Dietary patterns, gastrointestinal complaints, and nutrition knowledge of recreational triathletes.

Dietary habits, nutrition knowledge, and gastrointestinal complaints were evaluated in 21 female and 50 male triathletes; 30 completed hemoccult slides to determine the frequency of gastrointestinal bleeding. Triathletes trained 11 h/wk with weekly distances of 5.3, 116.5, and 40.9 km for swimming, biking, and running, respectively. Mean daily energy intake averaged 9058 and 11,591 kJ for women and men, respectively; 53.8% of the energy was from carbohydrates. Mean intakes of vitamins and most minerals exceeded the Recommended Dietary Allowances (RDAs), but many had intakes below RDAs for some nutrients; greater than 60% had low zinc and copper intakes. Because 39% took a daily multivitamin-mineral supplement, some had intakes 200-600% above the RDA. Although there were notable misconceptions about nutrition, nutrition knowledge was high. Upper-gastrointestinal complaints, reported by 50%, included bloating and abdominal gas; the incidence of positive hemoccult slides was 27%. The relation among performance, dietary patterns, nutrition knowledge, and gastrointestinal function remains to be established.

Prolonged whole-body cold water immersion: fluid and ion shifts.

To characterize fluid and ion shifts during prolonged whole-body immersion, 16 divers wearing dry suits completed four whole-body immersions in 5 degrees C water during each of two 5-day air saturation dives at 6.1 msw. One immersion was conducted at 1000 (AM) and one at 2200 (PM) so that diurnal variations could be evaluated. Fifty-four hours separated the immersions, which lasted up to 6 h; 9 days separated each air saturation dive. Blood was collected before and after immersion; urine was collected for 12 h before, during, and after immersion for a total of 24 h. Plasma volume decreased significantly and to the same extent (approximately 17%) during both AM and PM immersions. Urine flow increased by 236.1 +/- 38.7 and 296.3 +/- 52.0%, urinary excretion of Na increased by 290.4 +/- 89.0 and 329.5 +/- 77.0%, K by 245.0 +/- 73.4 and 215.5 +/- 44.6%, Ca by 211.0 +/- 31.4 and 241.1 +/- 50.4%, Mg by 201.4 +/- 45.9 and 165.3 +/- 287%, and Zn by 427.8 +/- 93.7 and 301.9 +/- 75.4% during AM and PM immersions, respectively, compared with preimmersion. Urine flow and K excretion were significantly higher during the AM than PM. In summary, when subjects are immersed in cold water for prolonged periods, combined with a slow rate of body cooling afforded by thermal protection and enforced intermittent exercise, there is diuresis, decreased plasma volume, and increased excretions of Na, K, Ca, Mg, and Zn.

Physiology of exercise in the cold.

Recreational and job requirements have increased the incidence in which humans exercise in cold environment. Understanding the physiological responses while exposed to cold entails knowledge of how exercise and cold interact on metabolic, cardiopulmonary, muscle and thermal aspects of human performance. Where possible, distinction are made between responses in cold air and cold water. While there is no consensus for diets most appropriate for working cold exposures, the evidence is strong that adequate amounts of carbohydrate are necessary. Carbohydrate loading appears to be efficacious, as it is for other athletic endeavours. Contrary to conventional wisdom, the combination of exercise and cold exposure does not act synergistically to enhance metabolism of fats. Free fatty acid (FFA) levels are not higher, and may be lower, with exercise in cold air or water when compared to corresponding warmer conditions. Glycerol, a good indicator of lipid mobilisation, is likewise reduced in the cold, suggesting impaired mobilisation from adipose tissue. Catecholamines, which promote lipolysis, are higher during exercise in cold air and water, indicating that the reduced lipid metabolism is not due to a lack of adequate hormonal stimulation. It is proposed that cold-induced vasoconstriction of peripheral adipose tissue may account, in part, for the decrease in lipid mobilisation. The respiratory exchange ratio (RER) is often similar for exercise conducted in warm and cold climates, suggesting FFA utilisation is equivalent between warm and cold exposures. The fractional portion of oxygen consumption (VO2) used for FFA combustion may decrease slightly during exercise in the cold. This decrease may be related to a relative decrease in oxygen delivery (i.e. muscle blood flow) or to impaired lipid mobilisation. Venous glucose is not substantially altered during exercise in the cold, but lactate levels are generally higher than with work in milder conditions. The time lag between production of lactate within the muscle and its release into the venous circulation may be increased by cold exposure. Minute ventilation is substantially increased upon initial exposure to cold, and a relative hyperventilation may persist throughout exercise. With prolonged exercise, though, ventilation may return to values comparable to exercise in warmer conditions. Exercise VO2 is generally higher in the cold, but the difference between warm and cold environments becomes less as workload increases. Increases in oxygen uptake may be due to persistence of shivering during exercise, to an increase in muscle tonus in the absence of overshivering, or to nonshivering thermogenesis. Heart rate is often, but not always, lower during exercise in the cold.(ABSTRACT TRUNCATED AT 400 WORDS)

Cardiovascular and thermal responses to SCUBA diving.

Recreational SCUBA diving exposes individuals to environmental stresses not often encountered in other types of activity. These stresses include increased ambient pressure, raised partial pressure of O(2), increased resistance to movement, added weight and drag of diving equipment, cold stress, and a higher breathing resistance. One means to understand how such stresses affect a diver is to employ the stress-strain-adaptive response model. Physiologic adaptations, like an increase in VO(2) in response to cold stress, will minimize the strain placed on thermal balance. Nonphysiologic adaptive responses include those behavioral and equipment interventions that isolate the diver from a particular stress. Self-contained underwater breathing apparatus (SCUBA) isolates the diver from the inability to extract O(2) from the water; dive garments minimize the stress of cold water immersion. This review will focus on cardiorespiratory and thermal responses to SCUBA diving, using the stress-strain-adaptive response model to illustrate the interaction between diver and environment. Some responses like hyperventilation, cardiac arrhythmias, or cold injury due to vasoconstriction are not considered adaptive but are realistic possibilities in diving environments.

Fluid ingestion during exercise in 25 degrees C water at the surface and 5.5 ATA.

To determine if drinking fluid altered exercise and thermal variables during 2-h immersions in 25 degrees C water, 11 male subjects were tested breathing air at 1 ATA and HeO2 at 5.5 ATA (PO2 = 0.42 ATA). Each immersion consisted of four periods of 5-min rest, 5-min leg exercise at 50 W, and 20 min of exercise at 68 +/- 3% VO2max. One test at each depth was done with no fluid (NF); one test was done with subjects consuming 125 ml of a 7% glucose polymer (GP) solution every 30 min (total intake = 500 ml). Compared with NF, GP increased urine volume by 469 +/- 118 and 443 +/- 82 ml at 1 and 5.5 ATA. Mean minute ventilation (VE) was reduced at depth by 10.1 +/- 0.6 and 5.1 +/- 0.6 l.min-1 for NF and GP, respectively (P < 0.05). GP trials reduced ventilatory equivalent (VEQ, VE/VO2) at 1 ATA, but increased it at 5.5 ATA, secondary to a small change in VE. Exercise heart rate was slightly lower at 5.5 ATA, but O2 pulse was the same among all conditions (17.2 +/- 0.1 ml O2.beat-1). Rectal temperature increased 0.71 +/- 0.04 degrees C for all conditions. Total body heat flux and insulation were unaffected by fluid or hyperbaric treatments. These results indicate that drinking fluid during immersed exercise does not change fluid balance or thermal status at depths to 5.5 ATA. The modest changes in VEQ noted with fluid intake or hyperbaric exposure reflect inconsequential alterations in cardiopulmonary efficiency.

Additive effects of caffeine and cold water during submaximal leg exercise.

Ten males exercised for 55 min at 1.5 W.kg-1 in 28 degrees C and 18 degrees C water to determine whether cold water plus caffeine (CF) ingestion had additive effects on energy production or core temperature. Two immersions were done at each water temperature, once with CF (5 mg.kg-1) and once with placebo (PL). Cold water alone (28 PL vs 18 PL) decreased free fatty acid (FFA, -13 +/- 8%) and glycerol (-37 +/- 15%) and increased lactate (18 +/- 12%), VO2 (11 +/- 3%), and minute ventilation (VE, 8 +/- 4%) but did not change glucose, heart rate (HR), respiratory exchange ratio (RER), or rectal temperature. CF alone (28 PL vs 28 CF) increased FFA (52 +/- 18%), glycerol (14 +/- 8%), lactate (28 +/- 10%), VO2 (9 +/- 3%), VE (7 +/- 5%), HR (4 +/- 1%), and rectal temperature (2 +/- 0.4%) but did not alter RER. Significant additive effects of cold water + CF (28 PL vs 18 CF) were noted for FFA, glycerol, lactate, VO2, and VE but not for RER and rectal temperature. These findings indicate that additive effects of cold water + CF alter substrate availability and increase energy production, but without a change in lipid utilization or core temperature. It may be concluded that use of CF during exercise in cold water has no physiological benefit.

Influence of self-induced hypnosis on thermal responses during immersion in 25 degrees C water.

The efficacy of self-induced post-hypnotic suggestion to improve thermogenic responses to head-out immersion in 25 degrees C water was evaluated in 12 males. An on-line computerized system permitted the change in body heat storage to be used as the independent variable and immersion time as the dependent variable. Test-retest reliability was good, exhibiting a coefficient of variation of less than 5% for exposure time. Immersion profiles consisted of the following: rest until 200 kJ of heat were lost, leg exercise at VO2 approximately 1.5 L.min-1 to regain 200 kJ, rest until 100 kJ were lost, and repeat the exercise to regain 100 kJ. A control immersion was done prior to two 1-h hypnotic training sessions. A second immersion (hypnotic) occurred within 24 h after training. There were no differences in rates of heat production, heat loss, mean skin temperature, or rectal temperature between control and hypnotic immersions. Individual hypnotic susceptibility scores did not correlate with changes in thermal status. Ratings of perceived exertion during exercise were similar for both immersions, but perceived sensation of cold was lower during the second rest period of the hypnotic immersion. Three subjects used images of warm environments during their hypnotic immersion and lost heat at a faster rate than during control immersions. These results indicate that brief hypnotic training did not enhance the thermogenic response to cool water immersion.

Effects of hyperbaric oxygen exposure at 31.3 ATA on spontaneously beating cat hearts.

Cardiovascular responses to raised ambient pressure and acute hyperbaric oxygen (HBO) exposure [HBO, partial pressure of oxygen (PO2) = 2 ATA)] were examined in anesthetized cats. Relatively normoxic [PO2 = 0.35 ATA] helium compression to 31.3 ATA decreased spontaneous heart rate, prolonged the P-R and Q-T intervals of the ECG independently of changes in rate, and significantly increased ventricular contractility. HBO while at depth produced further decreases in heart rate, with no change in the P-R interval. The Q-T interval decreased significantly in the presence of bradycardia, particularly during the first 15 min of exposure. During the first 10-15 min of HBO exposure myocardial contractility and developed ventricular pressure were reduced, but contractility increased significantly at 30 min. These results indicate that HBO interacts with depth effects to produce hyperbaric bradycardia but reverses pressure actions on ventricular repolarization. The inotropic effects of HBO are time dependent, with slightly negative effects occurring initially, followed by positive inotropic actions at 30 min.

Combined influence of temperature and hydrostatic pressure on cardiac conduction.

Interactions of temperature and hydrostatic pressure were examined in three types of cardiac tissue. Decreases in temperature from 37 degrees C to 27 degrees C and increases in pressure from 1 to 150 ATA slowed conduction significantly in rabbit atria, dog atria, and dog Purkinje fibers. The combination of lowest temperature and highest pressure produced the greatest decreases in conduction in all three tissues. Purkinje fiber action potentials revealed that slowing of conduction can be attributed to depressed excitability, decreased membrane potential, and reduced maximum upstroke velocity of the action potential. These action potential variables exhibited the greatest change at the lowest temperature and highest pressure. Cooling or elevations in pressure increased the duration of the action potential. At 27 degrees C, however, the duration was unaffected by increases in pressure. Aberrant conduction developed occasionally, at 27 degrees C and 150 ATA, and could be explained by changes in action potential variables. The significance of this finding is discussed with respect to the possible development of cardiac arrhythmias in divers exposed to cold hyperbaric stress.

Effects of hydrostatic pressure on conduction and excitability in rabbit atria.

The effects of hydrostatic pressure on cardiac conduction and excitability were studied in 39 rabbit right atrial preparations. Increases in pressure significantly increased atrial conduction time, i.e., 41% at 150 ATA. Excitability, determined from strength-duration (S-D) curves, was depressed by pressure. The slope constant of the S-D curve increased 64% at 150 ATA. Rheobase and the X-asymptote were not affected significantly. The depression of excitability partially accounted for the slowed conduction. Frequency stress and pressure had an additive effect to produce even greater decreases in excitability and increases in conduction time. Atrial refractoriness to premature stimuli also increased as a function of pressure. Nitrous oxide (2.5 ATA) partially reversed the effects of 150 ATA of pressure on conduction time and excitability. The results suggest that pressure acts by altering basic functional components of the myocardial cell membrane. Some of these changes, particularly when combined with frequency stress, i.e., rapid heart rate, may pose a serious threat to humans exposed to hyperbaric environments.

A computer analysis of cardiac electrograms.

Computer programs were constructed for the temporal analysis of 8 electrograms obtained from various regions of isolated dog hearts. The electrograms were recorded on photosensitive paper. A computer program written in the Fortran II language allowed an x-y digitizing table to be used to input the coordinates of each electrogram into a computer. Each electrode recording within a given cardiac cycle was digitized in this manner, and the temporal intervals between the 28 combinations of electrode pairs were calculated. The program also computed the heart rate and cardiac cycle length. Subroutine options permitted the grouping of data into experiment, record, and event numbers. All data were stored on a magnetic disk. A separate computer program written in the Fortran IV language performed basic statistical operations on each of the data files. The computer system described in the present report accurately and reliably processed over 10,000 cardiac cycles. Such a system can be used to analyze large amounts of data from sources other than electrograms. Temporal analysis of the standard electrocardiogram is offered as an example of the versatility of the programs.

Combined effect of beating rate and hydrostatic pressure on excitation in cardiac muscle.

Canine cardiac Purkinje fibers stimulated at frequencies from 1-4 Hz were exposed to hydrostatic pressures of 1 and 150 ATA. Both increases in stimulation frequency and pressure slowed conduction and decreased maximum upstroke velocity of the action potential. Depression of these variables was greatest when rapid rate and elevated pressure were combined. Membrane potential decreased at 150 ATA at all frequencies. Post-overdrive hyperpolarization was reduced 45% at 150 ATA. Action potential duration increased at 150 ATA at all frequencies. Elevated pressure had no effect on the interval dependence of repolarization time to -60 mV but had a slight effect on time to full repolarization. Pressure effect on resting potential was reduced in elevated extracellular potassium. Arrhythmias developed in more than 50% of the tissues at 150 ATA and a frequency of 4 Hz. The results are discussed with respect to ionic mechanisms underlying action potential propagation, to electrogenic pump activity, and to the arrhythmogenic nature of pressure-frequency interactions.

Hyperbaric exposures alter cardiac excitation-contraction coupling.

Cardiac excitation-contraction (E-C) coupling variables were measured in anesthetized cats during helium-oxygen dives to 1000 fsw (305 msw). At constant pacing rates of 190-300/min the delay between onset of ventricular excitation and onset of developed pressure increased with depth, the larger increases occurring at faster rates. The electrocardiographic Q-T interval increased with depth at all rates, but rate-related shortening of Q-T was less at depth. At a rate of 190, the indexes of contractile performance were enhanced by increases in depth, but they showed little change or were lessened at faster rates. Predive changes in contractility, time to peak pressure, and systolic duration correlated positively with rate-induced changes in the Q-T interval. The correlations were reduced or reversed at depth, however, suggesting a dissociation of E-C coupling. Several cases of conduction arrhythmias or pulsus alternans were encountered with increases in depth and heart rate. These results suggest cardiac E-C coupling is altered by interactions of depth and heart rate. Further, these findings may have relevance to the capacity of the cardiovascular system to support periods of work in a hyperbaric environment.

Lack of diurnal effects on periodic exercise during prolonged cold water immersion.

Diurnal effects on periodic exercise were examined in 8 male divers wearing passive thermal protection during whole body immersions in 5 degrees C water for periods of up to 6 h. Studies were done during the course of 5-day air saturation dives at a depth of 1.61 ATA, with immersions beginning at 1000 h (AM) and 2200 h (PM). During each hour of immersion, leg exercise was done for 3 min each at workloads of 50, 70, and 90 W. Heart rate (HR) at each workload increased uniformly with immersion time, without a change in slope of HR vs. workload. No AM or PM differences occurred. AM resting VO2 increased linearly, and to the same extent as PM, with exposure time. VO2 at 50 W also increased at the same rate as resting values. VO2 at 70 and 90 W were similar for AM and PM and did not vary significantly during the 6-h immersions. Temporal increases in exercise HR may reflect cardiac compensation of diminished plasma volume. Workloads greater than or equal to 70 W generate enough metabolic heat in this specific condition to meet the thermogenic requirement. Lack of diurnal effects on exercise variables may be due to environmental conditions suppressing circadian rhythms.

Action potential correlates of pressure-induced changes in cardiac conduction.

Microelectrode studies were undertaken to determine the cellular bases for hydrostatic pressure effects on impulse propagation and refractoriness in cardiac muscle. Canine Purkinje fibers, at 37 degrees C, were exposed to increases in hydrostatic pressure to 150 ATA. At 150 ATA membrane excitability was depressed and the maximum upstroke velocity (Vmax) of the action potential was reduced by 10%. Furthermore, the curve relating Vmax to takeoff potential (membrane responsiveness relation) shifted downward and to the right with the half inactivation voltage shifting in the hyperpolarizing direction by about 4 mV. Decreases in excitability and responsiveness occurred concomitantly with pressure-induced decreases in impulse conduction. Action potential duration (APD) increased significantly at 150 ATA. APD measured at -20 mV, -60 mV, and at maximum repolarization averaged 20.7, 15.5, and 13.5% longer than their respective 1-ATA values. The combined effects of increased APD and depressed responsiveness account for increased tissue refractoriness. The implications of the findings with regard to the arrhythmogenic nature of high hydrostatic pressure are discussed.

Prolonged whole body immersion in cold water: hormonal and metabolic changes.

To characterize metabolic and hormonal responses during prolonged whole body immersion, 16 divers wearing dry suits completed four immersions in 5 degrees C water during each of two 5-day air saturation dives at 6.1 meters of sea water. One immersion began in the AM (1000 h) and one began in the PM (2200 h) to evaluate diurnal variations. Venous blood samples were obtained before and after completion of each immersion. Cortisol and ACTH levels demonstrated diurnal variation, with larger increases occurring after PM immersions. A greater than three-fold postimmersion increase occurred in norepinephrine (NE). There were significant increases in triiodothyronine (T3) uptake and epinephrine, but no change in T3, thyroxine, thyrotrophic hormone, and dopamine. Postimmersion free fatty acid levels increased 409% from preimmersion levels; glucose levels declined, and lactate increased significantly. Only changes in NE correlated significantly with changes in rectal temperature. In summary, when subjects are immersed in cold water for prolonged periods, with a slow rate of body cooling afforded by thermal protection and intermittent exercise, hormonal and metabolic changes occur that are similar in direction and magnitude to short-duration unprotected exposures. Except for cortisol and ACTH, none of the other measured variables exhibited diurnal alterations.

Nutritional status of land-based U.S. Navy divers.

The nutritional status of 16 male, land-based U.S. Navy divers was assessed to collect baseline information for a cold water dive series. Diet records, blood samples, and 24-h urine collections were obtained and analyzed. The divers were deriving 17 +/- 1%, 40 +/- 2%, 32 +/- 2% of their calories from protein, carbohydrate, and fat, respectively. The remaining calories were furnished by alcohol (11 +/- 2%), an amount within the American Heart Association's guidelines. Crude fiber intake was low (3.7 +/- 0.4 g/d) whereas cholesterol (507 +/- 101 mg/d) and sodium intakes (4462 +/- 599 mg/d) were high. Mean intakes of vitamin B6 and folacin were below the Military Recommended Dietary Allowances. Mean blood concentrations and urinary excretion of minerals were normal but urinary sodium excretion was high. Results indicate that the divers' intakes of sodium and cholesterol were high, whereas intakes of complex carbohydrate and crude fiber were low. Whether these dietary patterns are suitable for extended dives, especially in cold water, remains to be determined.

Hyperbaric chamber for evaluating hydrostatic pressure effects on tissues and cells.

A chamber system is described for the study of pure hydrostatic pressure effects on tissues and cells. The small chamber has an internal volume of 7.6 liters and is rated for working pressures up to 400 ATA. Sliding doors at each end permit easy access and quick sealing. A cam-driven pump provides constant flow of physiological solution to the tissue bath containing the preparation. Connections to the pump allow a variety of test solutions to be used in the course of an experiment. The tissue bath is designed to prevent chamber gas from diffusing in to the perfusate, thus allowing for pure hydrostatic compression of the bath contents. The bath is coupled to a motorized stage to facilitate placement of recording devices once the bath is placed inside the chamber. Temperature is controlled within 0.05 degrees C of set point by thermoelectric modules coupled to a feedback amplifier. This system has been used for electrical and mechanical studies of cardiac muscle, but its versatility makes it suitable for a wide range of other biomedical applications.

Fluid and cation changes during head-out immersions in 25 degrees and 35 degrees C water.

To compare fluid and ion changes during cold (25 degrees C) and thermoneutral head-out immersion (HOI) 9 men were studied under 4 resting conditions lasting 3 h: 2 in 35 degrees C and 2 in 25 degrees C water. At each temperature, subjects consumed 250 ml of either water or a 7% glucose polymer solution every hour to evaluate possible differences in fluid composition. Plasma volume increased by 3.9% for 35 degrees C and decreased by 9.7% for 25 degrees C HOI after 3 h. Urine flow increased significantly during HOI, but there were no differences between water temperatures (35 degrees C: 8.37 +/- 0.44; 25 degrees C: 9.55 +/- 0.57 ml.min-1). Free water clearance and urinary sodium excretion were also elevated during HOI, but water temperature did not alter the magnitude of the response. No HOI-induced kaliuresis was noted. Finally, there was a significant cold-induced increase in serum potassium and sodium, but this reflected largely the decrease in plasma volume. In sum, differences in water temperature seemed to have minimal influence on fluid and cation changes, an indication that immersion is the primary stimulus. Whether greater differences would be noted with colder water remains to be determined.