Dietary effects on exercising muscle metabolism and performance by 31P-MRS.

To determine how diet modulates short-term exercise capacity, skeletal muscle pH and bioenergetic state were examined by 31P-magnetic resonance spectroscopy in nine healthy volunteers. Subjects performed incremental quadriceps exercise to exhaustion after 5 days of high-carbohydrate (HCHO) or high-fat (HFAT) diet randomly assigned in crossover fashion and separated by a 2.5-day period of ad libitum mixed diet. Simultaneous measurements were made of pulmonary gas exchange, minute ventilation, and quadriceps muscle pH and phosphorylation potential. At rest and peak exercise, respiratory exchange ratio and minute ventilation were higher after HCHO than after HFAT (P < 0.05), reflecting greater CHO utilization. Peak O2 consumption (VO2) was not increased after HCHO (P > 0.05), but exercise duration was (339 +/- 34 s for HCHO vs. 308 +/- 25 s for HFAT; P < 0.05). HCHO was associated with a blunted early fall of phosphocreatine (PCr)/Pi vs. VO2 (-4.1 +/- 0.7 x 10(-2) min/ml for HCHO vs. -5.6 +/- 1.2 x 10(-2) min/ml for HFAT; P < 0.05). On both study days, the slope of PCr/Pi vs. VO2, before and after the PCr threshold, was correlated with exercise time. The results suggest that a diet rich in CHO improves exercise efficiency through beneficial effects on intracellular phosphorylation potential.

Human cold air habituation is independent of thyroxine and thyrotropin.

Thyroxine (T4) is required in species possessing brown adipose tissue (BAT) for the maintenance of cold tolerance and adaptation. In humans, who possess negligible quantities of BAT, the importance of T4 has not been demonstrated. We studied the effects of decreased serum T4 and thyrotropin (TSH) on human cold habituation after repeated cold air exposures. Eight men (T3+) received a single daily dose of triiodothyronine (T3; 30 micrograms/day), and another eight men (T3-) received a placebo. All 16 normal thyroid men underwent a standardized cold air test (SCAT) under basal conditions in January and again in March after eighty 30-min 4.4 degrees C air exposures (10/wk). Measurements of basal metabolic rate (BMR), O2 consumption (VO2), mean arterial pressure (MAP), plasma norepinephrine (NE), serum TSH, free and total T4, and free and total T3 were repeated before and after 8 wk of exposure. TSH, free T4, and total T4 were 50% lower for T3+ than for T3- subjects. Total and free T3 were not different between groups. BMR was unchanged after habituation, whereas the cold-stimulated VO2, MAP, and NE were significantly reduced for all subjects in March. The relationship between VO2 and NE (r2 = 0.44, P less than 0.001) during the initial SCAT was unchanged with habituation. We suggest that human cold habituation is independent of major changes in circulating T4 and TSH.

Radio frequency (13.56 MHz) energy enhances recovery from mild hypothermia.

The rate of warming after hypothermia depends on the method of rewarming. This study compared the effectiveness of radio frequency (RF) energy against hot (41 degrees C) water immersion (HW) and an insulated cocoon (IC) for rewarming hypothermic men. Six men fasted overnight and were rewarmed for 1 h after attaining a 0.5 degree C reduction in rectal temperature (Tre). Tre and esophageal (Tes) temperature were recorded every 5 min with nonmetallic thermal probes. The base-line value for Tre and Tes just before rewarming was subtracted from each 5 min Tre and Tes during rewarming to give delta Tre and delta Tes. The 12 delta Tes values were averaged for each individual and were compared using analysis of variance. The average delta Tes for RF (1.15 +/- 0.22 degrees C/h) was faster (P less than 0.001) than either IC (0.37 +/- 0.16 degrees C/h) or HW (0.18 +/- 0.09 degree C/h). The present study shows the superiority of RF energy for rewarming mildly hypothermic men.

Adrenergic responses to cognitive activity in a cold environment.

Adrenergic responses during physical stress such as cold exposure have been reported to differ from those responses observed during cognitive activity. Both the separate and the combined effects of cold and cognitive activity on catecholamine activity were examined in six male subjects. Alterations in plasma epinephrine and norepinephrine showed different patterns as a function of exposure to a 4 degrees C cold environment, a cognitive performance assessment battery (PAB), and the two conditions combined. Plasma epinephrine was not altered by exposure to cold and only slightly increased by PAB performance when given at 23 degrees C. However, epinephrine was substantially elevated by exposure to combined cold and PAB. Heart rate changes paralleled observed changes in epinephrine. Norepinephrine release was predominantly increased by cold exposure and was not altered by PAB performance.

Hydration assessment during cold-weather military field training exercises.

To quantify the magnitude of dehydration and to identify predictors of dehydration by utilizing changes in total body water (TBW), 24 male Marines were studied during an 8d moderately cold-weather (1-3 degrees C) training exercise. Training consisted of approximately 12 h/d of moderately intense activity. Variables examined included body composition, TBW, and various blood and urinary parameters indicative of hydration. Dietary food and fluid records were also maintained. Body composition changes were consistently with the high energy expenditure and insufficient energy intake. Despite a decrease in TBW, significant dehydration as assessed by blood and urinary parameters did not occur over the 8d period, therefore no relationships were found between TBW and any of the clinical indicators of dehydration. The change in TBW was reflective of a decrease in lean body mass. These data suggest that during cold-weather military field exercise, despite high activity levels and a negative energy balance, body fluid balance can be maintained when particular attention is paid to fluid intake.

Norwegian military field exercises in the arctic: cognitive and physical performance.

In an effort to determine whether or not field living conditions degrade performance during cold weather military training, performance of 17 Norwegian Army soldiers living in tents in the field (FG) was compared with that of 13 soldiers living in barracks (GG). FG and GG performed similar tasks and were equipped and clothed identically. Each subject was tested prior to and following 9 days of field training. The tests consisted of marksmanship (score for a 5-shot group), snowshoe running (time to cover 1700 m), anaerobic power (Wingate test), and performance on 5 cognitive tests (preferred hand tapping, 4-choice reaction time, pattern recognition, memory search, and code substitution; each test scored as % correct and # completed). A subset of the subjects from each group wore watches which recorded heart rate during the day. During training GG had a lower average heart rate than FG, indicating lower physical activity level. Significant changes were not found in rifle shooting or in mean anaerobic power. Significant group (p less than .001) and time (p less than .001; pre vs. post) differences were found in snowshoe time, but a significant interaction was not found. Among the cognitive tests, a significant group by time interaction was found for % correct responses only for the Memory Search task, and represented a decrease in GG performance while FG performance was maintained. Time differences were found for # completed for Memory Search (p less than .002) and Pattern Recognition (p less than .001) suggesting incomplete learning of the task, but no group by time interactions were found.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of the Iditarod Sled Dog Race on serum thyroid hormones and body composition.

Cold exposure, activity and energy deficit have been associated with changes in body composition and/or changes in serum thyroid hormones. Because these conditions are present during the Iditarod Sled Dog Race, 10 volunteer participants were studied before and after the race. Body composition, total thyroxine (TT4), free thyroxine (FT4), total triiodothyronine (TT3), and free triiodothyronine (FT) levels were determined. There was a significant decrease in body weight (p < 0.05) and a decrease in body fat (p < 0.01). There were no significant changes in TT4, FT4, TT3 or FT3. On the basis of this study, it can be concluded that the strenuous activity of the Iditarod mitigates against the classic perturbation in thyroid axis brought on by the hypocaloric state.

Changes in serum triiodothyronine kinetics and hepatic type I 5'-deiodinase activity of cold-exposed swine.

Swine exposed to cold air have elevated serum values of total triiodothyronine (TT3) and free T3 (FT3). To characterize the mechanism of these increases, we measured in vivo kinetic parameters after a bolus intravenous injection of 125I-labeled T3 by use of both multicompartmental (MC) and noncompartmental (NC) methods and in vitro hepatic type I iodothyronine 5'-deiodinase (5'D-I) activity. Ten ad libitum-fed 5-mo-old boars were divided into two groups, living for 25 days in either control (22 degrees C) or cold (4 degrees C) conditions. Cold-exposed animals consumed 50% more calories than control animals but showed no difference in total body weight, percent body fat, or plasma volume. Thyroid gland weight was increased 86% (P < 0.004), as was serum total thyroxine (TT4) (48%), free T4 (FT4) (61%), TT3 (103%), and FT3 (107%), whereas serum thyrotropin (TSH) was not different in cold-exposed compared with control animals. The T3 plasma clearance rate was similar between groups when both MC and NC techniques were used. However, T3 plasma appearance rate (PAR) was elevated in cold-treated animals 110% over controls by MC (P < 0.001) and 83% by NC methods (P < 0.001). The animal total hormone pool of T3 was increased 76% (MC) and 53% (NC) compared with control (P < 0.01). The Michaelis constant of hepatic 5'D-I was not different between groups, but the maximum enzyme velocity increased (106%; P < 0.02). Therefore cold exposure for 25 days is associated with increased energy intake, thyroid size, T3 PAR, and hepatic 5'D-I activity with little change in serum TSH.