NUTRITION IN SPORT

Introduction

Athletes encounter heat stress from climatic con-
ditions (e.g. temperature, humidity, solar load)
and body heat production. Depending on the
climatic conditions, the relative contributions of
evaporative and dry (radiative and conductive)
heat exchange to the total heat loss will vary. The
hotter the climate, the greater the dependence
on evaporative heat loss and, thus, on sweating.
Therefore, a substantial volume of body water
may be lost via sweating to enable evaporative
cooling in hot environments. In addition, physi-
cal exercise will elevate metabolic rate above
resting levels, and thus increase the rate at which
heat must be dissipated to the environment to
keep core temperature from rising to dangerous
levels. Environmental heat stress and physical
exercise interact synergistically and may push
physiological systems to their limits (Sawka et al.
1996b).
Climatic heat stress and physical exercise will
cause both fluid and electrolyte imbalances that
need to be re-established (Marriott 1993, 1994;
Convertino et al. 1996). Athletes performing
exercise in the heat often incur body water
deficits. Generally, athletes dehydrate during
exercise because of fluid non-availability or a
mismatch between thirst and body water

requirements (Greenleaf 1992). In these

instances, the athlete starts exercise euhydrated,

but incurs an exercise-heat-mediated dehydra-

tion over a prolonged period. This scenario is

common for many athletic and occupational set-

tings; however, there are several sports (e.g.

boxing, power lifting, wrestling) where athletes

will purposely achieve hypohydration prior to

competition.

This chapter reviews fluid balance in the heat

and the effects of hydration status on tempera-

ture regulation and physical exercise perfor-

mance. Throughout this chapter, euhydration

refers to normal body water content, hypohydra-

tionrefers to body water deficit, and hyperhydra-

tionrefers to increased body water content.

Fluid and electrolyte balance

An athlete’s sweating rate is dependent upon the

climatic conditions, clothing worn and exercise

intensity (Molnar et al. 1946; Shapiro et al. 1982).

Figure 16.1 provides a range of sweating rates

expected from running in different climatic con-

ditions (Sawka & Pandolf 1990). Athletes per-

forming high-intensity exercise commonly have

sweating rates of 1.0–2.5 l · h–1while in the heat.

These high sweating rates, however, are not

maintained continuously and are dependent

upon the person’s need to dissipate body heat.

Daily fluid requirements range (for sedentary to

active persons) from 2 to 4 l · day–1in temperate

climates and from 4 to 10 l · day–1in hot climates

(Greenleaf 1994). Clearly, hot weather and

Chapter 16

Effects of Dehydration and Rehydration

on Performance*

MICHAEL N. SAWKA, WILLIAM A. LATZKA

AND SCOTT J. MONTAIN

216

• The views, opinions and/or findings contained in this
  chapter are those of the authors and should not be con-
  strued as an official Department of Army position or
  decision, unless so designated by other official
  documentation.