The effect of a carbohydrate beverage on the physiological responses during prolonged load carriage

Journal Title (Medline/Pubmed accepted abbreviation): Eur J Appl Physiol
Year: 2011
Volume: 111
Number: 8
Page numbers: 1901-1908
doi: 10.1007/s00421-010-1822-y

Summary of background and research design:

Background: An upward rise in oxygen uptake (VO2) and heart rate (HR) may occur over time with prolonged load carriage. Attenuation of these effects by increased glucose intake (for VO2) or fluid (for HR) may reduce the physiologic strain of load carriage and may benefit subsequent performance.

Hypothesis: Carbohydrate (CHO) supplementation can reduce the rise in VO2, but not in HR, compared with placebo during load carriage.

Subjects: Ten healthy males participated in this study. Mean age, height, body mass, body fat percentage, and maximal oxygen uptake (VO2max) were 28 ± 9 years, 1.82 ± 0.07 m, 81.5 ± 10.5 kg, 16.4% ± 3.2%, and 55.0 ± 5.5 mL/kg/min, respectively.

Experimental design: Randomized, cross-over

Treatments and protocol: Volunteers performed 2 treadmill walks for 2 hours each at 6.5 km/hr carrying a 25-kg backpack. During each walk, 500 mL (250 mL prior and 250 mL at 1 hr) of either water (placebo) or 6.4% CHO solution was consumed. Water (200 mL) was also consumed at 30 and 90 minutes of walking. A food diary was kept and a light meal was consumed at least 3 hours before each session. Plasma glucose and lactate, stride frequency, respiratory exchange ratio (RER), and VO2 were assessed at 5, 15, 30, 45, 60, 75, 90, 105, and 120 minutes of exercise. Heart rate was recorded every 5 seconds. Levels of perceived exertion were rated at 15-minute intervals. Electromyography activity was recorded for 60 seconds at Minutes 7 and 107.

Summary of research findings:
  • Main effect for time was VO2 increase between 5 and 120 minutes for both groups (P < .001).
    • Placebo increase of 22.7 to 25.8 mL/kg/min was higher than for CHO, 22.9 to 24.6 mL/kg/min (P = .036).
    • Increase in CHO group occurred between 5 and 30 minutes, and then plateaued.
    • No plateau in placebo group was observed.
  • Main effect for time was HR (P < .001) increase between 5 and 120 minutes in both placebo (16%) and CHO groups (12%) without any difference.
  • Plasma glucose levels at Minute 5 following beverage were higher in the CHO group than in the placebo group (4.49 vs 4.14 mmol/L; P = .021).
    • Peaked at 15 minutes before decreasing to baseline in the CHO group, and no change in the placebo group.
    • Plasma lactate levels decreased in both groups without any between-group differences.
  • There was a main effect for time for RER (P < .001) that was different for each group (P = .03).
    • RER decreased between Minutes 5 and 120 in the placebo group (P < .001), but not in the CHO group.
  • No changes in stride frequency or electromyography activity were observed for either group.
  • Perceived exertion increased between Minutes 15 and 120 for each group, with no differences over time between groups.

Interpretation of findings/Key practice applications:

A 6.4% CHO solution attenuated the VO2 rise observed during load carriage, but not HR. These results support observations of earlier studies. In addition, the RER decrease during placebo indicates an energy source change from CHO to fat, which has a lower energy yield per unit of O2 and may account, in part, for the VO2 rise. Maintaining a consistently higher plasma glucose level via shorter consumption intervals may be beneficial, although it must be balanced by side effects (intestinal discomfort and longer gastric emptying time limiting glucose uptake). A limitation of this study is that core body temperature was not measured, which could relate to HR changes.


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