Effects of caffeine on muscle glycogen utilization and the neuroendocrine axis during exercise.

To examine the effect of caffeine ingestion on muscle glycogen utilization and the neuroendocrine axis during exercise, we studied 20 muscle glycogen-loaded subjects who were given placebo or caffeine (6 mg/kg) in a double blinded fashion 90 min before cycling for 2 h at 65% of their maximal oxygen consumption. Exercise-induced glycogen depletion in the thigh muscle was noninvasively measured by means of 13C nuclear magnetic resonance spectroscopy (NMR) spectroscopy, and plasma concentrations of substrates and neuroendocrine hormones, including beta-endorphins, were also assessed. Muscle glycogen content was increased 140% above normal values on the caffeine trial day (P < 0.001). After cycling for 2 h, caffeine ingestion was associated with a greater increase in plasma lactate (caffeine: +1.0 +/- 0.2 mmol/L; placebo, +0.1 +/- 0.2 mmol/L; P < 0.005), epinephrine (caffeine, +223 +/- 82 pg/mL; placebo, +56 +/- 26 pg/mL; P < 0.05), and cortisol (caffeine, +12 +/- 3 mg/mL; placebo, +2 +/- 2 mg/mL; P < 0.001) levels. However, plasma free fatty acid concentrations increased (caffeine, +814 +/- 133 mmol/L; placebo, +785 +/- 85 mmol/L; P = NS), and muscle glycogen content decreased (caffeine, -57 +/- 6 mmol/L muscle; placebo, -53 +/- 5 mmol/L muscle; P = NS) to the same extent in both groups. At the same time, plasma beta-endorphin levels almost doubled (from 30 +/- 5 to 53 +/- 13 pg/mL; P < 0.05) in the caffeine-treated group, whereas no change occurred in the placebo group. We conclude that caffeine ingestion 90 min before prolonged exercise does not exert a muscle glycogen-sparing effect in athletes with high muscle glycogen content. However, these data suggest that caffeine lowers the threshold for exercise-induced beta-endorphin and cortisol release, which may contribute to the reported benefits of caffeine on exercise endurance.

A computational method for determination of the individual anaerobic threshold.

The individual anaerobic threshold (IAT) has received attention recently in the field of exercise physiology. The IAT is defined as the point during progressive exercise when lactate elimination from the blood is both maximal and equal to the diffusion from the working muscles. It has been theorized that an individual can maintain exercise for relatively long periods when working at the IAT. A common method for determination of the IAT is to perform a visual determination from plots of lactate concentration versus power output. This paper briefly describes the theoretical basis for determination of the IAT and presents an algorithm for its accurate calculation.

No effects of glycogen depleting exercise and altered diet composition on mood states.

To investigate the effects of glycogen depleting exercise and dietary composition on mood, 14 males completed the Profile of Mood States (POMS) questionnaire under four conditions. A control condition (CON) of ad libitum diet preceded any experimental manipulation. Subsequently, three treatments were administered randomly in a counterbalanced design: a) following a glycogen-depletion protocol and 3 d on a high-carbohydrate diet (93% of total caloric intake as carbohydrate [CHO]), b) following a glycogen-depletion protocol and 3 d on a low-CHO diet (23% of total caloric intake as CHO), and c) following 3 d of recorded ad libitum dietary consumption. Conditions a and b simulated phases of popular glycogen-loading protocols, and condition c served as a second control condition confirmed by records of dietary intake. The POMS measured tension-anxiety, depression-dejection, anger-hostility, vigor-activity, fatigue-inertia, and confusion-bewilderment. A composite score of total mood disturbance (TMD) was computed by summing the subscale scores (weighting vigor-activity negatively). Dietary composition (fat, protein, and CHO content) differed significantly (P less than 0.001) among the three treatments, but total caloric intake was not different. No significant differences were found among the treatment and control conditions for TMD or any POMS subscale score. We find no evidence that the 3-d glycogen loading protocol we employed with healthy, moderately fit, young males can be expected to confound performance measures by altering mood.

Differential effects of dietary carbohydrate on RPE at the lactate and ventilatory thresholds.

This study used manipulation of dietary intake and substrate utilization to dissociate the ventilatory (TVE) and lactate (TLAC) thresholds, and investigated the role of the thresholds in perception of effort as measured by the Borg 15-category rating of perceived exertion (RPE) scale. Thirteen males performed graded exercise tests following: a) glycogen depletion (GD) and 3 d on a high-carbohydrate diet (HC, 93% total daily energy intake as carbohydrate), b) GD and 3 d on a low-carbohydrate diet (LC, 21%), and c) a mixed-carbohydrate diet (NC, 51%). During submaximal exercise at intensities between 30 and 90% of peak oxygen uptake (VO2peak), significant differences among conditions (P less than 0.05) were obtained for carbon dioxide elimination, respiratory exchange ratio, and plasma lactate, with HC greater than NC greater than LC. Mean (+/- SD) TLAC occurred at different (P less than 0.05) percentages of VO2peak, with HC (55.6 +/- 2.9%) less than NC (59.6 +/- 2.9%) less than LC (63.8 +/- 2.8%). Means for TVE were not different. RPE at TLAC were significantly different (P less than 0.01) among conditions, with HC (12.6 +/- 0.6) less than NC (13.8 +/- 0.6) less than LC (14.3 +/- 0.7), but RPE at TVE were not different. It was concluded that the perception of exertion as becoming "somewhat hard" to "hard" is more closely linked to TVE than to TLAC.

Effects of pyridostigmine bromide on human thermoregulation during cold water immersion.

This study examined the effects of an oral 30-mg dose of pyridostigmine bromide (PYR) on thermoregulatory and physiological responses of men undergoing cold stress. Six men were immersed in cold water (20 degrees C) for up to 180 min on two occasions, once each 2 h after ingestion of PYR and 2 h after ingestion of a placebo. With PRY, erythrocyte cholinesterase inhibition was 33 +/- 12% (SD) 110 min postingestion (10 min preimmersion) and 30 +/- 7% at termination of exposure (mean 117 min). Percent cholinesterase inhibition was significantly related to lean body mass (r = -0.91, P less than 0.01). Abdominal discomfort caused termination in three of six PYR experiments but in none of the control experiments (mean exposure time 142 min). During immersion, metabolic rate, ventilatory volume, and respiratory rate increased significantly (P less than 0.05) over preimmersion levels and metabolic rate increased with duration of immersion (P less than 0.01) in both treatment but did not differ between conditions. PYR had no significant effect on rectal temperature, mean body temperature, thermal sensations, heart rate, plasma cortisol, or change in plasma volume. It was concluded that a 30-mg dose of PYR does not increase an individual's susceptibility to hypothermia during cold water immersion; however, in combination with cold stress, PYR may result in marked abdominal cramping and limit cold tolerance.

A comparison of methods used to determine VO2 of exercising humans and animals.

The purpose of these experiments was to compare two systems for measuring oxygen uptake (VO2): the open circuit method of indirect calorimetry (Rv), which is commonly used in human studies, and the open flow method, which has been adopted in a variety of animal studies. VO2 measured using the Rv system (procedure 1) was accepted as the criterion value. Two procedures were used to calculate VO2 in the open flow system: MCO2 (where O2 and CO2 were measured in mixed expired gas) and MO2 (O2 measured only). VO2 was measured in ten human subjects during three continuous incremental cycling tests to peak power output administered on three separate occasions--once using each of the three procedures. The results indicated a relatively small difference in VO2 between the Rv and MCO2 procedures (6.2 +/- 4.5%) and between the Rv and MO2 procedures (3.7 +/- 1.7%), across a broad range of power outputs (40-100% VO2 peak). The difference between the Rv and MCO2 procedures decreased as power output increased, while that between the Rv and MO2 procedures remained relatively constant. Therefore, the open flow method is shown to be a valid and reproducible technique for assessment of the metabolic activity of humans and animals.

Precise water vapor pressure value calculations.

Precise values for water vapor pressure in saturated systems may be computed using the Goff-Gratch equation. This equation has been adapted for convenient use on microcomputers. A Microsoft BASIC listing of the programming version of the equation is presented. This BASIC version may be easily translated into other high level programming languages and thus is suitable for use on many computer systems for routine laboratory computations.