Sodium Balance and Exercise

INTRODUCTION Dietary sodium consumption has been the focus of much scientific and media attention, primarily because of the health-related implications associated with a chronically high dietary sodium intake (1). Yet sodium is a vital nutrient, and not surprisingly, the healthy body is well equipped to precisely regulate its sodium content. In times of sodium surplus or deficit, renal regulation of sodium and water balance helps ensure the preservation of extracellular and intravascular fluid volumes, as well as the maintenance of blood pressure. In sedentary individuals not exposed to heat stress, daily sodium losses are typically quite small, with urinary output the primary avenue for sodium loss. Depending upon sodium intake, sodium in the urine can range from 15 to 250 mmol·d−1, amounting to 0.345 to 5.75 g of sodium (2). This amount of sodium is easily acquired in the diet of most individuals; survey data indicate that the sodium content of American and Canadian diets usually exceeds 5.8 g·d−1 (2). In 2004, the U.S. National Academy of Sciences Food and Nutrition Board established the adequate intake (AI) standard for sodium in adults as 1500 mg·d−1 (equivalent to 3.8 g of table salt), a value designated to reflect normal urinary sodium losses as well as modest sweat sodium losses. They also suggest an upper limit sodium intake of 5800 mg·d−1, based upon the suggestion that high sodium intakes are associated with hypertension in some individuals. While the AI guidelines indicate that highly active individuals who lose large volumes of sweat on a regular basis will require higher sodium intakes to replace their losses, no specific guidance is provided. When sweating is profuse - as a result of physical activity, high environmental heat exposure, or both - daily sodium losses can be prodigious, especially in those with high sweating rates or high sweat sodium concentrations. For example, an athlete who excretes sweat containing 40 mmol of sodium per liter of sweat (an average value) will excrete 1840 mg of sodium by losing 2 L of sweat per day. In some athletes, sodium losses in sweat can exceed 3000 mg·h−1 (3), twice the dietary AI for sodium. Sodium concentration in sweat can range from 20 to over 80 mmol·L−1, depending upon an athlete's genetics, aerobic fitness, and state of acclimatization (3). Some athletes can lose sweat at rates exceeding 2 L·h−1, predisposing them to large sodium losses. In one documented case, a triathlete lost 13 g of sodium in one 4-hour exercise session (4). In the summer of 2007, the Gatorade Sports Science Institute (GSSI) convened a meeting of international scientists and clinicians with expertise in the area of sodium balance and exercise. The purpose of the meeting was to address a variety of issues related to sodium balance in athletes and other individuals who may regularly experience large sweat sodium losses. The articles in this supplement issue of Current Sports Medicine Reports are based upon the proceedings of that 2007 summer GSSI meeting held in Vail, Colorado. In this supplement, the authors address key questions related to sodium balance and exercise, including the following: How does the body regulate its sodium content? Can sodium be stored in the body without osmotic consequences? How does sodium intake affect thirst and cardiovascular function? What determines the sodium content of sweat? How do perturbations in sodium and fluid balance affect health and performance? What are the root causes of those perturbations, and how can they be prevented? How great are the risks of sodium- and fluid-related imbalances? How can these risks be minimized? What is the most effective way of replacing sodium and fluid losses? Sodium is a critical nutrient for all humans, but especially so for those who sweat on a regular basis. These articles illustrate the vital importance of sodium in maintaining voluntary drinking, plasma sodium concentration, vascular volume, total body water, and cardiovascular function during rest and physical activity. Accordingly, dietary sodium intake should match daily sodium loss - low during those periods when sweat loss is low, and elevated when sweat loss is high. Precision in this regard typically is not required because the body's sodium-regulatory responses will conserve sodium over time when intake is inadequate and excrete sodium in times of surplus. However, acute sodium deficits may create more pressing needs in some athletes, when large sodium losses compromise hydration status, increase the risk of severe muscle cramps, and predispose to hyponatremia.

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