Replacement of body fluid losses

The body loses more water than electrolytes during heavy sweating. This raises the osmotic pressure in the body fluids because the electrolytes become more concentrated. The need to replace body water is greater than the need for electrolytes because only by replenishing water content can the electrolytes return to normal concentrations. But how does the body know when this is necessary?

Thirst

When people feel thirsty, they drink. The thirst sensation is regulated by the hypothalamus. It triggers thirst when the plasma’s osmotic pressure is increased. Unfortunately, the body’s thirst mechanism doesn’t precisely gauge its state of dehydration. It does not sense thirst until well after dehydration begins. Even when dehydrated, people might desire fluids only at intermittent intervals.

The control of thirst is not fully understood. When permitted to drink water as their thirst dictates, people can require 24 to 48h to completely replace water lost through heavy sweating. In contrast, dogs and burros can drink up to 10% of their total body weight within the first few minutes after exercise or heat exposure, replacing all lost water. Because of our sluggish drive to replace body water and to prevent chronic dehydration, we are advised to drink more fluid than our thirst dictates. Because of the increased water loss during exercise, it is imperative that athletes’ water intake be sufficient to meet their bodies’ needs, and it is essential that they rehydrate during and after an exercise bout.

Benefits of fluids during exercise

Drinking fluids during prolonged exercise, especially in hot weather, has obvious benefits. Water intake will minimize dehydration, increases in body temperature, cardiovascular stress, and declines in performance. As seen in the figure below, when subjects became dehydrated during several hours of treadmill running in the heat(40°C, or 104°F) without fluid replacement, their heart rates increased steadily throughout the exercise. When they were deprived of fluids, the subjects became exhausted and couldn’t complete the 6h exercise. Ingesting either water or a saline solution in amounts equal to weight loss prevented dehydration and kept subjects’ heart rates lower. Even warm fluids(near body temperature) provide some protection against overheating, but cold fluids enhance body cooling because some of the deep body heat is used to warm cold drinks to body temperature.

Hyponatremia

Fluid replacement is beneficial, but too much of a good thing could potentially be bad. In the 1980s, the first cases of hyponatremia were reported in endurance athletes. Hyponatremia is clinically defined as a serum sodium concentration below the normal range of 135 to 145 mmol/L. Symptoms of hyponatremia generally appear when serum sodium levels drop below 130 mmol/L. Early signs and symptoms include bloating, puffiness, nausea, vomiting, and headache. As the severity increases, due to increasing cerebral edema(swelling of the brain), the symptoms include confusion, disorientation, agitation, seizures, pulmonary edema, coma, and death. How likely is hyponatremia to occur?

The processes that regulate fluid volumes and electrolyte concentrations are highly effective, so consuming enough water to dilute plasma electrolytes is difficult under normal circumstances. Marathoners who lose 3 to 5 L of sweat and drink 2 to 3 L of water maintain normal plasma concentrations of sodium, chloride, and potassium. And distance runners who run 25 to 40 km(15.5-24.9 mi) per day in warm weather and do not salt their food don’t develop electrolyte deficiencies.

Some research has suggested that during ultramarathon running(more than 42 km, or 26.2 mi), athletes can experience hyponatremia. A case study of two runners who collapsed after an ultramarathon race(160km, or 100 mi) in 1983 revealed that their blood sodium concentrations had decreased from a normal value of 140 mmol/L to values of 123 and 118 mmol/L. One of the runners experienced a grand mal seizure; the other became disoriented and confused. Examining the runners’ fluid intakes and estimating their sodium intakes during the run suggested that they had diluted their sodium contents by consuming too much fluid that contained too little sodium.

The ideal resolution to prevent hyponatremia would be to replace water at the exact rate at which it is being lost or to add sodium to the ingested fluid. The problem with the latter approach is that the most sport drinks contain no more than 25 mmol/L of sodium and are apparently too weak to prevent sodium dilution alone, but very strong concentrations cannot be tolerated. Exercise hyponatremia appears to be the result of a fluid overload due to overconsumption, underreplacement of sodium losses, or both. Only a small number of cases have been reported. Thus, it is probably inappropriate to form conclusions from this information to design a fluid replacement regimen for people who must exercise for long periods in the heat.