DIETARY-SODIUM AND PLASMA-VOLUME LEVELS WITH EXERCISE

Sodium is the major cation of the extracellular fluid and has a potent influence on fluid movement. Sodium has been likened to a sponge that draws fluids into the extracellular space, including the plasma volum e, to equalise gradients in concentration. Conventional wisdom suggest s limiting dietary intake of Na+ to decrease risk of hypertension. How ever, there are some extreme occupational or exercise-related conditio ns where sweat losses are great and Na+ losses may exceed normal dieta ry intake. This can occur acutely such as in an ultra-endurance event or chronically as in hard manual work in the heat. In such cases, addi tional Na+ in the form of a higher Na+ diet or adding Na+ to beverages used for fluid replacement may be warranted. A higher Na+ diet also a ppears to accelerate the cardiovascular and thermoregulatory adaptatio ns that accompany heat acclimation or short term exercise training. Sa line ingestion before exercise causes an expansion of plasma volume at rest and throughout the subsequent exercise bout. This expansion of p lasma volume alters cardiovascular and thermoregulatory responses to e xercise in ways that may lead to beneficial changes in endurance exerc ise performance. Plasma volume expansion also occurs with saline infus ion during exercise, but exercise performance advantages have yet to b e reported. The purpose of this article is to review the literature co ncerning dietary sodium and its influence on fluid balance, plasma vol ume and thermoregulation during exercise. It contains 2 major sections . First, we will discuss manipulations in daily Na+ intake initiated b efore or throughout an exercise regime. Second, Eve will examine studi es where an acute Na+ load was administered immediately before or duri ng an exercise trial. The dependent Variables that we will discuss per tain to: (i) body water compartments, i.e. plasma volume; (ii) thermor egulatory variables, i.e. core temperature and sweat rate; (iii) cardi ovascular variables, i.e. heart rate and stroke volume; and (iv) perfo rmance, i.e. time trial performance and time to exhaustion. Particular attention will be given to the route by which Na+ was administered. t he environmental conditions. the level of acclimation of the participa nts and specifics relating to Na+ administration such as the osmolalit y of the Na+-containing beverage.