Nutritional Needs for Hot Weather Exercise
You don’t need special sports drinks or power bars. 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. However, they can prolong their endurance by taking:
- a source of sugar when they race or exercise very intensely for more than an hour
- a source of water when they exercise intensely for more than a half hour in very hot weather or several hours in cooler weather
- a source of salt when they exercise for more than three hours
Mild Dehydration Does Not Impair Exercise Performance
Most competitive athletes can maintain their speed and efficiency by drinking when they feel thirsty. 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.
The Gatorade Sports Science Institute in Barrington, Illinois published a study showing that 46 percent of recreational exercisers are dehydrated. 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. A person must lose a tremendous amount of fluid before it affects his performance. On average, a world-class marathon runner drinks less than a cup an hour during a race. This is far less than the amount recommended by the American College of Sports Medicine just a few years ago. 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: 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 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 and lie on the ground unconscious.
Hitting the Wall: 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.
During Competitions Longer than an Hour, Take Caffeine with Sugar
Taking caffeine with sugar during athletic competitions can increase endurance and improve your performance (Medicine & Science in Sports & Exercise, July 2010). Caffeine works by increasing the absorption of sugar from your intestines and by increasing your exercising muscles’ uptake of sugar by as much as 26 percent (Journal of Applied Physiology, June 2006). However, taking sugar and caffeine when you are not exercising doubles your rise in blood sugar (Journal of Caffeine Research, April 16, 2011). High rises in blood sugar can damage every cell in your body to increase risk for weight gain, diabetes and heart attacks.
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 do not 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 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 used to recommend fluid intake of 1200 cc (five cups, 2.5 pints or 2 average-size water bottles) per hour, but for a person who is not exercising near his or her maximum, this can be too much. A person exercising near his capacity and not slowed down by fatigue probably does not have to worry about limiting fluid intake. He is working so hard at maintaining intensity, he doesn’t 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 fewer than two large 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 more than three hours 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.
- When you exercise casually in hot weather, listen to your body. Drink water when you are thirsty, eat fruit when you are hungry, and eat salted peanuts, nuts or potato chips if you are exercising in the hot sun for more than three hours.
- If you are competing in sports that last more than 70 minutes, take a source of sugar, such as jelly beans or any sugared drink, a few minutes before you start and during your event. There is no significant advantage to special sports drinks.
- If you are competing for more than two hours, take a food source of sugar such as fruit, cookies or candy bars. You don’t need special energy bars because no sugar source is better for you than one that contains both glucose and fructose and almost all types of candy and pastries contain these two sugars.
- If you are going to exercise or compete for more than three hours, add salty foods such as salted nuts or potato chips.
By Gabe Mirkin