Carbohydrate oxidation from a drink during running compared to cycling exercise
Journal Title (Medline/Pubmed accepted abbreviation): Med Sci Sports Exerc
Year: 2010 Epub ahead of print
Page numbers:
doi: 10.1249/MSS.0b013e3181ebc488

Summary of Background and Research Design

Background:Carbohydrate (CHO) intake improves the endurance capacity and performance of runners. However, the recommended dosage rate for CHO during prolonged exercise is not established. The American College of Sports Medicine recommends 30 to 60 g/hour of a 6% to 8% CHO solution, with glucose being the primary form of CHO. Recent evidence suggests that athletes should ingest CHO blends, instead of a single source, and at rates up to 90 g/hour during intensive exercise. This evidence is based almost entirely on cycling studies and is not validated in runners. Because movements and muscle recruitment patterns are different between the 2 types of exercise, energy metabolism may also be different, with less-effective exogenous CHO oxidation in runners.

Hypothesis/purpose of study: To determine whether recent recommendations for CHO intake based on exogenous CHO oxidation from a CHO drink (13C/12C ratios) during cycling can be extrapolated to running.

Subjects: Male endurance athletes (N = 8) with comparable cycling and running backgrounds (age, 37 ± 7 yr; mass, 75 ± 7 kg; height, 1.77 ± 0.05 m; maximal oxygen consumption [VO2max] cycling, 63 ± 3 mL/kg/min; VO2max running, 65 ± 4 mL/kg/min) who trained >= 3 times/week for > 2 hours a session. All athletes had been involved in running or cycling training for >= 2 years.

Experimental design:Randomized, cross-over exercise trials

Treatments and protocol:Two preliminary exercise exhaustion tests (>= 3 days apart) were conducted 1 week before study start to correlate oxygen consumption with power output or speed, as well as VO2max and lactate threshold (LT). For the study, each athlete performed 4 exercise tests (>= 5 days apart) consisting of 120 min of cycling or running at ~60% VO2max while ingesting either 1.5-g/min CHO blend (13.3% maltodextrin and fructose [naturally 13C enriched]) or plain water. Three to 7 days before testing, athletes performed a strenuous training session to deplete 13C-enriched glycogen stores. Additionally, 24 hours before each exercise test, athletes were advised to follow a CHO intake > 4 g/kg of body weight that was low in foods containing endogenous 13C (to enrich 12C glycogen stores) and keep a food and activity diary. Respiratory and blood samples were collected at rest and every 15 min until exercise end. Breaths for 13C/12C ratios were collected at rest and during the last 60 sec of each 3-min exercise period. An initial 300 mL drink was given during the first 2 to 3 min of exercise, and a 150-mL drink was ingested every 15 min thereafter. Every 30 min, the athletes answered a questionnaire regarding gastrointestinal (GI) symptoms.

Summary of research findings:
  • Similar exercise intensities were achieved for running and cycling (VO2max, 59.7 and 59.2, respectively).
  • Energy expenditure was similar between CHO blend and water within the running and cycling tests.
    • Energy expenditure was higher for running vs cycling during water intake (P = .003), but not CHO blend intake (P = .08).
  • Exogenous CHO oxidation rates increased similarly between all tests, with no significant differences between peak rates of running and cycling.
  • Lower endogenous CHO oxidation rates were reported for running vs cycling, but did not achieve statistical significance.
  • Plasma measures of glucose, insulin, and lactate levels were not different between running and cycling during the exercise test.
  • Higher total fat oxidation was observed during running vs cycling, but only reached significance in the water ingestion test (P = .02).
  • Reports of GI symptoms were not different between running and cycling.

Interpretation of findings/Key practice applications:

Peak and average exogenous CHO oxidation rates were demonstrated to be similar between running and cycling, in contrast to the hypothesis that they may be different. An expectation that GI distress during running may alter absorption, and therefore exogenous CHO intake, was not observed. However, a study limitation is that the VO2max was 60% instead of a more intensive exercise test with 75%. As GI distress increases with exercise intensity, differences may become more apparent under the high-intensity competitive conditions. Although running resulted in a trend for higher fat oxidation and lower endogenous CHO oxidation vs cycling, the overall results suggest that the recommendations for CHO intake (based on cycling) are transferable to running.
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