If you weigh yourself before and after an hour or two of exercise, the difference is likely to be fluid loss. However, in events lasting several hours or even several days, measurable fat loss can occur. At a competitive 12-hour indoor stationary bicycle marathon, one athlete took fluids and food throughout the entire competition, and still lost 2.64 pounds (Schweizerische Rundschau für Medizin Praxis, July 2007).
Of this weight loss, 1.98 pounds was due to loss of fat. His calculated muscle weight increased by 1.46 pounds due to damage to the muscle cells, which results in fluid retention in the cells.
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How fast you can move and how long you can exercise intensely depends on the amount of sugar (glycogen) stored in your muscles. The same rule applies in all sports: when muscles run out of their stored sugar supply, they require more oxygen and you have to slow down.
Fluid is less important than muscle sugar because dehydration will not cause you to slow down until your blood volume is reduced. As you lose fluid from sweating, interstitial fluid stored around cells is released into the blood to maintain blood volume. When you compete is sports at a very high intensity, your muscles run out for stored sugar long before your blood volume is reduced, and you slow down from lack of muscle sugar before you slow down from reduced blood volume (Sports Medicine, April- May 2007).
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A study from the University of North Carolina at Chapel Hill shows that drinking fluids earlier can improve performance more than taking them later (International Journal of Sport Nutrition and Exercise Metabolism, December 2006).
Seven highly-trained male triathletes, aged 18 to 35 years, were tested during two simulated Olympic-distance triathlons. They took a full glass of water at 8, 16, 24, and 32 kilometers, and this was compared to taking the same drink 2, 4, 6 and 8 kilometers later in the event (at 10, 20, 30, and 40 kilometers).
As you would expect, opening swim times for 1500 meters were similar between trials; as were the second event (40-km cycling) times, but the third event (10-km run) times were faster when the athletes took food and drink earlier. Dehydration does not harm an athlete’s performance until he lacks a large amount of water and his blood volume is depleted significantly. That explains why the athletes’ performance was not harmed until the third event of the three-event competition.
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In general, marathon races are among the most physiologically demanding endurance events in the world with runners competing for a distance of 42 kilometers (26.2 miles). (3,6) Competitive marathon runners often maintain a pace equaling 70-90% of their VO2max (max output) for over two hours. (6) Both biological and environmental factors affect the thermoregulatory balance in marathon runners.
Marathon races are considered mass participation events, and heat injuries occur in less extreme conditions. When running in closer proximity to other runners, it has been reported that the physiological heat stress is three times more straining on the body compared to someone running solo in identical weather conditions. (6) As mentioned earlier biological and environmental factors play a role in maintaining thermoregulatory balance in marathon runners. Biological factors such as dehydration, metabolic rate and gender limit thermoregulatory control. (3)
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For the last 40 years, sports medicine experts have told athletes in endurance events that they should take fluids frequently during events lasting more than one hour. However, three years ago, a 28-year-old woman collapsed and died after finishing the Boston Marathon. Her blood salt levels were extremely low and she died from a condition called hyponatremia.
A few weeks ago, a policeman training for bicycle duty died of the same condition. On July 26, 2005, sports medicine experts issued a warning to all athletes from the First International Exercise-Associated Hyponatremia Consensus Development Conference.
I have never seen this syndrome in well-conditioned athletes. It has been reported almost exclusively in very thin, less-fit, slower and novice athletes, and is far more common in women. This condition is caused by drinking too much fluid and is not caused by excessive loss of salt in sweat or by exercising. When people with psychiatric problems force themselves to drink huge amounts of water while sitting still, they also can die of hyponatremia, only in this case, it is called water intoxication.
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It may. During hard exercise, particularly in hot weather, you sweat and lose a lot of water and salt. Your performance does not suffer until you lose a very large amount of fluid, about five percent of your body weight or more.
However, when you lose so much fluid that your performance suffers, you cannot catch up during that competition. That’s why you need to take in fluids during events that last more than 90 minutes. If your event lasts more than 2.5 hours, you also need to take in calories and salt.
According to a study from The University of Birmingham in England, cold drinks taste better so you will drink more (Experimental Physiology, September 2006). The researchers concluded that cold drinks also improve endurance during hot weather by acting as a heat sink, decreasing the rise in body temperature and therefore reducing the effects of heat stress.
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A study from the Netherlands shows that sugar in sports drinks slows absorption and increases stomach cramping in running races shorter than 12 miles.
Fluids pass through your stomach and are absorbed almost immediately in your intestines. Exercise slows fluid passage from the stomach but does not affect intestinal absorption. Sugar added to drinks can delay stomach emptying to increase risk for cramps.
Another study from the University of Utah, reported in the same journal, shows that taking a salty drink just before exercise increases endurance. Dehydration is the most common cause of fatigue during exercise in fit men and women. This study used salted drinks or placebo (unsalted) drinks with two groups of cyclists, and demonstrated a significant improvement in an endurance time trial as well as better maintenance of blood volume in the group that had the salted drinks.
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The Gatorade Sports Science Institute in Barrington, Illinois published a study showing that 46 percent of recreational exercisers are mildly dehydrated (Applied Physiology, Nutrition and Metabolism, June 2006).
However with good reason, the study does not say that they are harmed. There is no data anywhere to show that this mild dehydration affects health or athletic performance. Another study from the University of Connecticut shows that a person must lose a tremendous amount of fluid before it affects his performance (Medicine & Science in Sports & Exercise, October 2006).
When you exercise for more than an hour, you [need to] take in fluid … but not too much. Excessive fluid can cause a potentially fatal condition called hyponatremia. Normally, the amount of salt and other minerals in your bloodstream should equal the same total mineral content in every tissue in your body. If the mineral concentrations are not equal, they try to become equal. Fluid moves from the area of lower mineral content to that of the higher concentration.
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