Experimental models for the investigation of water and solute transport in man. Implications for oral rehydration solutions

Effect of beverage glucose and sodium content on fluid delivery

BACKGROUND

Rapid fluid delivery from ingested beverages is the goal of oral rehydration solutions (ORS) and sports drinks.

OBJECTIVE

The aim of the present study was to investigate the effects of increasing carbohydrate and sodium content upon fluid delivery using a deuterium oxide (D2O) tracer.

DESIGN

Twenty healthy male subjects were divided into two groups of 10, the first group was a carbohydrate group (CHO) and the second a sodium group (Na). The CHO group ingested four different drinks with a stepped increase of 3% glucose from 0% to 9% while sodium concentration was 20 mmol/L. The Na group ingested four drinks with a stepped increase of 20 mmol/L from 0 mmol/L to 60 mmol/l while glucose concentration was 6%. All beverages contained 3 g of D2O. Subjects remained seated for two hours after ingestion of the experimental beverage, with blood taken every 5 min in the first hour and every 10 min in the second hour.

RESULTS

Including 3% glucose in the beverage led to a significantly greater AUC 60 min (19640 ± 1252 δ per thousand vs. VSMOW.60 min) than all trials. No carbohydrate (18381 ± 1198 δ per thousand vs. VSMOW.60 min) had a greater AUC 60 min than a 6% (16088 ± 1359 δ per thousand vs. VSMOW.60 min) and 9% beverage (13134 ± 1115 δ per thousand vs. VSMOW.60 min); the 6% beverage had a significantly greater AUC 60 min than the 9% beverage. There was no difference in fluid delivery between the different sodium beverages.

CONCLUSION

In conclusion the present study showed that when carbohydrate concentration in an ingested beverage was increased above 6% fluid delivery was compromised. However, increasing the amount of sodium (0-60 mmol/L) in a 6% glucose beverage did not lead to increases in fluid delivery.

Animal and human models for studying effects of drugs on intestinal fluid transport in vivo

The understanding of the physiology and pathophysiology of intestinal fluid transport has been derived from animal and human models of normal and perturbed intestines. This understanding helped in designing drugs and changing the composition of oral rehydration solutions in a targeted manner to affect intestinal fluid absorption/secretion that was tested both in vitro and in vivo before embarking on clinical trials. In this review, in vivo techniques used to study water transport in both animal and human models are described. In particular, steady state intestinal perfusion techniques, closed segment techniques, fistulous animal models, balance study models, enteropooling models, and isotope tracer models are reviewed. Advantages and drawbacks of each technique and examples where drug effects have been studied in a particular model are provided.

Gastric emptying and fluid availability after ingestion of glucose and soy protein hydrolysate solutions in man

The double sampling gastric aspiration method was used to measure the effect of energy content on the rate of gastric emptying of glucose and soy protein hydrolysate solutions. The net rate of absorption of water from these solutions was assessed using deuterium oxide as a tracer for water. Six healthy male subjects were each studied on four separate occasions using a test drink volume of 600 ml. The half emptying times (t 1/2, median (range)) of the iso-energetic soy protein hydrolysate (6P, 60 g l(-1), 36 (14-39) min) and glucose (7G, 70 g l(-1), 25 (19-29) min) solutions were similar. These two solutions (6P, 7G) delivered energy to the small intestine at similar rates, and resulted in similar rates of accumulation of the deuterium tracer in the circulation. The dilute glucose solution (LG, 23 g l(-1)) was emptied faster (t 1/2 13 (11-19) min) and resulted in a faster rate of tracer accumulation in the circulation than any of the other solutions, including the iso-osmotic soy protein solution (LG 311 +/- 5 mosmol kg(-1), 6P 321 +/- 24 mosmol kg(-1)). The concentrated soy protein hydrolysate solution (12P, 120 g l(-1)) emptied more slowly (t 1/2 80 (44-120) min) than the more dilute solutions. The rate of energy delivery to the small intestine from 12P was similar to that from 6P for the first 50 min after ingestion, and similar to that from 7G at all sample points. These results indicate that the iso-energetic solutions of glucose and soy protein hydrolysate used in this study are emptied from the stomach at similar rates and result in similar rates of fluid availability after ingestion.

Functional foods and food supplements for athletes: from myths to benefit claims substantiation through the study of selected biomarkers

The development of the sports food market and industrial involvement have led to numerous nutritional studies to define the type of nutrients that are most suited to support energy metabolism, fluid balance and muscle function. The key question in many of these studies was: 'Does the product lead to a significant product/consumer benefit that can be used as a claim on the package?' New methods and techniques have been developed, partly with sponsorship of the food industry, with the goal of measuring the effects of specific nutrients and supplements on athletic performance and metabolism. In line with this development, a wide variety of supplements and sports foods/drinks labelled with various performance or health benefit statements have been launched on the sports nutrition market. Although a variety of products have been tested clinically, there are also many products on the market with benefit claims that cannot be supported by sound nutritional and sports physiological science. The current short review highlights some of the methods and biomarkers that are used to substantiate product/consumer benefit claims for foods and drinks that are marketed as functional foods for athletes.

Gastric emptying of oral rehydration solutions at rest and after exercise in horses

We examined the gastric emptying (GE) of oral rehydration solutions (ORS) at rest and after exercise in four Standardbred horses. In one study isotonic, cold isotonic (5 degrees C), isotonic containing glucose and hypertonic fluid were tested at rest. In another study, isotonic fluid was given following a bout of treadmill exercise at 70 per cent VO2 max until exhaustion or at rest. In both studies, a single dose of 8 litres was given via nasogastric tube. GE and electrolyte concentrations (Na+, K+ and Cl-) of the stomach content were measured at 15 minutes intervals for one hour. In both studies, 90 per cent of the fluid was emptied within 15 minutes of administration. There was no treatment effect on electrolyte concentrations in either study but significant changes did occur over time. The data showed that GE is unlikely to significantly affect rehydration.

Exercise in the heat: strategies to minimize the adverse effects on performance

Exercise in the heat is usually associated with reduced performance; both dehydration and hyperthermia adversely affect mental and physical performance. For athletes from temperate climates, the negative effects of heat had humidity can be attenuated by a period of acclimatization. This requires up to 10-14 days. Endurance-trained individuals already show some of the adaptations that accompany acclimatization, but further adaptation occurs with training in the heat. Prior dehydration has a negative effect even on exercise of short duration where sweat losses are small. The athlete must begin exercise fully hydrated and regular ingestion of fluids is beneficial where the exercise duration exceeds 40 min. Dilute carbohydrate-electrolyte (sodium) drinks are best for fluid replacement and also supply some substrate for the exercising muscles. Post-exercise rehydration requires electrolyte as well as volume replacement. In extreme conditions, neither acclimatization nor fluid replacement will allow hard exercise to be performed without some risk of heat illness.

The hyponatremia of exercise

The hyponatremia of exercise may exist in symptomatic and asymptomatic forms. Symptomatic hyponatremia is usually characterized by severe alterations in cerebral function including coma and grand mal seizures; it develops especially in less competitive athletes who have maintained high rates of fluid intake during endurance events lasting at least 5 hours. The hyponatremia becomes symptomatic when the volume of excess fluid retained exceeds 2 to 3 liters. The etiology of the condition is unknown. Possibly as many as three or more pathologies (abnormal fluid retention possibly due to inappropriate ADH secretion, abnormal regulation of the extracellular fluid volume, translocation of sodium into a "third space") must be present for symptomatic hyponatremia to develop. The avoidance of overhydration would appear to be the only certain way that susceptible individuals can prevent symptomatic hyponatremia. Sodium chloride containing solutions ingested in physiologically significant concentrations would likely prevent a possible "third space" effect.

Fluid replacement and exercise stress. A brief review of studies on fluid replacement and some guidelines for the athlete

Fluid ingestion during exercise has the twin aims of providing a source of carbohydrate fuel to supplement the body's limited stores and of supplying water and electrolytes to replace the losses incurred by sweating. Increasing the carbohydrate content of drinks will increase the amount of fuel which can be supplied, but will tend to decrease the rate at which water can be made available; where provision of water is the first priority, the carbohydrate content of drinks will be low, thus restricting the rate at which substrate is provided. The composition of drinks to be taken will thus be influenced by the relative importance of the need to supply fuel and water, this in turn depends on the intensity and duration of the exercise task, on the ambient temperature and humidity, and on the physiological and biochemical characteristics of the individual athlete. Carbohydrate ingested during exercise appears to be readily available as a fuel for the working muscles, at least when the exercise intensity does not exceed 70 to 75% of maximum oxygen uptake. Carbohydrate-containing solutions appear to be more effective in improving performance than plain water. Water and electrolytes are lost form the body in sweat: although the composition of sweat is rather variable, it is invariably hypotonic with respect to plasma. Sweat rate is determined primarily by the metabolic rate and the environmental temperature and humidity. The sweat rate may exceed the maximum rate of gastric emptying of ingested fluids, and some degree of dehydration is commonly observed. Excessive replacement of sweat losses with plain water or fluids with a low sodium content may result in hyponatraemia. Sodium replacement is essential for postexercise rehydration. The optimum frequency, volume and composition of drinks will vary widely depending on the intensity and duration of the exercise, the environmental conditions and the physiology of the individual. The athlete must determine by trial and error the most suitable regimen.

Fluid and electrolyte loss and replacement in exercise

Prolonged exercise leads to a progressive water and electrolyte loss from the body as sweat is secreted to promote heat loss. The rate of sweating depends on many factors and is increased in proportion to the work rate and the environmental temperature and humidity. Sweat rate is highly variable between individuals, and can exceed 21 h-1 for prolonged periods. Since it is established that dehydration will impair exercise capacity and can pose a risk to health, the intake of fluid during exercise to offset sweat loss is important. Fluid intake is also aimed at providing a source of substrate, usually in the form of carbohydrate. The availability of ingested fluids may be limited by gastric emptying or by intestinal absorption. Gastric emptying of liquids is slowed by the addition of carbohydrate in proportion to the carbohydrate concentration and osmolality of the solution. With increasing glucose concentration, the rate of fluid delivery to the small intestine is decreased, but the rate of glucose delivery is increased. Water absorption in the small intestine is a passive process and is stimulated by the active absorption of glucose and sodium. The optimum fluid for rehydration during exercise depends on many factors, particularly the intensity and duration of the exercise, the environmental conditions, and the individual physiology of the athlete. There is no advantage to fluid intake during exercise of less than 30 min duration. The composition of fluids to be used will depend on the relative needs to replace water and to provide substrate. Where rehydration is a priority the solution should contain some glucose and sodium and should not exceed isotonicity: this will require the glucose concentration to be low (20-309 g l-1) or the substitution of glucose polymers, and the sodium content to be high (perhaps as much as 60 mmol l-1). Where substrate provision is more important, a more concentrated solution, incorporating large amounts of glucose polymers in concentrations of 150-200 g l-1, is to be preferred. To minimize the limitation imposed by the rate of gastric emptying, the volume of fluid in the stomach should be kept as high as is comfortable by frequent ingestion of small amounts of fluid. Addition of sodium, and perhaps also of potassium, may be important for rehydration after exercise.