Carl V. Gisolfi

Blood and urinary measures of hydration status during progressive acute dehydration

PURPOSE To determine whether: a) plasma osmolarity (Posm) is sensitive to small incremental changes in hydration status, b) urine specific gravity (Usg) can accurately identify a state of euhydration, c) Usg is a sensitive indicator of a change in hydration status, and d) Usg correlates with Posm. METHODS Euhydrated (Posm = 288 +/- 4 mOsm.L-1) subjects (N = 12) were dehydrated by 5% of their body weight via exercise in the heat (40 degrees C, 20% RH). Posm, urine osmolarity (Uosm), and Usg were measured at 1%, 3%, and 5% dehydration, and 30 and 60 min of recovery (rec). Subjects consumed water in recovery equal to their loss of body weight. RESULTS Posm increased incrementally with each successive increase in percent body weight loss (%BWL). Usg was not significantly different from baseline until 3% BML. Uosm was not significantly different from baseline until 5% BWL. Usg correlated moderately (r = 0.46, P > 0.10) with Posm but reasonably well (r = 0.68, P < 0.02) with Uosm. CONCLUSIONS Posm accurately identifies a state of euhydration and is sensitive to changes in hydration status during acute dehydration and rehydration. Usg and Uosm are also sensitive to changes in hydration status but lag behind during periods of rapid body fluid turnover and therefore correlate only moderately with Posm during acute dehydration.

Guidelines for optimal replacement beverages for different athletic events.

During exercise, especially endurance exercise in the heat, vital body fluids and essential ions are lost in sweat, and the body may deplete its glycogen stores. Unless essential body fluids are restored, these conditions can produce hypovolemia, hypoglycemia, hyponatremia, hyperthermia, and dehydration. Performance may also be reduced. This paper briefly reviews pertinent aspects of gastric emptying and intestinal absorption and proposes the formulation of beverages for ingestion both pre-event and during exercise. For events lasting less than 1 h, 300-500 ml of a 6-10% CHO beverage is recommended pre-event (0-15 min), and cool (5-15 degrees C) water in a volume approximately half the subjects sweat rate is recommended during exercise. For events between 1-3 h long, 300-500 ml of water is recommended pre-event, and 800-1600 ml.h-1 of a 6-8% CHO solution with 10-20 mEq Na+ is recommended during exercise. For events longer than 3 h, 300-500 ml of water is recommended pre-event, and 500-1000 ml.h-1 of a 6-8% CHO beverage with 20-30 mEq Na+ is recommended during exercise. In recovery, a beverage containing 5-10% CHO with 30-40 mEq Na+ should be ingested to achieve euhydration. A minimum of 50 g.h-1 of CHO should be ingested in the first 2 h to maximize glycogen repletion. Guidelines to follow in formulating these beverages are reviewed and the rationale for each formulation is provided.

Effects of carbohydrate type and concentration and solution osmolality on water absorption

We studied intestinal absorption of solutions containing either one (glucose, Glu, or maltodextrin, Mal) or two (fructose, Fru, and Glu or sucrose, Suc) transportable carbohydrate (CHO) substrates using segmental perfusion technique in eight healthy male subjects. These CHO were either free or directly transportable monosaccharides (Glu, Fru), bound as the disaccharide (sucrose, Suc), or as oligomers (maltodextrins, Mal). [CHO] was varied from 6% to 8% (120-444 mmol.1(-1)). All solutions contained low [Na+] (15-19 mEq) and [K+] (3-4 mEq). Solutions osmolalities varied from 165 to 477 mOsm.kg(-1). Osmolalities in the test segment ranged from 268 to 314 mOsm.kg(-1). The regression line of osmolality with water absorption differed for single as compared with multiple substrate solutions. The significantly different intercepts of these two regression lines suggest that solutions with multiple substrates produce greater water absorption at a given osmolality than those with one. Comparing all solutions, test segment solute flux (partial r = 0.69) was more important than mean osmolality (partial r = 0.32). In conclusion, solutions with multiple substrates stimulate several different solute absorption mechanisms yielding greater water absorption than solutions with only one substrate.

Thermal effects of prolonged treadmill exercise in the heat

ABSTRACTSix trained men 21–38 yrs old ran 19–29 km (1.5 to 2.5 hrs) approximately 75% of their Vo2 max on a level treadmill in the heat (33.5/21.5°C db/wb, wind velocity 36 m/min). Every 20 min while running they (a) drank 200 ml of 10°C water, (b) drank 200 ml of water at core body temperature, or

Splanchnic tissues undergo hypoxic stress during whole body hyperthermia

Exposure of conscious animals to environmental heat stress increases portal venous radical content. The nature of the observed heat stress-inducible radical molecules suggests that hyperthermia produces cellular hypoxic stress in liver and intestine. To investigate this hypothesis, conscious rats bearing in-dwelling portal venous and femoral artery catheters were exposed to normothermic or hyperthermic conditions. Blood gas levels were monitored during heat stress and for 24 h following heat exposure. Hyperthermia significantly increased arterial O2 saturation, splanchnic arterial-venous O2 difference, and venous PCO2, while decreasing venous O2 saturation and venous pH. One hour after heat exposure, liver glycogen levels were decreased approximately 20%. Two hours after heat exposure, the splanchnic arterial-venous O2 difference remained elevated in heat-stressed animals despite normal Tc. A second group of rats was exposed to similar conditions while receiving intra-arterial injections of the hypoxic cell marker [3H]misonidazole. Liver and intestine were biopsied, and [3H]misonidazole content was quantified. Heat stress increased tissue [3H]misonidazole retention 80% in the liver and 29% in the small intestine. Cellular [3H]misonidazole levels were significantly elevated in intestinal epithelial cells and liver zone 2 and 3 hepatocytes and Kupffer cells. This effect was most prominent in the proximal small intestine and small liver lobi. These data provide evidence that hyperthermia produces cellular hypoxia and metabolic stress in splanchnic tissues and suggest that cellular metabolic stress may contribute to radical generation during heat stress.

Effects of ingesting carbohydrate beverages during exercise in the heat.

Ingesting carbohydrate beverages during exercise in cool temperatures can improve endurance performance. However, because hyperosmotic solutions leave the stomach more slowly than water, carbohydrate beverages could be less effective in minimizing the dehydration and hyperthermia that accompany exercise in the heat. To determine the effect of osmotically different beverages on prolonged (2 h) treadmill exercise (65% VO2max) in the heat (T db = 35 degrees C), five male runners (age 24 to 41 yr) performed three separate runs drinking 200 ml every 20 min of either 10% glucose polymer (GP), 10% glucose (G), or saccharin-sweetened water (WH). A fourth run was performed in a cool (T db = 25 degrees C) room and included drinking saccharin-sweetened water (WC). Drink osmolalities (Osm) for runs GP, G, WH, and WC were 194, 586, 94, and 71 mmol . kg-1, respectively. No significant differences were observed between runs in the heat (GP, G, and WH) for heart rate, rectal and mean skin temperatures, sweat rate, percent change in plasma volume, and gastric residue volume. When compared to the WC run, both the GP and G runs yielded greater (P less than 0.05) declines in percent change in plasma volume, but only the G run had a greater (P less than 0.05) gastric residue volume. Neither plasma osmolality, total protein, nor [Na+] varied between runs. Plasma glucose, insulin, and respiratory exchange ratios were similar between the GP and G runs. However, the GP run yielded the lowest (P less than 0.05) plasma glycerol values. Although gastric residue volume (r = 0.68) and final percent change in plasma volume (r = 0.69) were significantly correlated with drink osmolality, thermoregulation was similar between runs in the heat despite the beverage consumed.

Effects of acute graded exercise on human colonic motility

Whether physical exercise stimulates colonic motility is unclear. Our aim was to determine the immediate effects of graded exercise on colonic motility. Colonic motility was recorded at six sites in 11 untrained subjects, by colonoscopically placing a solid-state probe. Subjects were free to ambulate. The next day, subjects exercised on a bicycle at 25, 50, and 75% of peak oxygen uptake for 15 min, with each followed by a 15-min rest. Motor patterns, motility indexes, and regional variations before, during exercise, during rest, and during postexercise periods were compared. During exercise, there was an intensity-dependent decrease ( P < 0.001) in the number and area under the curve of pressure waves. The incidence of propagated or simultaneous pressure waves and cyclical events also decreased ( P < 0.05). After exercise, the pressure activity reverted to baseline, but the number and amplitude of propagated waves increased ( P < 0.01), whereas the simultaneous waves and cyclical events remained lower. Acute graded exercise decreases colonic phasic activity. This may offer less resistance to colonic flow, whereas the postexercise increase in propagated activity may enhance colonic propulsion.Whether physical exercise stimulates colonic motility is unclear. Our aim was to determine the immediate effects of graded exercise on colonic motility. Colonic motility was recorded at six sites in 11 untrained subjects, by colonoscopically placing a solid-state probe. Subjects were free to ambulate. The next day, subjects exercised on a bicycle at 25, 50, and 75% of peak oxygen uptake for 15 min, with each followed by a 15-min rest. Motor patterns, motility indexes, and regional variations before, during exercise, during rest, and during postexercise periods were compared. During exercise, there was an intensity-dependent decrease (P < 0.001) in the number and area under the curve of pressure waves. The incidence of propagated or simultaneous pressure waves and cyclical events also decreased (P < 0.05). After exercise, the pressure activity reverted to baseline, but the number and amplitude of propagated waves increased (P < 0.01), whereas the simultaneous waves and cyclical events remained lower. Acute graded exercise decreases colonic phasic activity. This may offer less resistance to colonic flow, whereas the postexercise increase in propagated activity may enhance colonic propulsion.

Fluid and Carbohydrate Replacement During Intermittent Exercise

SummaryMost studies relating to fluid replacement have addressed the problem of drinking during prolonged exercise. Fluid replacement is also very important for intermittent exercise, although it has not been extensively studied. More studies in this area would help coaches and athletes understand the importance of fluid balance and carbohydrate supplementation during intermittent exercise. Based on available data, it can be concluded that: (i) because of high exercise intensity, sweat loss and glycogen depletion during intermittent exercise are at least comparable with those during continuous exercise for a similar period of time. Therefore, the need to ingest a sport drink or replacement beverage during intermittent exercise may be greater than that during continuous exercise in order to maintain a high level of performance and to help prevent the possibility of thermal injury when such activity occurs in a warm environment; (ii) the volume of ingested fluid is critical for both rapid gastric emptying and complete rehydration; and (iii) osmolality (250 to 370 mOsm/kg), carbohydrate concentration (5 to 7%), and carbohydrate type (multiple transportable carbohydrates) should be considered when choosing an effective beverage for rehydration and carbohydrate supplementation during intermittent exercise.

Gastrointestinal permeability following aspirin intake and prolonged running.

We sought to evaluate the effects of exercise and aspirin on gastroduodenal and intestinal permeability. Seven volunteers (age = 29 +/- 3 yr, VO2max = 56.8 +/- 4.1 ml.kg-1.min-1) rested or performed treadmill exercise (60 min at approximately 68% VO2max), with or without aspirin ingestion. Placebo (glucose) or aspirin (1.3 g) was taken the night before and prior to rest or exercise (total 2.6 g). A permeability test solution (approximately 1300 mOsm.kg-1), containing 10 g lactulose (L), 5 g mannitol (M), and 10 g sucrose (S), was ingested prior to rest or exercise. Urinary excretion rates (6.h-1), expressed as a percentage of ingested dose, were used to quantify intestinal (L/M ratio) or gastroduodenal (S) permeability. Ingestion of aspirin before running increased (P < 0.05) intestinal permeability compared to placebo+running and placebo+rest, but not compared to aspirin+rest; mean (+/-SE) values for the L/M ratio were 0.248 +/- 0.046, 0.029 +/- 0.012, 0.012 +/- 0.004, and 0.104 +/- 0.057, respectively. Gastroduodenal permeability following aspirin+running (3.25 +/- 1.21%) was also elevated (P < 0.05) compared to placebo+running (0.43 +/- 0.15%) and placebo+rest (0.24 +/- 0.11%), but not compared to aspirin+rest (0.66 +/- 0.27%). Neither running nor aspirin ingestion was associated with gastrointestinal (GI) complaints. Thus, GI permeability while running can be markedly elevated by aspirin ingestion.

Upper limit for intestinal absorption of a dilute glucose solution in men at rest.

We studied gastric and intestinal function by gastric intubation/intestinal perfusion in six healthy male volunteers to evaluate optimal use of a 6% glucose-electrolyte (GES) solution. Gastric volume, residual volume, emptying rate, and secretion were measured for an initial 763 +/- 19 ml gastric load of GES and at the beginning and end of four additional gastric loads (2.2 ml.kg-1; approximately 180 ml) given at 10-min intervals. The relatively high gastric (713 +/- 58 ml) and residual (507 +/- 26 ml) volumes maintained a high gastric emptying rate (19.5 +/- 1.4 ml.min-1). Composition of the GES emptied into the duodenum was also measured in this first experiment. In a second experiment, this modified solution was infused (triple lumen tube) into the duodenum at a rate equal to gastric emptying rate, or at 38 or 77% greater rates. Absorption of water (11.3-12.9 ml.h-1.cm-1) and glucose 4.3-5.6 mmol.h-1.cm-1) were similar at all perfusion rates during the second experiment. We conclude that duodenojejunal segmental absorption rates of water and glucose produced by a rapid, sustained gastric emptying rate cannot be increased by delivering a greater load of glucose and water by intestinal perfusion.