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THE LATEST SCIENCE OF REHYDRATION

Posted by Dr. John Seifert on 5/18/2017

Fluid balance and rehydration play critical roles in delaying fatigue and enhancing athletic performance.  Rehydration has three components:

1) How fast fluids empty from the stomach

2) The rate at which fluid is absorbed from the small intestine

3) How much fluid is retained after ingestion

Is it Better to Drink a Larger Volume or to Meter Fluid Intake?

In spite of the best prepared schedules, endurance athletes in the heat of competition often forget to drink on schedule.  To compensate, they drink larger amounts in single gulps, which can lead to stomach discomfort – that feeling of sloshiness or fullness.  And the large gulp strategy also leads to less effective rehydration.  Researchers have shown that ingesting fluids in a metered way produces more effective rehydration and less GI upset than drinking large volumes.  Drinking large volumes of fluid rapidly dilutes the blood leading to increased urine output.  The key is to take your hourly fluid needs and break it into six segments and drink one segment of fluid every 10 minutes.  Of course we cannot ignore that certain activities, such as cycling, make it easier to drink more frequently.

Why Drink a Sport Drink

For athletes, water is the most common rehydration beverage.  And for millions of people who exercise at low to moderate intensity for shorter intervals of time, water is an ideal choice.  However, for individuals who exercise more frequently at greater intensities, properly formulated sports drink have been shown in research conducted over 40 years to be superior to water.  Unfortunately, there is no standard formula for a sports drink.  The question is what are the most essential elements?  For decades it was thought the only ingredients needed were water and salt.  The modern era of sports drink began when exercise scientists added carbohydrate to the salt water formula. Although the carbohydrate was added to provide energy for muscle contraction, it was soon shown that carbohydrate actually improved rehydration.  One ingredient, however, that was overlooked was protein, even though there was significant research showing that protein improved absorption of water.  This led to the first sports drink that contained carbohydrate, protein and sodium.  Our labs showed that the protein provided a synergistic effect to sodium and carbohydrate to enhance absorption as well as fluid retention over a conventional carbohydrate sports drink.

Do the Fluid Rules Change When it is Cold Out Vs. Warm?

There’s a popular misconception that dehydration seldom occurs when exercising in the cold.  That is just not true.  Studies involving cross country and downhill skiers have shown dehydration does occur and it can negatively impact performance.  It appears, however, that dehydration occurs differently when exercising in the cold vs. what normally happens during exercise in warm temperatures.  In the summer, we feel the heat and sweat production is obvious and thirst mechanisms as activated.  In the cold, however, our thirst mechanism is suppressed.  A case in point is a recent study showing downhill skiers experienced significant dehydration during a morning of skiing, and yet were unable to rehydrate back to a normal range during a long lunch break even when given free access to fluids.  Not only are the thirst mechanisms altered in cold, but we also experience greater water loss through breathing.  This is due to breathing cold dry air, which forces our body to work harder to humidify and warm the cold air.  Although most of the studies have been conducted in warm weather, cold weather dehydration can negatively impact performance.  One studies in skiers demonstrated that drinking water was more effective in reducing muscle stress compared to not drinking.  This same study also reinforced the benefits of a sports drink.  Ingestion of a carbohydrate protein sports drink not only was most effective in minimizing muscle stress, but was also shown to improve skiing performance.

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