The Effects of Consuming Carbohydrate-Electrolyte Beverages on Gastric Emptying and Fluid Absorption During and Following Exercise

Summary

A variety of beverages formulated to provide fluid, carbohydrates, and electrolytes during and following exercise are commercially available. Such ‘sport drinks’ commonly contain 4 to 8% carbohydrate (as glucose, fructose, sucrose or maltodextrins) and small amounts of electrolytes (most often sodium, potassium, and chloride). The efficacy of consuming such beverages has been questioned primarily because of concern that beverage carbohydrate content may inhibit gastric emptying rate and fluid absorption during exercise, thereby jeopardising physiological homeostasis and impairing exercise performance.

Gastric motor activity, and consequently gastric emptying rate, is governed by neural and humoral feedback provided by receptors found in the gastric musculature and proximal small intestine. Gastric emptying rate may be influenced by a variety of factors including, but not limited to, the caloric content, volume, osmolality, temperature, and pH of the ingested fluid, diurnal and interindividual variation, metabolic state (rest/exercise), and the ambient temperature. The caloric content of the ingested fluid appears to be the most important variable governing gastric emptying rate, providing a mean caloric efflux from the stomach of 2.0 to 2.5 kcal/min for ingested fluid volumes less than 400ml.

At rest, gastric emptying is inhibited by solutions containing calories in a manner independent of the nutrient source (i.e. carbohydrate, fat or protein). Consequently, plain water is known to empty from the stomachs of resting subjects at rates faster than solutions containing calories. Gastric emptying is increasingly inhibited as the caloric content of the ingested fluid increases.

During moderate exercise (< 75% V̇_2max), gastric emptying occurs at a rate similar to that during rest; more intense exercise appears to inhibit gastric emptying. When fluids are consumed at regular intervals throughout prolonged exercise (> 2 hours), postexercise aspiration of stomach contents reveals that solutions containing up to 10% carbohydrate empty at rates similar to plain water.

There is ample physiological justification for the addition of glucose, fructose, sodium, potassium and chloride to fluid replacement beverages. Fluid absorption in the small intestine is stimulated by glucose and sodium (and to a lesser extent by fructose and other electrolytes). Glucose and sodium are absorbed via a common membrane carrier in the mucosal epithelium of the proximal small intestine. The potentiation of sodium uptake by glucose establishes an osmotic gradient for fluid absorption.

It is well established that the electrolyte requirements of most physically active individuals can be adequately met by consuming a balanced diet. However, fluid/electrolyte imbalances may occur during the early stages of acclimatisation to a hot environment, in physically active individuals on sodium restricted diets, during repeated exposure to exercise or heat, and during prolonged exercise (> 2 hours). Many commercially available carbohydrate-electrolyte beverages provide electrolytes in quantities that may reduce the risk of fluid/electrolyte imbalances. Yet, the electrolyte content of most carbohydrate-electrolyte beverages is sufficiently dilute that hyperelectrolytaemia in response to consuming even large quantities of these beverages is highly unlikely.

Cardiovascular and thermoregulatory function during prolonged exercise are as well maintained when subjects consume carbohydrate-electrolyte beverages as with plain water alone. Core temperature, mean skin temperature,.plasma volume, heart rate, plasma osmolality, and plasma electrolyte content change similarly during prolonged exercise in the heat whether subjects consume plain water or a carbohydrate-electrolyte beverage.

Performance, as assessed by exercise time to exhaustion or by time to complete a predetermined exercise task, has frequently been shown to be enhanced by carbohydrate feeding during prolonged exercise. Whether this ergogenic effect is linked to sparing of muscle and liver glycogen, maintenance of plasma glucose concentration or other factors remains to be determined.

The organoleptic properties of fluid replacement beverages are important and often overlooked elements in preventing dehydration. Voluntary fluid consumption during exercise in the heat is known to be greater with sweetened, flavoured beverages than with plain water.