Fluids and Electrolytes

Perhaps the single most important factor associated with sustaining a high level of athletic performance is maintenance of fluid balance during exercise. Despite this, most athletes experience deterioration in hydration state (with a resultant drop in blood volume) during training and competition. Studies demonstrate that, even in the presence of available fluids, athletes experience a degree of voluntary dehydration that has an inevitably negative impact on performance. Given the tremendous amount of heat that must be dissipated during exercise through sweat evaporation, athletes have no reasonable alternative for sustaining exercise performance other than to pursue strategies that can sustain the hydration state. A failure to do so will result in premature fatigue and may also lead to potentially life-threatening heatstroke. This chapter discusses the strategies related to achieving and sustaining an optimal hydration state and reviews studies that have assessed the optimal concentration of carbohydrate and electrolytes for the "ideal" sports beverage.

Water is the main component of blood, which delivers oxygen, nutrients, hormones, and a multitude of other substances to cells and removes metabolic by-products from cells. Water also has a protective function, cushioning the spinal cord and brain from sudden-impact injury, and is a critical component of our temperature regulation mechanism. Water and its electrolyte components are involved in the control of osmotic pressure, regulating the amount of fluid inside and outside cells.

Well-hydrated athletes are referred to as being euhydrated or normohydrated; those with below-normal body water levels are referred to as hypohydrated or, if severe, dehydrated; and those with above-normal body water levels are referred to as hyperhydrated. We have systems for the normal control of body water levels, involving an increased retention of body water or an increased loss of body water, all mediated through a series of hormones stimulated by osmoreceptors that monitor blood osmolality and volume receptors that monitor the volume of extracellular water.

Excretion of fluids and metabolic by-products is a main function of the kidneys, which are stimulated by hormones and enzymes to adjust the volume of water and electrolytes excreted or retained. The concentration of sodium is a primary influence on the osmolality of extracellular fluid, which is maintained within a narrow range. Because sweat is hypotonic, prolonged exercise results in a higher plasma osmolality (more water is lost than sodium). As a means of preserving body water volume, urine production during and shortly after exercise is slightly decreased.^{1, 2}

If the blood has a relatively high concentration of sodium, protein, or glucose per unit volume of fluid (i.e., is hypertonic), water is drawn from cells to normalize the concentration of electrolytes. Receptors in the hypothalamus detect the fact that the blood is hypertonic through its osmoreceptors, which leads to the release of antidiuretic hormone (ADH) from the pituitary gland. ADH forces the kidneys to reabsorb more water by producing a more concentrated urine.³ For this reason, a common test for adequate hydration status is urine color, with dark urine indicating a greater degree of underhydration than light urine. The osmoreceptors can also induce the sensation of thirst, although this sensation rarely occurs before the loss of 1.5 to 2.0 liters of water (1 liter equals approximately 1 quart;. Since it is nearly impossible for athletes to consume sufficient fluids during physical activity to maintain the body water level, waiting for the thirst sensation before drinking fluids guarantees that the athlete will be exercising in a progressively worsening state of underhydration.

Table 3.1 Where's the Water?

• 66% of a person's total body weight is from water.
• 65% of total body water is intracellular.
• 35% of total body water is extracellular.
• Well-hydrated muscles are about 75% water.
• Bones are about 32% water.
• Fat is essentially anhydrous, having only about 10% water content.
• Blood is about 93% water.
• Average males are about 60% water weight.
• Average females are about 50% water weight.
• Obese individuals are about 40% water weight.
• Athletes are about 70% water weight.

Note: The higher the musculature and the lower the body fat, the higher the contribution of body water to total body mass.

Table 3.2 Common Conversions

• To convert Fahrenheit to Celsius, subtract 32 degrees and divide by 1.8.
• To convert Celsius to Fahrenheit, multiply by 1.8 and add 32 degrees.
• To convert quarts to liters, multiply quarts by .946.
• To convert liters to quarts, multiply liters by 1.057.

In a state of hyperhydration, the concentrations of electrolytes, protein, and glucose are lower than normal in the blood. This condition shuts down the production of ADH so that diluted urine is produced. Fluid tends to migrate from blood to cells to adjust for this hypotonic state.

Blood volume is affected by the concentration of sodium, the main extracellular electrolyte. A high sodium concentration is associated with an eventual enlargement of the blood volume, which results from the body's attempt to normalize the concentration of sodium per unit of fluid volume. The reverse situation, a low sodium concentration, is typically associated with an eventual reduction of the blood volume. To adjust for the natural variations in sodium intake, the hormone aldosterone is produced to retain more sodium in a low-sodium environment, and aldosterone production ceases when sodium concentrations are high so as to cause the excretion of more sodium.

Under normal circumstances, the combination of volumetric controls, osmo receptors, antidiuretic hormone, and aldosterone maintains a relatively steady blood volume even with variations in fluid and sodium consumption. Exercise leads to an increased production of ADH and aldosterone, both of which conserve body water and sodium. This system is sufficiently effective that fluid deficiencies leading to physiological problems are rare, even in athletes. However, exercise at high intensity or of long duration (or both), particularly in a hot and humid environment, places the athlete at hydration risk because fluid loss (through sweat) may exceed the athlete's capacity to consume and absorb fluids. This can lead to a progressive reduction in blood volume, a reduced sweat rate, and other problems that negatively affect performance and health; therefore, it is important for athletes to always maintain fluid balance.

Table 3.3 Benefits of Maintaining Fluid Balance

Maintaining fluid balance during exercise helps sustain athletic performance through the following:

• Attenuation of increased heart rate
• Attenuation of increased core temperature
• Improvement in stroke volume
• Improvement in cardiac output
• Improvement in skin blood flow
• Attenuation of higher plasma sodium, osmolality, and adrenaline
• Reduction in net muscle glycogen usage

A Balance of Fluid Loss and Intake

Athlete Conditioning, Adaptation, and Age

Hydration Strategies