Research

• Carbohydrates and Work Rate
• Supplemental Carbohydrates
• Carbohydrate and Cognitive Function
• Carbohydrate and Protein Supplement
• Sleep and Energy Deprivation

Carbohydrates and Work Rate

Our laboratory at the University of Montana has demonstrated that the total energy expenditure of extended wildfire suppression may exceed 6,000 kilocalories/day (Medicine and Science in Sports and Exercise 34(6): 1048–54, 2002). The purpose of this study was to determine the effects of supplemental carbohydrate (CHO) feedings on self-selected work rates during extended arduous wildland fire suppression. Subjects included wildland firefighters (n = 20 hotshots) during two wildfires in Idaho and Oregon. Subjects consumed 200 milliliters (6.8 ounces) per hour of a 20- percent maltodextrin solution (40-gram carbohydrate) or a placebo (PLA) drink in a counterbalanced crossover design and were allowed to drink water freely during the day. Blood samples were collected at 2-hour intervals with an automated glucometer. The self-selected hourly work rate was determined using accelerometers as established by Heil (Applied Ergonomics 33: 405–13, 2002). Data were analyzed using a priori planned comparisons across trials.

 

During the 20-percent carbohydrate trials, blood glucose (millimoles) was significantly higher immediately before lunch, after lunch, and after shift (figure 1).

Figure 1—Changes in blood glucose levels during work
performed after breakfast and lunch.

The self-selected work rate was similar across trials during the initial hours after breakfast, but was significantly different later in the morning (mean work rate 4 to 6 hours after breakfast: CHO=58,088, PLA=40,191 activity counts/hour). Work rate was also similar across trials during the early hours after lunch but was significantly different later in the afternoon (mean work rate 4 to 6 hours after lunch: CHO=64,172, PLA=47,528 activity counts/hour, figure 2).

Figure 2—Average activity counts in the hours after lunch
(PLA = placebo, CHO = carbohydrate drink).

The results indicate that carbohydrate supplements maintain blood glucose and work rate between meals. With carbohydrates, workers selected a higher work rate, as indicated by the activity monitors. The effects were most pronounced 4 to 6 hours after breakfast and lunch. Although the work rate for wildland fire suppression is moderate, the long work shift creates a need for supplemental carbohydrates. The supplement provides the carbohydrates needed to fuel working muscles and to maintain blood glucose levels and cognitive function.

Conclusions—Self-selected work rate is higher during arduous wildland fire suppression when carbohydrates are delivered at a rate of 40 grams/hour (160 kilocalories/hour) in a 20-percent solution administered at the rate of 200 milliliters/hour.

This study was supported by MTDC.

B. Ruby, S. Gaskill, D. Lankford, D. Slivka, D. Heil, and B. Sharkey. 2003. Carbohydrate Feedings Increase Self-Selected Work Rates During Arduous Wildfire Suppression. Paper accepted for presentation at the June 2003 meeting of the American College of Sports Medicine, San Francisco.

Supplemental Carbohydrates

The effectiveness of consuming carbohydrates during endurance exercise has been widely documented. However, carbohydrate ingestion during extended exercise at lower intensities has received little attention. This study attempted to determine the effects of carbohydrate feeding on blood glucose and self-selected hiking speed during a prolonged submaximal exercise. Subjects (n = 12) completed two 24-kilometer hikes 7 days apart in a single-blind double-crossover design. Blood glucose concentration was measured with an automated glucometer before the hike, hourly during the hike, and after the hike. Subjects consumed 200 milliliters of a carbohydrate (40 grams/hour, 20-percent maltodextrin) or placebo drink hourly. The subjects' ratings of perceived exertion were recorded hourly using the Borg 6–20 scale. Data were analyzed using a priori planned comparisons across trials.

Blood glucose (millimoles) was similar for both groups before exercise and for the first 8 kilometers of the hike. Afterward, blood glucose remained significantly higher (p < 0.05) during the carbohydrate trial (figure 3).

Figure 3—Blood glucose levels during a 24-kilometer hike.

Subjects self-selected a faster hiking speed during the carbohydrate trial, demonstrated by a significant difference in return time (124 and 136 minutes for the carbohydrate and placebo trials, respectively, p < 0.05) despite no differences in the ratings of perceived exertion (11.92 vs. 11.75, p < 0.05). These data demonstrate that supplemental carbohydrate feedings can improve self-selected exercise performance during low- to moderate-exertion extended hiking, regardless of the subject's rating of perceived exertion.

B. Ruby, S. Gaskill, and others. 2003. Liquid Carbohydrate Feeding Improves Self-Selected Exercise Performance During a 24 km Hike. Paper accepted for presentation at the June 2003 meeting of the American College of Sports Medicine, San Francisco.

Carbohydrate and Cognitive Function

Carbohydrate and Cognitive Function The brain requires a continuous supply of glucose to function adequately. During aerobic exercise, peripheral glucose requirements increase, and carbohydrate supplementation improves performance. This study investigated the effects of carbohydrate supplementation on cognitive function. A double-blind, placebo-controlled, between-subjects design was used. Young men (n = 143) were randomly assigned to one of three groups: a 6-percent (by volume) carbohydrate, 12- percent carbohydrate, or placebo beverage. Each group received six doses. All subjects consumed two meals.

During the 10-hour study, the subjects performed physically demanding tasks, including a 19.3-kilometer (12-mile) road march and two 4.8-kilometer (3-mile) runs, interspersed with rest and other activities. Subjects responded as rapidly as possible to auditory stimuli (20 per hour) from vigilance monitors they wore on their wrists. A self-report mood questionnaire was used to assess cognitive function. Vigilance consistently improved in a dose-related manner when supplemental carbohydrates were administered. Vigilance was highest for those receiving 12-percent carbohydrate doses and lowest for those receiving the placebo. Mood results agreed with the results from the vigilance monitors. Subjects who received carbohydrates reported less confusion and greater vigor than did those who received the placebo.

Supplemental carbohydrates enhance vigilance and improve mood during sustained exertion. This study also shows that monitoring devices can be used to assess the effects of nutritional factors on cognition during field activities.

H. Lieberman, C. Falco, and S. Slade. 2002. Carbohydrate Administration During a Day of Sustained Aerobic Activity Improves Vigilance, as Assessed by a Novel Ambulatory Monitoring Device, and Mood. American Journal of Clinical Nutrition, 76:120–127.

Carbohydrate and Protein Supplement

Will a carbohydrate and protein supplement be more effective in replenishing muscle glycogen after exercise compared to a supplement containing as much carbohydrate, but no protein? After 2.5 hours of intense cycling to deplete muscle glycogen stores, subjects (n = 7) received, in a rank order design, a supplement with 80 grams of carbohydrates, 28 grams of protein, and 6 grams of fat (CHO-Pro), 80 grams of carbohydrates and 6 grams of fat (LCHO), or 108 grams of carbohydrates and 6 grams of fat (HCHO) 10 minutes after exercise and 2 hours after exercise. Muscle glycogen of the vastus lateralis muscle on the front of the thigh was determined using nuclear magnetic resonance spectroscopy before exercise and during 4 hours of recovery. Exercise significantly reduced the muscle glycogen stores (40 to 41 millimoles/liter). After 240 minutes of recovery, muscle glycogen levels were significantly higher for the CHO-Pro treatment (88.8 millimoles/liter, compared to 70 and 75.5 millimoles/liter for the LCHO and HCHO treatments, respectively). Glycogen storage did not differ significantly between the LCHO and HCHO treatments. These results suggest that a carbohydrate-protein supplement is more effective for rapidly replenishing muscle glycogen after exercise than a supplement with as much carbohydrate, but no protein.

J. Ivy and others. 2002. Early Post-Exercise Muscle Glycogen Recovery is Enhanced with a Carbohydrate-Protein Supplement. Journal of Applied Physiology, 93:1337–1344.

Sleep and Energy Deprivation

Studies examining the effects of prolonged physiological stress have relied primarily on tests that were not occupationally relevant and often did not establish test-retest reliability. This study evaluated changes in occupational task performance and body composition during 72 hours of military occupational stress. Ten male subjects (22 years old, 183 centimeters tall, weighing 87 kilograms) participated in physical performance tests during days 1 to 4 of a control and experimental period. During the period, subjects engaged in sustained physical exertion of 4,500 kilocalories per day while sleeping for only 2 hours per day and receiving only 1,600 kilocalories per day of nutrients. Military occupational physical performance was measured by:

• Time to complete a six-station indoor obstacle course
  
• Number of 20.5-kilogram boxes lifted to 1.3 meters during a 10-minute box-lifting trial
  
• Landing distance from a target for a 35-millimeter grenade throw

Subjects were provided with strong verbal encouragement. Body mass (–3.1 percent), fat-free mass (–2.3 percent), and fat mass (–7.3 percent) all declined significantly by the end of the 4-day experimental period. The obstacle course and box-lifting trials showed temporal changes as performance was significantly lower on day 3 as compared to day 1, but showed some recovery on day 4. The grenade throw showed no change during the period. The data show that soldiers are able to perform militarily relevant physical performance tasks at a similar capacity to their normally rested and fed state, despite having lost fat-free mass, after 72 hours of military operational stress (physical exertion, sleep deprivation and restricted caloric intake).

C. Pandorf and others. 2002. Physical Performance Responses to 72 Hours of Prolonged Work, Sleep Deprivation and Caloric Restriction. Medicine and Science in Sports and Exercise, 34: s194. U.S. Army Institute of Environmental Medicine.

Editors note: This study confirms the ability of trained and motivated workers to sustain performance of meaningful physical work despite fatigue caused by prolonged work and deprivation of food and sleep.