Hydration during exercise

What and when athletes drink depends upon exercise duration and intensity

By Elizabeth Quinn (from Sports Medicine)

Water is the most essential ingredient to a healthy life. Water has many important functions in the body including:

* Transportation of nutrients / elimination of waste products.
* Lubricating joints and tissues.
* Temperature regulation through sweating.
* Facilitating digestion.

Importance of Water During Exercise
Proper hydration is especially important during exercise. Adequate fluid intake for athletes is essential to comfort, performance and safety. The longer and more intensely you exercise, the more important it is to drink the right kind of fluids.

Dehydration
Athletes need to stay hydrated for optimal performance. Studies have found that a loss of two or more percent of one's body weight due to sweating is linked to a drop in blood volume. When this occurs, the heart works harder to move blood through the bloodstream. This can also cause muscle cramps, dizziness and fatigue and even heat illness including:

* Heat Exhaustion
* Heat Stroke

Causes of Dehydration

* Inadequate fluid intake
* Excessive sweating
* Failure to replace fluid losses during and after exercise
* Exercising in dry, hot weather
* Drinking only when thirsty

Hyponatremia - Water Intoxication
Although rare, recreational exercisers are also at risk of drinking too much water and suffering from hyponatremia or water intoxication. Clearly, drinking the right amount of the right fluids is critical for performance and safety while exercising.

Adequate Fluid Intake for for Athletes
Because there is wide variability in sweat rates, losses and hydration levels of individuals, it is nearly impossible to provide specific recommendations or guidelines about the type or amount of fluids athletes should consume.

Finding the right amount of fluid to drink depends upon a variety of individual factors including the length and intensity of exercise and other individual differences. There are, however, two simple methods of estimating adequate hydration:

1. Monitoring urine volume output and color. A large amount of light colored, diluted urine probably means you are hydrated; dark colored, concentrated urine probably means you are dehydrated.
2. Weighing yourself before and after exercise. Any weight lost is likely from fluid, so try to drink enough to replenish those losses. Any weight gain could mean you are drinking more than you need.

Things that Affect Fluid Loss in Athletes

* High altitude. Exercising at altitude increases your fluid losses and therefore increases you fluid needs.
* Temperature. Exercising in the heat increases you fluid losses through sweating and exercise in the cold can impair you ability to recognize fluid losses and increase fluid lost through respiration. In both cases it is important to hydrate.
* Sweating. Some athletes sweat more than others. If you sweat a lot you are at greater risk for dehydration. Again, weigh yourself before and after exercise to judge sweat loss.
* Exercise Duration and Intensity. Exercising for hours (endurance sports) means you need to drink more and more frequently to avoid dehydration.

To find the correct balance of fluids for exercise, the American College Of Sports Medicine suggests that "individuals should develop customized fluid replacement programs that prevent excessive (greater than 2 percent body weight reductions from baseline body weight) dehydration. The routine measurement of pre- and post-exercise body weights is useful for determining sweat rates and customized fluid replacement programs. Consumption of beverages containing electrolytes and carbohydrates can help sustain fluid-electrolyte balance and exercise performance."

According to the Institute of Medicine the need for carbohydrate and electrolytes replacement during exercise depends on exercise intensity, duration, weather and individual differences in sweat rates. [They write, "fluid replacement beverages might contain ~20–30 meqILj1 sodium (chloride as the anion), ~2–5 meqILj1 potassium and ~5–10% carbohydrate."] Sodium and potassium are to help replace sweat electrolyte losses, and sodium also helps to stimulate thirst. Carbohydrate provides energy for exercise over 60-90 minutes. This can also be provided through energy gels, bars, and other foods.

What about Sports Drinks?
Sports drinks can be helpful to athletes who are exercising at a high intensity for 60 minutes or more. Fluids supplying 60 to 100 calories per 8 ounces helps to supply the needed calories required for continuous performance. It's really not necessary to replace losses of sodium, potassium and other electrolytes during exercise since you're unlikely to deplete your body's stores of these minerals during normal training. If, however, you find yourself exercising in extreme conditions over 3 or 5 hours (a marathon, Ironman or ultramarathon, for example) you may likely want to add a complex sports drink with electrolytes.

General Guidelines for Fluid Needs During Exercise
While specific fluid recommendations aren't possible due to individual variability, most athletes can use the following guidelines as a starting point, and modify their fluid needs accordingly.

Hydration Before Exercise

* Drink about 15-20 fl oz, 2-3 hours before exercise
* Drink 8-10 fl oz 10-15 min before exercise

Hydration During Exercise

* Drink 8-10 fl oz every 10-15 min during exercise
* If exercising longer than 90 minutes, drink 8-10 fl oz of a sports drink (with no more than 8 percent carbohydrate) every 15 - 30 minutes. RECOMMEND THE REV 3 ENERGY DRINK

Hydration After Exercise

* Weigh yourself before and after exercise and replace fluid losses.
* Drink 20-24 fl oz water for every 1 lb lost.
* Consume a 4:1 ratio of carbohydrate to protein within the 2 hours after exercise to replenish glycogen stores.

Tips for athletes

Sports Nutrition - How Foods Fuel Exercise?

Elizabeth Quinn (Feb 2008)

How the body converts food to fuel relies upon several different energy pathways. Having a basic understanding of these systems can help athletes train and eat efficiently for improved sports performance.

Sports nutrition is built upon an understanding of how nutrients such as carbohydrate, fat, and protein contribute to the fuel supply needed by the body to perform exercise. These nutrients get converted to energy in the form of adenosine triphosphate or ATP. It is from the energy released by the breakdown of ATP that allows muscle cells to contract. However, each nutrient has unique properties that determine how it gets converted to ATP.

Carbohydrate is the main nutrient that fuels exercise of a moderate to high intensity, while fat can fuel low intensity exercise for long periods of time. Proteins are generally used to maintain and repair body tissues, and are not normally used to power muscle activity.

Energy Pathways
Because the body can not easily store ATP (and what is stored gets used up within a few seconds), it is necessary to continually create ATP during exercise. In general, the two major ways the body converts nutrients to energy are:

* Aerobic metabolism (with oxygen)
* Anaerobic metabolism (without oxygen)

These two pathways can be further divided. Most often it's a combination of energy systems that supply the fuel needed for exercise, with the intensity and duration of the exercise determining which method gets used when.

ATP-CP Anaerobic Energy Pathway
The ATP-CP energy pathway (sometimes called the phosphate system) supplies about 10 seconds worth of energy and is used for short bursts of exercise such as a 100 meter sprint. This pathway doesn't require any oxygen to create ATP. It first uses up any ATP stored in the muscle (about 2-3 seconds worth) and then it uses creatine phosphate (CP) to resynthesize ATP until the CP runs out (another 6-8 seconds). After the ATP and CP are used the body will move on to either aerobic or anaerobic metabolism (glycolysis) to continue to create ATP to fuel exercise.

Anaerobic Metabolism - Glycolysis
The anaerobic energy pathway, or glycolysis, creates ATP exclusively from carbohydrates, with lactic acid being a by-product. Anaerobic glycolysis provides energy by the (partial) breakdown of glucose without the need for oxygen. Anaerobic metabolism produces energy for short, high-intensity bursts of activity lasting no more than several minutes before the lactic acid build-up reaches a threshold known as the lactate threshold and muscle pain, burning and fatigue make it difficult to maintain such intensity.

Aerobic Metabolism
Aerobic metabolism fuels most of the energy needed for long duration activity. It uses oxygen to convert nutrients (carbohydrates, fats, and protein) to ATP. This system is a bit slower than the anaerobic systems because it relies on the circulatory system to transport oxygen to the working muscles before it creates ATP. Aerobic metabolism is used primarily during endurance exercise, which is generally less intense and can continue for long periods of time.

During exercise an athlete will move through these metabolic pathways. As exercise begins, ATP is produced via anaerobic metabolism. With an increase in breathing and heart rate, there is more oxygen available and aerobic metabolism begins and continues until the lactate threshold is reached. If this level is surpassed, the body can not deliver oxygen quickly enough to generate ATP and anaerobic metabolism kicks in again. Since this system is short-lived and lactic acid levels rise, the intensity can not be sustained and the athlete will need to decrease intensity to remove lactic acid build-up.

Fueling the Energy Systems
Nutrients get converted to ATP based upon the intensity and duration of activity, with carbohydrate as the main nutrient fueling exercise of a moderate to high intensity, and fat providing energy during exercise that occurs at a lower intensity. Fat is a great fuel for endurance events, but it is simply not adequate for high intensity exercise such as sprints or intervals. If exercising at a low intensity (or below 50 percent of max heart rate), you have enough stored fat to fuel activity for hours or even days as long as there is sufficient oxygen to allow fat metabolism to occur.

As exercise intensity increases, carbohydrate metabolism takes over. It is more efficient than fat metabolism, but has limited energy stores. This stored carbohydrate (glycogen) can fuel about 2 hours of moderate to high level exercise. After that, glycogen depletion occurs (stored carbohydrates are used up) and if that fuel isn't replaced athletes may hit the wall or "bonk." An athlete can continue moderate to high intensity exercise for longer simply replenishing carbohydrate stores during exercise. This is why it is critical to eat easily digestible carbohydrates during moderate exercise that lasts more than a few hours. If you don't take in enough carbohydrates, you will be forced to reduce your intensity and tap back into fat metabolism to fuel activity.

As exercise intensity increases, carbohydrate metabolism efficiency drops off dramatically and anaerobic metabolism takes over. This is because your body can not take in and distribute oxygen quickly enough to use either fat or carbohydrate metabolism easily. In fact, carbohydrates can produce nearly 20 times more energy (in the form of ATP) per gram when metabolized in the presence of adequate oxygen than when generated in the oxygen-starved, anaerobic environment that occurs during intense efforts (sprinting).

With appropriate training these energy systems adapt and become more efficient and allow greater exercise duration at higher intensity.

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