Poor Hydration Will Affect Exercise Performance


Congratulations if you are continuing on with your New Year resolution to exercise on a regular basis.  If you notice there are days where your performance is not up to par, yet you can’t relate it to anything in particular, it may be inadequate hydration.  Thirst drive is not stimulated until 1-2% water loss—therefore, exercise enthusiasts probably train in a state of hypohydration most times.  When given free access to fluids, subjects tend to consume only about ½ of the fluid they lost through sweat.

Water is the largest single constituent of the human body and is essential for cellular homeostasis and life. Total water intake includes drinking water, water in beverages, and water that is part of food. Although a low intake of total water has been associated with some chronic diseases, this evidence is insufficient to establish water intake recommendations as a means to reduce the risk of chronic diseases. Instead, an Adequate Intake (AI) for total water is set to prevent deleterious, primarily acute, effects of dehydration, which include metabolic and functional abnormalities.

The primary indicator of hydration status is plasma or serum osmolality, but not a realistic option unless you have access to a lab.   Because normal hydration can be maintained over a wide range of water intakes, the AI for total water (from a combination of drinking water, beverages, and food) is set based on the median total water intake from U.S. survey data. The AI for total water intake for young men and women (ages 19 to 30 years) is 3.7 L and 2.7 L per day, respectively. Fluids (drinking water and beverages) provided 3.0 L (101 fluid oz; ≈ 13 cups) and 2.2 L (74 fluid oz; ≈ 9 cups) per day for 19- to 30-year-old men and women, respectively, representing approximately 81 percent of total water intake in the U.S. survey.

Dehydration refers to an imbalance in fluid dynamics when fluid intake does not replenish water loss from either hyper hydrated or normally hydrated states. It impairs physical work capacity, physiologic function, and  predisposes one to heat injury when exercising in a hot environment   So what’s the best way to increase voluntary fluid intake?   Fluids should be cool…not cold.   Sodium in the drink (.5-.7 g/L) will stimulate the thirst response.  Thirst is triggered by a decrease in plasma volume and an increase in plasma sodium.  Consumption of plain water may remove the osmotic drive to drink.  Studies show that sports drinks are consumed at 2 times the rate of water.   Sodium though does not enhance fluid absorption because the amount in the beverage is miniscule compared to the amount of Na in the bloodstream. Palatability is a big factor.

There appears to be little physiologic need to replace electrolytes during a single exercise session of moderate duration (eg, less than 3 to 4 hours), particularly if sodium was present in the previous meal. However, including sodium in amounts between 0.5 and 0.7 g/L is recommended during exercise lasting longer than 1 hour because it may enhance palatability and the drive to drink, therefore increasing the amount of fluid consumed.    It should be noted that this amount of sodium exceeds that typically available in commercial beverages. Including sodium in fluid replacement beverages may also help prevent hyponatremia in susceptible people.  Although most athletes who drink more fluid than they lose as sweat simply excrete the excess fluid as urine, in some people it is retained . If the fluid contains sodium, it could help prevent the dilution of serum sodium levels, thereby decreasing the risk of hyponatremia. Limiting fluid intake so that it does not exceed sweat rate can also decrease risk of hyponatremia.

Colder air contains less moisture than air at warmer temperature, particularly at higher altitudes. Greater fluid volumes leave the respiratory passages as the incoming cold, dry air becomes fully humidified and warmed to body temperature. Cold stress also increases urine production, which adds to total-body fluid loss.

So what is an appropriate amount of fluid to consume?  You can easily calculate your hydration needs based on your sweat rate.

Sweat Rate:

  • Wt before exercise = 176#
  • Wt post exercise = 174#
  • Difference = 2# X 16 oz. (32 oz)
  • Fluids consumed during exercise = 70 oz.
  • Total fluids used during exercise =  102 oz.
  • Fluids used/hours of activity (102 oz/2hr)
  • Sweat Rate = 51 oz/hour

Hydrating based on sweat rate:

  • Drink every 10-12 min  10 oz – 5X/hr
  • Drink every 15 min  13 oz – 4X/hr
  • Drink every 20 min  17 oz – 3X/hr

In most cases athletes do not consume enough fluids during exercise to balance fluid losses, and thus complete their exercise sessions dehydrated to some extent. Consuming up to 150% of the weight lost during an exercise session may be necessary to cover losses in sweat plus obligatory urine production. Including sodium either in or with fluids consumed postexercise reduces the diuresis that occurs when only plain water is ingested . Sodium also helps the rehydration process by maintaining plasma osmolality and thereby the desire to drink. Because most commercial sport drinks do not contain enough sodium to optimize postexercise fluid replacement, athletes can rehydrate in conjunction with a sodium-containing meal.   High-sodium items include soups, pickles, cheeses, processed meats, pizza, pretzels, and popcorn. Use of condiments such as soy sauce and ketchup, as well as salting food at the table, also increase sodium intake

The overall take home message is to  monitor your hydration status.  Check the following:
  • Urine color- the lighter the better
  • Sweat rate
  • Pre-exercise :  16-24 ounces fluid prior to 2-3 hours prior to work out
  • During exercise:  6-12 ounces fluid every 15-20 minutes
  • Post-exercise:  24 ounces per pound lost
  • Possible inclusion of sodium foods or sports drink

In addition to increasing fluid intake 24 hours before strenuous exercise in the heat, it is recommend to consume 400 to 600 mL (13-20 oz) of cool water about 20 minutes before exercise.

Caution with hyperhydration.  There was one documented case at Ft. Benning, GA when a Soldier from Alaska drank 10+ canteens of water the evening prior to hyperhydrate and he ended up with severe hyponatriemia resulting in the Soldiers death.

Good luck with continuing your pursuit of a new you for the new year.

Signature  Joanne Slyter, dietitian, Westminster, CO   Interest in sports nutrition




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