During exercise, being appropriately hydrated contributes to performance. Water is needed to cool the body, transport oxygen and nutrients, and remove waste products from the muscles.  Water needs are increased during exercise due to the extra water losses experienced through evaporation and sweat. Dehydration can occur when there are inadequate water levels in the body and can be very hazardous to the health of an individual. As the severity of dehydration increases, the exercise performance of an individual will begin to decline (see Figure 13.9 “The effects of dehydration on exercise performance”). It is important to continue to consume water before, during, and after exercise to avoid dehydration as much as possible.

During exercise, thirst is not a reliable short-term indicator of the body’s needs as it is typically not enough to replace the water loss. Even with the constant replenishment of water throughout an exercise, it may not be possible to drink enough water to compensate for the losses. Dehydration occurs when the total loss of water is so significant that the total blood volume decreases, which leads to the reduction of oxygen and nutrients transported to the muscle cells. Decreased blood volume also reduces the blood flow to the skin and the production of sweat, which can increase the body temperature. As a result, the risk of heat-related illnesses increases.

Heat cramps are one of the heat-related illnesses that can occur during or after exercise. Heat cramps are involuntary muscle spasms that usually involve the muscle being exercised, and are caused by an imbalance of electrolytes, usually sodium. Heat exhaustion is caused by the loss of water decreasing the blood volume so much that it is not possible to cool the body or provide oxygen and nutrients to the active muscles. Symptoms that arise from heat exhaustion may include low blood pressure, disorientation, profuse sweating, and fainting. Heat exhaustion can progress further if exercise continues into a heat stroke. A heat stroke is the most serious form of heat-related illness that can occur. During a heat stroke, the internal body temperature rises above 105℉ which causes the brain’s temperature-regulatory center to shut down. When the brain’s temperature regulatory center shuts down, an individual cannot sweat despite their internal body temperature rising. Other symptoms that arise are dry skin, extreme confusion, and unconsciousness. A heat stroke requires immediate medical attention.

The external temperature during exercise can also play a role in the risk of heat-related illnesses. As the external temperature increases, it becomes more difficult for the body to dissipate heat. As humidity also increases, the body is unable to cool itself through evaporation. The Heat Index is a measure of how hot the body feels when humidity is added to the air temperature (see Figure 13.10 “The Heat Index”).

Hyponatremia

Sweating during exercise helps our bodies stay cool. Sweat consists mostly of water but it also causes losses of sodium, potassium, calcium and magnesium. During most exercises, the amount of sodium lost is very small. However, during long endurance exercises such as a marathon or triathlon, sodium losses are greater and must be replenished. If water is replenished without sodium, the sodium already present in the body will become diluted. These low levels of sodium in the blood will cause a condition known as hyponatremia (see Figure 13.11 “The effect of exercise on sodium levels”). When sodium levels in the blood decrease, water moves into the cell through osmosis, which causes swelling. Accumulation of fluid in the lungs and the brain can cause serious life-threatening conditions such as seizures, coma and death.

In order to avoid hyponatremia, athletes should increase their consumption of sodium in the days leading up to an event and consume sodium-containing sports drinks during their race or game. The early signs of hyponatremia include nausea, muscle cramps, disorientation, and slurred speech.  To learn more about sports drinks that can optimize your performance, refer to the chapter on water and electrolytes.

Fluid Intake Recommendations

The following recommendations were obtained from ^[1] and ^[2]

Before Exercise:

1. Prior to exercise, the individual should slowly drink beverages (for example, ~5-7 mL·kg−1 per body weight) at least 4 h before the exercise task.
2. If the individual does not produce urine, or the urine is dark or highly concentrated, the individual should slowly drink more beverage (for example, another ~3-5 mL·kg−1) about 2 h before the event.
3. Consume beverages with sodium (20-50 mEq·L⁻¹) and/or small amounts of salted snacks or sodium-containing foods

During Exercise:

1. The goal of drinking during exercise is to prevent excessive dehydration (>2% BW loss from water deficit) and excessive changes in electrolyte balance to avert compromised exercise performance. So drink early and regularly.
2. Predicted sweating rates range from ~0.4 to ~1.8 L·h⁻¹ depending on a number of factors, so predicting individual fluid volume intake is difficult. Therefore, weighing yourself before and after a range of activities in a variety of conditions can help you estimate fluid intake requirements.
3. For events lasting longer than 1 hr, aim for beverages containing 6-8% carbohydrates, 0.5-0.7 g of sodium, and 0.8-2.0 g of potassium per litre of water.

Following Exercise:

1. For every 1 kg of body weight lost during exercise, consume 1.5 L of fluid.
2. Consuming beverages and snacks with sodium will help expedite rapid and complete recovery.