Semantics are important when it comes to hydration to distinguish between two common terms associated with sweating: dehydration and hyponatremia. Differentiating terminology related to over and under consuming fluids and the concern for each is important. Consuming an insufficient amount of water that results in total body water weight loss is a condition called dehydration. Drinking too much water during exercise can cause water intoxification, (exercise associated hyponatremia (EAH)), a condition defined by dangerously low serum sodium levels. The media has used the terms dehydration and exercise associated hyponatremia (EAH) to mean the same and in doing so it has falsely led readers to assume that the symptoms for the two different conditions are the same. Due to specific events like the Boston Marathon which gained a lot of hype and press in 2002 when athletes over consumed water and/or total fluid including carbohydrate beverages, guidelines have been created and recommendations on fluid intake have been updated.
Dehydration, or insufficient water status in the body, is important because water makes up 45% – 75% of our body weight (60% average) and is tightly regulated to within 0.2 – 0.5% of our daily body mass. A reference range is used because each person’s water mass percentage will vary depending on fluid consumption and body composition. The more lean mass and glycogen stores a person has, the higher the percentage of water in body composition. The main symptom of mild dehydration is thirst, but as the percentage of water loss increases, perceived exertion, core temperature, and heart rate may increase.
The goal of hydration plans is not to prevent weight loss but to reduce the amount of it. There has been debate whether or not dehydration affects performance, and each person's tolerance to it will vary. It is recommended that athletes minimize weight loss to 2 – 3% of body weight in exercise lasting longer than 90 minutes. Many professional marathon runners accumulate much higher percentages of body weight loss than the recommendation without perceived decrement to their performance. However, the physiological adaptations that take place as a result of their training in combination with the elevated threshold for discomfort elite athletes exhibit; make them the minority, not the norm.
Determining accurate water loss using sweat rates is difficult since there are several sources of error, including respiratory loss of water and water as a byproduct of the breakdown of glycogen, fat, and protein. In addition, sweat rates focus on the loss of body weight which can be altered without a change in body water status. However, sweat rates are the least burdensome for athletes to use to measure body water status and can provide approximations for a variety of environments and intensities of training.
Exercise associated hyponatremia (EAH) can occur when a person is in a state of water balance or water deficit (dehydration), but it is usually associated with excessive water intake. In general, if a person consumes an electrolyte beverage when they are thirsty, EAH can be avoided. However, if a person drinks electrolyte containing beverages excessively (as if trying to prevent sweat loss), they can experience EAH. Exercise associated hyponatremia is usually asymptomatic and even though we usually associate excessive intake of water with weight gain, many times there is no change in body weight. The only way to diagnose EAH is to have your blood drawn for evaluation of sodium levels. Exercise associated hyponatremia is diagnosed when serum sodium levels fall below 135 mmol/L. Symptoms of EAH include feeling bloated, a ‘puffy’ appearance, nausea, and vomiting. These are all symptoms that can be experienced from simply participating in an endurance event. EAH is not a condition one would experience after a normal training day, so just because you feel bloated, you most likely are not experiencing EAH. Studies have found that even a moderate intake of sodium as part of a carbohydrate electrolyte drink prevented dramatic changes in serum sodium levels, and sodium intake while exercising in the heat is necessary in preventing EAH due to sodium loss that occurs when an athlete tries to match fluid intake with sweat loss.
The balance of water inside and outside of cells in relation to the concentration of sodium and chloride determines sodium balance. Hew-Butler et al. found that athletes who lost up to 3.8% of their body weight after Ironman South Africa (2006) still maintained serum sodium and plasma volume, showing us that our bodies work to maintain the balance of water, sodium and chloride inside and outside of cells. Weight loss >4% may result in a disturbance to sodium balance however, this depends on sex, amount and type of fluid ingested during event, and sweat rate. Pahnke et al. found that men had a higher overall sweat rate than women during the 2003 Hawaii IronmanⓇ Triathlon World Championship in Kailua-Kona, Hawaii and women on average drank more of a sodium containing beverage than the men, which attenuated their decline in sodium levels. Out of the men and women who completed this race, those that lost >4% body weight did not show a significant change in their serum sodium concentrations. The average concentration of sodium per pound of sweat is 400 – 500 mg, but it can range as high as 1100 mg. This large degree of variability is why it is difficult to make definitive recommendations about the use of salt tablets.
Some research has shown sodium supplementation does not affect sodium loss or performance during a 12.5 hour Ironman competition but it did minimize total weight loss in subjects who consumed 700 mg/hr salt compared to those that did not during Ironman South Africa. In addition, in the same race, none of the athletes that finished the race had EAH defined as < 135 mmol/L sodium regardless if they consumed a salt tablet or not. Most athletes did not fully replace or match fluid intake with loss. This is important because if they did try to match loss through intake, serum sodium levels most likely would have decreased. However, by not matching sweat loss with fluid intake, their kidneys preserved sodium concentration levels while total body mass (mostly water) decreased. Many professional Ironman athletes positively attribute their success in Kona, HI to salt supplementation during the race. The training and acclimatization that a professional athlete does in preparation for Kona cannot be compared to an age group triathlete with less fitness, experience, and who trains in a different environment.
- Monitoring your sweat rate is a great way to follow trends in your rate of water loss from different types and duration of workouts and also comparing the same workout in two different environments; however, it is very easy to become caught up in the numbers and almost obsessed with fluid in and fluid out. Don’t try to make this equation equal to zero.
- Instead of fighting your sweat rate, accept it and learn to become comfortable with it. Yes, we want to drink as much fluid and electrolytes as is comfortable within reason, but the other half of this is that you also have to train. You have to train enough that your body acclimates to the environment (e.g., hot and humid) and you mentally learn to manage the discomfort of sweating.
- Hydrating in advance of training and rehydrating 24 – 48 hours in advance and after workouts in which you lose > 2% body weight is important. You can work with a sports nutritionist or dietitian to determine how much and what types of fluid to use, but in general, you should replace 24 oz (750 ml) per pound of weight lost. Drink these fluids throughout the day and not in one sitting. During this time, urine color is not necessarily an accurate indicator of hydration status.
- To enhance hydration status the week preceding a half or full Ironman distance race in a hot and humid environment, consume electrolytes (calcium, magnesium, potassium, sodium and chloride) in addition to water. These will enhance total body water status.
- Replenishing glycogen stores will also enhance hydration status since up to three grams of water is drawn into cells and stored in muscle bound to a gram of glycogen. Carbohydrate loading may not be necessary to accomplish this. It depends what kind of nutrition plan you are currently following (e.g., low carbohydrate or paleolithic) and the training you completed the week before your event.
- There are different opinions on using thirst as a reminder to drink. Consuming a carbohydrate beverage that contains sodium has shown to increase palatability and stimulate thirst (McKinley). From our experience we have noticed that thirst works for lower intensity workouts completed in moderate environments for shorter durations. When it comes to a half or full ironman distance triathlon, fatigue and fitness of the athlete can affect their ability to recognize when they are thirsty. Sometimes their mental status is skewed due to fatigue or low energy intake and they don’t recognize thirst.
- The efficiency at which you sweat increases with fitness, so you may notice that as soon as you step outside on a hot and humid day, you begin to sweat buckets. This is good – your natural AC is turning on!
Regina Hammond has a Master’s Degree in Sports Nutrition and works for Trismarter Triathlon Coaching and Nutrition (http://www.trismarter.com). Staying abreast of the latest research she believes in an individualized approach to nutrition. With a background in competitive swimming, biking and running, she understands what it takes to be a competitive triathlete and works with clients on performance fueling plans, periodized nutrition plans, weight loss, and behavior change.
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