Just a decade ago, recommendations emphasized drinking as much as one could handle during exercise, however it is now recommended to replace at least 75%  but not more than 100% of sweat losses.  But don't depend on your thirst mechanism to tell you when and how much to drink. Thirst is a poor indicator of hydration status, as an athlete can lose over 1.5 L of body water before becoming thirsty.  

Sweat rates vary amongst individuals, and sweat losses of 1-2% of body weight compromises physiologic function, and harms exercise performance. A loss of 3% of one's body weight increases the risk of heat cramps and heat exhaustion, where losses of 5% or more of body weight can cause heat stroke and a trip to the medical tent or closest hospital.

Hyponatremia

Hyponatremia  is a potentially fatal condition of low blood sodium levels. This can happen during prolonged exercise if sodium levels are not properly replenished, with salty snacks or electrolyte sport drinks. It can also happen from drinking too much water, which effectively dilutes the sodium content of the blood. Sodium losses range from 2.25-3.4 g/L of sweat (which is equivalent to about ½ tsp of salt). Conditioned athletes may be more efficient with their sodium losses, however this varies from person to person.

Some drugs such as NSAIDS (aspirin, ibuprofen) or diuretics have been shown to alter kidney function, which may exacerbate the risk of hyponatremia during long duration events. Athletes should be aware of this increased risk, know the signs and symptoms, ensure that they are not limiting dietary salt intake when in training, and ensure proper rehydration strategies are in place.

Signs and Symptoms

The signs and symptoms of dehydration and hyponatremia are very similar, and if incorrectly diagnosed can be detrimental, possibly even causing death. For example, an athlete suffering from hyponatremia can easily be mistaken as dehydrated, and drinking water, as one would do to treat dehydration, will only exacerbate the problem. Water will dilute the blood, thereby further decreasing its concentration of sodium, and worsening the symptoms of hyponatremia.

In this next section you will learn the theory behind sport drink formulas, to help you better plan your hydration strategy and optimize your performance safely.

The "What" and the "Why" of Sport Drinks

Composition of our blood: Plasma Osmolality

Plasma osmolality is a measure of the concentration of sodium, chloride, potassium, glucose and other ions (molecules with a positive or negative electric charge) in the blood. This osmolality is affected by changes in water content, increasing with dehydration and decreasing with overhydration. Normal plasma osmolality ranges from 275-300 mOsm/kg. This is similar to the osmolality of sport drinks.

The kidney plays a protective role in the hydration status of the body. It helps to regulate plasma osmolality by reabsorbing water when we are dehydrated, and increases its filtration rate when we are overhydrated. Although the composition of sweat varies from person to person, it is always hypotonic (less concentrated) compared to body fluids, and therefore sweat loss increases plasma osmolality.

Composition of Sport drinks

There are three types of sport drinks: isotonic, hypotonic, and hypertonic.

Sport drinks are a convenient way to replace water, carbohydrate, and sodium losses during and after exercise. Replacing the large amounts of sodium lost in sweat may help endurance athletes to avoid hyponatremia. Sodium also helps to maintain extracellular fluid volume, and stimulate the uptake of water and glucose into the small intestine. Dilute glucose-electrolyte solutions, with a hypotonic osmolality compared to plasma, help to maximize the rate of water uptake.

The type of sugar in sport drinks may also have an effect on absorption. Research shows that glucose stimulates more net water and sodium absorption than does fructose. Maltodextrin, a glucose polymer often found in sport drinks, may be the most effectively absorbed. It is less sweet than glucose or fructose, and is often mixed with other sugars to increase palatability. Maltodextrin refers to a family of glucose polymers, is composed of between 3-19 molecules of glucose, and is commercially manufactured from potato, corn or rice.

How to make your own isotonic sport drink

1)     Mix 200 mL of concentrated orange juice + 1 g (a pinch) of salt + 1 L water
2)     Mix kool aid (to your taste)  + 8 tsp sugar + ½ tsp salt + 250 mL water
3)     Mix your favorite fruit juice + 8 tsp sugar + ½ tsp salt + 250 mL water

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