By Dr. Gabe Mirkin

When you exercise in hot weather, you don’t need special sports drinks or power bars. A review of 46 studies shows that a person can prolong endurance by eating before and during exercise (Scand J Med Sci Sports, 2018 May;28(5):1476-1493). However, even the most elite athletes can get the nutrients they need from ordinary foods, water and salt. Healthy and fit people usually don’t need to drink or eat when they exercise at a casual pace for less than two hours. They can prolong their endurance by taking:

Mild Dehydration Does Not Impair Exercise Performance

Most competitive athletes can maintain their speed and efficiency by drinking when they feel thirsty (J Hum Kinet, 2021 Jul; 79: 111–122). Racers do not slow down until they have lost enough fluid to lose at least two percent of their body weight. For a 150-pound person, that’s at least three pints or three pounds of water. Losses greater than five percent of body weight can decrease work capacity by about 30 percent (Am J Clin Nutr, August 2000;72(2):564s-572s).

The Gatorade Sports Science Institute in Barrington, Illinois published a study showing that 46 percent of recreational exercisers are dehydrated (Applied Physiology, Nutrition and Metabolism, June 2006). However, the study did not say that they were harmed, with good reason. There is no data anywhere to show that mild dehydration affects health or athletic performance. People must lose a tremendous amount of fluid before it affects their performance (Medicine & Science in Sports & Exercise, October 2006). On average, a world-class marathon runner drinks less than a cup an hour during a race (a cup of water has 250 milliliters). This is far less than the amount formerly recommended by the American College of Sports Medicine. That amount could dilute blood salt levels to cause brain swelling. that could kill you. On the basis of our present knowledge, it may not be safe for mediocre athletes to take in more than 800 cc per hour (3.5 cups). Recent studies show that fit humans can tolerate significant fluid loss before their performance suffers, and that most cases of muscle cramps are not caused by dehydration or salt loss. They are caused by muscle or nerve damage and can be controlled by stopping exercise and stretching the cramped muscle.

Lack of Sugar

Low levels of sugar can cause muscle pain and weakness called “hitting the wall” that occurs in long distance runners, and confusion and passing out (called “bonking”) that occurs in cyclists. Your muscles use primarily sugar and fat for energy. You have an almost infinite amount of fat stored in your body, but you start to run out of sugar stored in your liver after 70 minutes of intense exercise. Bonking (in cyclists): There is only enough sugar in your bloodstream to last three minutes at rest. To maintain blood sugar levels, your liver constantly releases sugar into your bloodstream. However, there is only enough sugar stored in your liver and muscles to last about twelve hours at rest and less than 70 minutes when you exercise intensely. Your brain has almost no stored energy, so it gets almost all of its energy from the sugar carried to it in your bloodstream. When liver sugar levels drop, your blood sugar levels must also drop and your brain has lost its main source of energy. Your brain then cannot function normally and you feel weak, tired, and confused, and can even pass out. Hitting the Wall (in runners): Muscles have only a limited amount of sugar stored in their cells. When muscles run out of their stored sugar supply, they hurt as you exercise and become difficult to coordinate. A limiting factor to how fast you can move is the time it takes to bring oxygen into your muscles. Since sugar requires less oxygen than fat does to power your muscles, you will slow down when your muscle sugar and blood sugar levels start to drop.

Sugar before Competition or Intense Exercise

Take sugar no more than five minutes before you start your competition. Do not take sugar earlier than that because when you eat sugar and your muscles are not contracting, you can get a high rise in blood sugar that causes the pancreas to release large amounts of insulin. This can cause a drop in blood sugar levels that can tire you. On the other hand, exercising muscles draw sugar rapidly from the bloodstream without needing insulin, so taking sugar during exercise or just before you start usually does not cause the high rise in blood sugar levels.

During your competition or exercise session, take sugar before you feel hungry. Hunger during exercise is a very late sign of not getting enough calories. By the time you feel hungry, your body will be so depleted of sugar that you will have to stop or slow down so you can eat a lot of carbohydrate-rich food just to restore your sugar supplies. On days that are not too hot, your exercise performance will be harmed far earlier by lack of sugar than from lack of water. The rule of thumb is that athletes should take a source of sugar during all competitions lasting longer than an hour.

Lack of Salt

The only mineral that you need to take during prolonged exercise is sodium, found in regular table salt. Potassium, calcium and magnesium deficiencies almost never occur in healthy athletes (Medicine & Science in Sport & Exercise, October 1999). Just about everyone agrees that you need to take in extra salt during extended athletic competitions in hot weather. Salt is necessary to hold water in your body, prevent muscle cramps, and help keep your muscles contracting with great force. However, excess intake of salt may raise blood pressure and increase risk for heart attacks, particularly in people who have big bellies and high blood sugar levels. Most processed foods are full of extra salt, so people who eat the typical North American diet will almost always get all the salt they need from their food and should restrict adding extra salt from a salt shaker.

If you do not meet your needs for salt during extended exercise in hot weather, you will tire earlier and increase your risk for heat stroke, dehydration and cramps. During vigorous exercise lasting longer than three hours, you should eat salty foods such as salted nuts, peanuts or potato chips. Some sports drinks contain salt, but since salted drinks taste awful, the amount added is so small that it may not be enough to meet your needs. One study showed that you cannot replace all of the salt lost through exercise by drinking the sports drinks that contain salt, since they typically contain very little salt (British Journal of Sports Medicine, April 2006).

You Can Take In TOO MUCH Fluid

The American College of Sports Medicine formerly recommended fluid intake of 1200 cc (five cups or two average-size water bottles) per hour, but for people who are not exercising near their maximum, this can be too much (Clinical Journal of Sport Medicine, July-August 2005). People exercising near their capacity, and not slowed down by fatigue, probably do not have to worry about limiting fluid intake. They are working so hard at maintaining intensity that they do not have enough time to drink too much. On the other hand, people slowed down by fatigue or those who are out of shape should limit fluid intake, probably to no more than two water bottles per hour.

Some people develop hyponatremia (low salt) when they drink too much. It almost always occurs in people who attempt events that are beyond their training levels. They run out of energy, slow down and focus on drinking fluids instead of maintaining their pace. Hyponatremia is caused by drinking too much fluid, not by excessive loss of salt in sweat or by the stress of exercising. The extra fluid expands blood volume and dilutes blood salt levels. This causes blood salt levels to drop to low levels, while brain salt levels remain normal. Fluid moves from an area of low salt concentration into areas with high salt levels, so in hyponatremia, fluid moves from the bloodstream into the brain to cause brain swelling. Since the brain is enclosed in the skull, which is a tight box, the expanding brain has nowhere to go and the squashing can cause a headache, nausea and blurred vision. Since these are the same symptoms caused by pure dehydration with normal blood salt levels, the only way to diagnose the condition is with blood tests. As blood salt levels drop even lower, the person becomes confused, develops seizures and falls unconscious. You should suspect hyponatremia if the person has been exercising for a long time and has been drinking often. Under any circumstances, a person who is confused, has seizures or has passed out should be sent to a hospital immediately. Hyponatremia requires skilled management because the first impulse is to give intravenous fluids, which would dilute blood salt levels further to cause more brain swelling that could kill the patient.

My Recommendations

© Copyright 2024, Gabe Mirkin MD, all rights reserved.r>