Updated Fluid Recommendation: Position Statement From the International Marathon Medical Directors Association (IMMDA)

Abstract

Controversy exists regarding optimal fluid guidelines for athletes engaging in different sports. Most published recommendations emphasize the detrimental consequences of dehydration, while more recent reports warn of the morbid consequences of hyperhydration. Accordingly, individualized recommendations emphasizing a balance between the two extremes have evolved. These revised guidelines, however, continue to promote static recommendations for what in reality are very dynamic athletic situations. Marathon running epitomizes a dynamic situation that requires a constant adjustment to constantly changing homeostatic requirements. Real-time assessments of fluid and sodium homeostasis are physiologically represented by changes in plasma osmolality (POsm). Pituitary arginine vasopressin (AVP) secretion is stimulated when POsm increases by only 1% to 2%, representing the body’s attempt to prevent dehydration by decreasing kidney water excretion. After maximal antidiuresis is achieved, thirst is then stimulated to replace water losses that are in excess of the ability of AVP-stimulated antidiuresis to conserve body water. In general, the osmotic threshold for thirst is 5 to 10mOsm/g H2O higher than that of AVP secretion. Thus, thirst is stimulated with decreases in body water of approximately 1.7% to 3.5%. Concomitant performance decrements and cardiovascular strain are also documented when baseline body fluid losses exceed approximately 2%. The failure of athletes to replace 100% of bodyweight losses from ad libitum fluid intake has been well-described as ‘‘involuntary’’ or ‘‘voluntary’’ dehydration; this phenomenon has resulted in the devaluation of thirst as a ‘‘poor’’ indicator of body fluid needs. The combination of laboratory with recent field data (see Table 1 and subsequent discussion), however, suggests that the body primarily defends POsm, and not blood or extracellular fluid volume during prolonged endurance exercise. Because it is well established that thirst is stimulated in response to changes in tonicity in most land mammals, it seems reasonable to conclude that thirst would be the predominant physiologic and dynamic regulator governing fluid balance during exercise. This article will focus on the physiology of normal fluid balance as the ultimate guide toward the evolution of optimal fluid recommendations. Six physiologic considerations of fluid and sodium balance will be detailed followed by 6 practical recommendations. The neuroendocrine regulation of homeostasis will be emphasized in defense of the behavioral drives for thirst and sodium palatability during dynamic activities such as prolonged endurance exercise. The behavioral drives to maintain fluid and sodium balance are evolutionary stable, essential for the safety and survival of the species, and deeply rooted within the human genetic makeup.