Monthly Archives: January 2012

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




The Gym is Crowded

The gyms must love this time of year, because of increased memberships.  Those who frequent the gym may find the equipment they like to work out on in constant use.   New Years resolutions have good intentions but my bet is the equipment will become more available sooner than later.  I have no stats to back up my claim – just observation and hear-say.  Exercise  is  encouraged since it is vital to help obtain and maintain overall health.  It’s therefore  important when starting out to know how much is enough versus overdoing it so you don’t cause injury and quickly grow discouraged .

The first step is to  pick an exercise that you  enjoy and willing to do on a regular basis versus dread.  Not everyone loves the gym so consider walking, swimming, or bike riding.   The point is that all adults should avoid inactivity. Some physical activity is better than none, and adults who participate in any amount of physical activity gain some health benefits.  The 2008 Physical Activity Guideline ( guidelines) for adults 18-64 is:

  • For substantial health benefits, adults should do at least 150 minutes (2 hours and 30 minutes) a week of moderate-intensity, or 75 minutes (1 hour and 15 minutes) a week of vigorous-intensity aerobic physical activity, or an equivalent combination of moderate- and vigorous-intensity aerobic activity. Aerobic activity should be performed in episodes of at least 10 minutes, and preferably, it should be spread throughout the week.
  • For additional and more extensive health benefits, adults should increase their aerobic physical activity to 300 minutes (5 hours) a week of moderate-intensity, or 150 minutes a week of vigorous-intensity aerobic physical activity, or an equivalent combination of moderate- and vigorous-intensity activity. Additional health benefits are gained by engaging in physical activity beyond this amount.
  • Adults should also include muscle-strengthening activities that involve all major muscle groups on 2 or more days a week.

When I see someone having an extended conversation with the person next to them I know they are not getting the benefit they  seek with the workout. The same goes with watching someone who is pushing themselves to the point of exhaustion and can barely take a deep breathe.  The goal is to burn fat, but if you are under or over-exercising you are burning carbohydrates.  The only thing this will do is create  hunger at the end since your body wants to replace the immediate source of energy it just lost.  My guideline is to evaluate your breathing.  If you are able to carry on a full conversation without the need for a breathe after a few words you are under-exercising.  On the other hand, if you cannot say any words in-between breathes  you are over-exercising.  A moderate-intensity aerobic activity means you’re working hard enough to raise your heart rate and break a sweat. One way to tell if you’re working at a moderate intensity is if you can still talk but you can’t sing the words to a song.  A vigorous-intensity aerobic activity means you’re breathing hard and fast, and your heart rate has gone up quite a bit. If you’re working at this level, you won’t be able to say more than a few words without pausing for a breathe.

Think of the FITT principle as a set of rules that must be adhered to in order to benefit from any form of fitness training program. These rules relate to the Frequency, Intensity, Type and Time (FITT) of exercise…  These four principles of fitness training are applicable to individuals exercising at low to moderate training levels and may be used to establish guidelines for both cardiorespiratory and resistance training.   The FITT principle is used to guide the development of fitness plans that cater to an individual’s specific needs.   The aerobic fitness goal using FITT for weight loss is Frequency of 4-5 x per week.   Intensity of moderate to vigorous, and Time of 30-45 minutes per workout.

Exercise should be a way to train and improve athletic performance, and physical health, but not a means of purging calories.  Make exercise a lifestyle change and not a temporary fix.  If you decide to add exercise to you New Years resolution think first about your goal.

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