed on day 1) was given when they requested it.  During lunch, blood glucose and gas exchange were monitored.  The glucose monitoring system was removed after the last gas exchange measurement on day 3.  Hunger/satiation questionnaires were completed throughout the morning until lunch commenced. 

Summary of research findings:
  • Frequency data from the glucose monitoring system suggested a shift from sympathovagal balance towards sympathetic activation during exercise compared to rest.   The sympathetic nervous system is associated with “fight or flight” responses and could reduce glucose-stimulated insulin release from the pancreas.
  • Exercise did not change the amount of time between breakfast and the time lunch was requested.
  • Mean blood glucose levels were significantly lower during exercise than during rest, but were not significantly different for the 30 min before lunch request.
  • The majority of lunch requests were preceded by a decline in blood glucose.  There was no noted difference in these data between exercise or rest trials.
  • The peak glucose concentration peaked later after exercise than after rest (67 ± 9 min s. 33 ± 4 min, p < 0.005).  However, the blood glucose response to the meal was significantly higher after exercise than after rest (p < 0.05).
  • At the time of the meal request and 3 hrs after lunch, carbohydrate oxidation was less (about 37-46%) and fat oxidation was greater (about 46%) post-exercise compared to post-rest.

Key practice applications:

Exercise did not change timing of hunger for lunch in these participants, but it significantly changed the mobilization of fat and the blood glucose response to a meal. The mechanisms by which exercise prevents and helps control diabetes are still being understood.
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