Sodium-facilitated hypervolemia, endurance performance, and thermoregulation.

The purpose of this study was to investigate the effects of an immediate pre-exercise, orally ingested, sodium load (164 mEq Na+) (IPOSL), equivalent to 10 ml per kilogram of body weight, on plasma volume, endurance performance, and thermoregulation. Fourteen male participants consumed a nearly isotonic (255 mOsm . kg (-1)) IPOSL and a hypotonic (94 mOsm . kg (-1)), no-sodium, placebo beverage (Pl) equivalent to 10 ml . kg (-1) body weight in a randomized design. Subjects cycled at 70 % of maximal work rate, in a 21.0 - 23.3 degrees C lab, for 45 min while cardiovascular and thermoregulatory variables were measured. This was followed by a 15-min performance time trial. IPOSL and Pl ingestion lead to a 3.1 % expansion and a 4.7 % reduction in resting baseline plasma volume, respectively. IPOSL maintained plasma volume during exercise to a greater extent than the Pl at 15 and 30, but not 45 min. There was a significant improvement ( approximately 7.8 %; p < 0.05) in time trial performance following IPOSL. No significant differences were observed for heart rate, core temperature, rate of perceived exertion or total body sweat rate (p > 0.05). In conclusion, IPOSL ingestion increased pre-exercise plasma volumes, maintained 15- and 30-min exercise plasma volumes, and improved an endurance performance time trial better than the Pl with no apparent compromise in thermoregulation.

Renal responsiveness to aldosterone during exposure to simulated microgravity.

We measured renal functions and hormones associated with fluid regulation after a bolus injection of aldosterone (Ald) during head-down tilt (HDT) bed rest to test the hypothesis that exposure to simulated microgravity altered renal responsiveness to Ald. Six male rhesus monkeys underwent two experimental conditions (HDT and control, 72 h each) with each condition separated by 9 days of ambulatory activities to produce a crossover counterbalance design. One test condition was continuous exposure to 10 degrees HDT; the second was a control, defined as 16 h per day of 80 degrees head-up tilt and 8 h prone. After 72 h of exposure to either test condition, monkeys were moved to the prone position, and we measured the following parameters for 4 h after injection of 1-mg dose of Ald: urine volume rate (UVR); renal Na(+)/K(+) excretion ratio; renal clearances of creatinine, Na(+), osmolality, and free water; and circulating hormones [Ald, renin activity (PRA), vasopressin (AVP), and atrial natriuretic peptide (ANP)]. HDT increased Na(+) clearance, total renal Na(+) excretion, urine Na(+) concentration, and fractional Na(+) excretion, compared with the control condition, but did not alter plasma concentrations of Ald, PRA, and AVP. Administration of Ald did not alter UVR, creatinine clearance, Ald, PRA, AVP, or ANP but reduced Na(+) clearance, total renal Na(+) excretion, urinary Na(+)/K(+) ratio, and osmotic clearance. Although reductions in Na(+) clearance and excretion due to Ald were greater during HDT than during control, the differential (i.e., interaction) effect was minimal between experimental conditions. Our data suggest that exposure to microgravity increases renal excretion of Na(+) by a natriuretic mechanism other than a change in renal responsiveness to Ald.

The physiological effects of beverage ingestion during cross country ski training in elite collegiate skiers.

The purpose of this study was to investigate the effects of beverage ingestion on fluid balance during 1.5 hr of low intensity cross country skiing. In Part I, 6 skiers drank water ad libitum during ski training. In Part II, 10 skiers were matched by body weight (BW) and assigned to ingest 2.5 ml.kg-1 BW of water or a carbohydrate/electrolyte (CE) beverage every 2.5 km. Skiing speed averaged 11.5 km.hr-1 for 90 min around a 5 km groomed track. Following 20 min of seated rest, blood samples were collected immediately before and approximately 30 min after skiing. Part I data indicated that subjects ingested 576 +/- 189 ml of fluid and produced 266 +/- 205 ml of urine; BW, plasma and urine osmolality, and plasma protein decreased significantly. In Part II, the CE group produced less urine (135u75 vs. 450 +/- 262 ml) and had smaller decreases in plasma osmolality (-1.0 +/- 1.0 vs. -7.0 +/- 2.4 mOsm.kg H2O) and protein (-0.11 +/- 08 vs. -0.42 +/- 0.24 gL-1) than the water group. No differences were observed for BW loss, % change in PV, FWC, or change in urine osmolality. It was concluded that ad lib water ingestion was inadequate to minimize fluid balance disruption. Plain water ingestion also led to significant dilution of the plasma and increased urine output. However, the ingestion of CE led to attenuation of fluid balance disruption, presumably due to the maintenance of osmotic balance in the plasma.

Dietary sodium and plasma volume levels with exercise.

Sodium is the major cation of the extracellular fluid and has a potent influence on fluid movement. Sodium has been likened to a sponge that draws fluids into the extracellular space, including the plasma volume, to equalize gradients in concentration. Conventional wisdom suggests limiting dietary intake of Na+ to decrease risk of hypertension. However, there are some extreme occupational or exercise-related conditions where sweat losses are great and Na+ losses may exceed normal dietary intake. This can occur acutely such as in an ultra-endurance event or chronically as in hard manual work in the hear. In such cases, additional Na+ in the form of a higher Na+ diet or adding Na+ to beverages used for fluid replacement may be warranted. A higher Na+ diet also appears to accelerate the cardiovascular and thermoregulatory adaptations that accompany heat acclimation or short term exercise training. Saline ingestion before exercise causes an expansion of plasma volume at rest and throughout the subsequent exercise bout. This expansion of plasma volume alters cardiovascular and thermoregulatory responses to exercise in ways that may lead to beneficial changes in endurance exercise performance. Plasma volume expansion also occurs with saline infusion during exercise, but exercise performance advantages have yet to be reported. The purpose of this article is to review the literature concerning dietary sodium and its influence on fluid balance, plasma volume and thermoregulation during exercise. It contains 2 major sections. First, we will discuss manipulations in daily Na+ intake initiated before or throughout an exercise regime. Second, we will examine studies where an acute Na+ load was administered immediately before or during an exercise trial. The dependent variables that we will discuss pertain to: (i) body water compartments, i.e. plasma volume; (ii) thermoregulatory variables, i.e. core temperature and sweat rate; (iii) cardiovascular variables, i.e. heart rate and stroke volume; and (iv) performance, i.e. time trial performance and time to exhaustion. Particular attention will be given to the route by which Na+ was administered, the environmental conditions, the level of acclimation of the participants and specifics relating to Na+ administration such as the osmolality of the Na(+)-containing beverage.

The effects of mountain bike suspension systems on energy expenditure, physical exertion, and time trial performance during mountain bicycling.

The purpose of this 3-Phase study was to investigate the effects of suspension systems on muscular stress, energy expenditure, and time trial performance during mountain biking. Three suspension systems were tested, a rigid frame bike (RIG), a suspension fork bike (FS), and a front and rear suspension bike (FSR). Phase I and II consisted of cycling at 16.1 km.hr-1 over a flat, bumpy course for 63 min. Phase III consisted of ascending (ATT), descending (DTT), and cross country (XTT) time trials. Phase I assessed muscular stress by 24 h change in CK, Phase II assessed HR, VO2, VE, and Phase III assessed performance responses to the suspension systems. The 24 hr change in CK was greater for RIG than FS and FSR (+91.9 +/- 79.5 IU vs +8.6 +/- 17.5 IU and +9.7 +/- 21.8 IU). Mean HR was greater for RIG than FS and FSR (153.7 +/- 15.6 bpm vs 146.7 +/- 15.4 bpm, 146.3 +/- 16.2 bpm). Subjects rode significantly faster on FS than FSR and RIG during the XTT (30.9 +/- 2.0 min vs 32.3 +/- 3.6 min, 32.3 +/- 3.2 min). Subjects RPE was lower for FSR than FS and RIG, however, no differences were observed for VO2, VE, ATT, or DTT. Cyclists incurred less muscular stress, indicated by CK and HR, when riding the FS and FSR. Although the FS and FSR weigh from 0.7 to 2.2 kg more than RIG, no differences were observed for energy expenditure and that riding the FS in a XTT resulted in a faster finishing time than FSR or RIG.

Health-related fitness and quality of life in organ transplant recipients.

BACKGROUND: The purpose of this study was to describe the levels of health-related fitness and quality of life in a group of organ transplant recipients who participated in the 1996 U.S. Transplant Games. METHODS: A total of 128 transplant recipients were selected on a first reply basis for testing. Subjects with the following organ types were tested: kidney (n=76), liver (n=16), heart (n=19), lung (n=6), pancreas/kidney (n=7), and bone marrow (n=4). Cardiorespiratory fitness (peak oxygen uptake) was measured using symptom-limited treadmill exercise tests with expired gas analysis. The percentage of body fat was measured using skinfold measurements, and the Medical Outcomes Short Form questionnaire (SF-36) was used to evaluate health-related quality of life. RESULTS: Participants achieved near age-predicted cardiorespiratory fitness (94.7+/-32.5% of age-predicted levels). Scores on the SF-36 were near normal. The active subjects (76% of total sample) had significantly higher levels of peak VO2 and quality of life and a lower percentage of body fat compared with inactive subjects (P<0.01). CONCLUSIONS: Although this is a highly select group which is not representative of the general transplant population, the data suggest that near-normal levels of physical functioning and quality of life are possible after transplantation and that those who participate in regular physical activity may achieve even higher levels.

Evidence of a secondary hypervolemia in trained man following acute high intensity exercise.

Exercise induced hypervolemia is well documented following the initial onset of chronic exercise. In trained man, however, it might be difficult to induce further PV expansion with exercise induced hypervolemia already present. Therefore the primary goal of this study was determine the effect of acute high intensity exercise on plasma volume (PV), cardiac output (CO), stroke volume (SV), heart rate (HR), and oxygen consumption (VO2), in fourteen male competitive runners (VO2max 60.7 +/- 6.4 ml.kg-1.min-1) in the middle of their season. In this well trained state, subjects twice performed three steady state runs at different speeds, and one maximal graded exercise test (baseline). After this baseline evaluation, subjects exercised at a high intensity (approximately 90-95% of VO2max) on 2 consecutive days (short-term high intensity STHI exercise). Subjects then repeated the submaximal runs and maximal exercise test (post high intensity exercise, PHIE). Following the intense exercise period, the subjects experienced an increase in PV of 4.4% (p < 0.05), HR was significantly reduced for any given running speed (p < 0.05) as was blood lactate concentration (p < 0.05), and SV was significantly increased by 4% (p < 0.05). Both CO and VO2 at submaximal running speeds were unaffected by the acute increase in exercise intensity. Maximal HR was also significantly reduced following the intense exercise (p < 0.05), but VO2max was unchanged. These data illustrate that STHI exercise can induce a secondary hypervolemia at a given work rate in trained man. These findings support scientific and anecdotal reports of a reduced HR response at a given work rate PHIE.

Dietary sodium intake and changes in plasma volume during short-term exercise training.

This study was a retrospective examination of the relationship between estimated dietary sodium intake and training-induced changes in plasma volume (PV). It was undertaken to explore one possible explanation for the large individual differences in PV shifts accompanying 3 d of endurance cycling. Ten healthy males rode a stationary cycle for an average time of 94 min/day at an average relative intensity of 68% VO2max. During the training period, the subjects were allowed to eat a diet of their own choosing and dietary sodium intake was estimated from diet records. Plasma volume was determined before and after short-term training by dye dilution using Evan's blue dye. The mean increase in PV was 4.53 ml.kg body weight (BW)-1 (i.e. 11%). However, the individual shifts in PV ranged from -1.44-14.30 ml.kg BW-1. The correlation coefficient between estimated dietary sodium intake and changes in PV was 0.81. It was concluded that dietary sodium intake was strongly associated with training-induced shifts in PV and may be an influential factor in determining the magnitude of PV expansion derived from short-term exercise training. Further studies are needed, however, to examine this hypothesis.

Spironolactone administration and training-induced hypervolemia.

The purpose of this study was to determine the contribution of aldosterone to plasma volume expansion accompanying short-term exercise training. Twelve healthy males (age = 26.5 +/- 5.8) cycled for 120 min on three consecutive days at a relative exercise intensity of 65% VO2max. Half of the subjects were treated with 25 mg of spironolactone taken four times daily to suppress the effects of aldosterone. Resting plasma volume increased significantly (501 +/- 83 ml, p < 0.05) from pretraining in control subjects, but not for drug-treated subjects (163 +/- 94 ml). Spironolactone attenuated gains in resting plasma volume, serum sodium content, and serum osmolal content by 67%, 79% and 66%, respectively. Spironolactone-treated and non-treated subjects experienced similar increases in total serum protein content sufficient to increase the plasma volume by approximately 290 ml. It was not determined why drug-treated subjects attained less of a plasma volume expansion than that expected by the increase in oncotic pressure. In conclusion, two-fifths of the plasma volume expansion induced by 3 d of endurance cycling could be attributed to aldosterone activity, and the remaining three-fifths could be explained by the expansion of intravascular protein mass. Other neurohormonal influences may have contributed to the overall plasma volume expansion but this experiment did not allow for their exclusion.

A justification for high resolution hematocrit measurement.

The purpose of this study was to determine the feasibility of using a 6" digital caliper and a 20x viewing microscope to measure hematocrit (HCT) from microhematocrit tubes. The reliability and validity of the digital caliper technique (DC) was simultaneously compared with that of the conventional "turntable" style microhematocrit method (MC) and the Coulter Counter hematocrit (CC) for a comprehensive comparison of the three methods. The reliability of the three methods was assessed by computing the standard error of the measurement (SEm) on triplicate readings of human blood samples. The SEm for MC, CC, and DC methods were 0.3555, 0.3004, and 0.1491, respectively. Validity was assessed by comparing the average of the triplicate HCT readings for each method with HCT determined by densitometry. Average HCT values (+/- SE) for the MC, CC, DC, and densitometry methods were 42.3 +/- 3.2, 42.4 +/- 3.4, 43.1 +/- 3.4, 43.7 +/- 3.3, respectively. Only the DC HCT values were not different from the densitometry HCT (P > 0.05). The MC and CC values were significantly lower (P < 0.01). It is concluded that accurate, highly precise measurements of HCT are obtainable using the digital caliper. The significance for this is the increased ability to perform extremely accurate measurement of changes in plasma volume. With this information, research labs can reliably measure smaller changes in plasma than was previously possible with commercial procedures.

Hypervolemia and cycling time trial performance.

Ten experienced cyclists rode three simulated time trials to determine whether hypervolemia was associated with improvements in cycling time trial performance. The conditions were: exercise-induced hypervolemia (ExH), dextran-induced hypervolemia (DxH), and euvolemia (Eu). ExH was induced by 3 d of submaximal cycling lasting an average of 92.9 min at an average relative intensity of 68%. DxH was induced by acute plasma volume expansion with 400 +/- 121 ml of a 6% dextran solution. Compared with Eu, ExH and DxH were associated with 9.4% and 8.7% elevations in blood volume as well as 11.1% and 12.4% elevations in plasma volume, respectively. Performance was significantly improved (P < 0.05) (i.e., target work goal reached earlier) during ExH (81.41 +/- 5.52 min) and DxH(81.36 +/- 5.06 min) than during Eu (90.87 +/- 5.27 min). Average power was significantly higher during E x H (246 +/- 13 W) and DxH (245 +/- 14 W) than during Eu (221 +/- 15 W). There were no significant differences in performance time or average power between the two hypervolemic conditions. Average sweat rates were significantly elevated during ExH (22.6 +/- 1.4 ml.min-1) and DxH (22.2 +/- 1.6 ml.min-1) than during Eu (20.4 +/- 1.7 ml.min-1). Rectal temperatures rose from approximately 37.2-39.2 degrees C during each time trial but there were no significant differences in T(re) between trials. In conclusion, hypervolemia, whether induced by short-term training or dextran-infusion, had a beneficial effect on performance and average power during simulated time trials lasting approximately 90 min. These improvements in performance were related to hypervolemia rather than other short-term training adaptations.