Wednesday, February 16, 2011

How to Avoid Water Intoxication....a.k.a hyponatremia

There is on question that the popularity of endurance and ultra-endurance sports is on the rise. The health benefits of participating in such endeavors is hard to argue with, increased HDL (good cholesterol), decreased triglycerides, decreased blood pressure, increased feelings of well being, the list goes on and on and on...

However, there are considerations that individuals participating in and training for these events must be aware of in order to decrease the risk of bodily harm. One such consideration is proper hydration. A common consequence of participating in endurance sports is hyponatremia, otherwise known as water intoxication. Hyponatremia is a metabolic condition in which the concentration of sodium (salt) is lower outside the cells with in the body. As you may have read in past posts the word homeostasis has been mentioned. In short, it's the scientific word for "balance" and something the body is consistently striving for at the cellular level. So as a consequence of having less sodium outside the cell, water begins to move into the cell in order to balance the concentration of sodium on each side. When this happens the cells begin to swell.

Most cells in the body can handle some of this swelling, but the cells in brain cannot and it is the swelling of the brain that causes most of the outward signs/symptoms of hyponatremia. Mild/Moderate symptoms include: Confusion, headache, irritability, fatigue, muscle spasms and even hallucinations. If not treated and corrected in time more severe symptoms develop including: decreased consciousness, possible coma and death.

Obviously, we would like to prevent any and all of the above listed symptoms from occurring. With a little science and Know-how we can understand how it can be done, but I will tell you that it is easier in theory than in practice (just ask any seasoned ultra-endurance athlete). One of the main reasons why it's difficult in practice comes down to human physiology. As our bodies exert themselves, blood flow to the stomach and intestines decreases and makes digestion more difficult. Furthermore, the body's hunger and thirst mechanisms are inhibited so the body simply doesn't have the same drive to consume fluids and/or solids as compared to resting state.

So, here are some keys to success when working to avoid hyponatremia.

Determine your sweat rate: The sweat rate is simply the amount of bodily fluid loss/unit time. On average humans sweat 27.4 to 47.3 oz./hour of exercise. Obviously variables such as speed/intensity of exercise, environment (hot/cold) along with others will influence this number. Here is a pretty simply strategy to follow in order to zero in on your sweat rate (as adapted from Runners World Article).

1) Measure your pre-exercise weight (naked).
2) Exercise at race or training pace for one hour, keeping track of how much fluid (in ounces) is taken in during the activity.
3) After the activity, strip down, towel off any sweat, and weigh yourself nude again.
4) Subtract your post activity weight from your pre-activity weight and convert to ounces. Then add to that number however many ounces of liquid you consumed on your activity. (For example, if you lost a pound and drank 16 ounces of fluid, your total fluid loss is 32 ounces.)
5) To determine how much you should be drinking about every 15 minutes, divide your hourly fluid loss by 4 (in the above example it would be 8 ounces).
6) Because the test only determines your sweat losses for the environmental conditions you were active in that day, you should retest on another day when conditions are different to see how your sweat rate is affected. You should also redo the test during different seasons, in different environments (such as higher or lower altitudes), and as you become faster, since pace also affects your sweat rate.

I should tell you that even though you may be adequately replacing fluids you may not be adequately replacing electrolytes like sodium, without which can lead to hyponatremia. So we still have to consider salt intake during long bouts of exercise. This is where many questions arise! The American Heart Association recommends that we limit salt intake to 1.5 grams/day. Remember, this is for an inactive individual trying to prevent cardiovascular disease. This is not a recommendation for the ultra-endurance athlete.

Sweat typically contains between 2.25 - 3.4 grams of sodium/liter, and the sweat rate in a long, hot race can easily average 1 liter per hour. So, for a 12 hour race (like that of an ultra-marathon or triathlon), one could lose approximately 27 to 41 grams of salt. If the athlete only replaces the water and has little salt intake, hyponatremia can result. One study which looked at athletes completing a 160km race found that those who consumed more fluids had a higher incidence of symptoms associated with hyponatremia vs. those who consumed less fluid. Do not read this as saying that you should limit your fluid intake! Instead be sure you are ingesting the proper amounts of water and salt together.

There are no specific guidelines on salt intake/hour, but some sources discuss 1-2 grams/hour. If you were counting on sports drinks like gatorade to supply this amount of sodium you would be falling well short. For example, the nutritional facts state that gatorade has 110mg of sodium per 8 oz serving. So if we do the math and assume an individual consumed 32 oz of gatorade/hour (by the way...good luck trying to consume 32 oz of gatorade per may be seeing it on your shoes!) you would only be taking in 440mg of sodium.

This is not even 1/2 the amount your body may need. Other ways to increase salt intake are utilizing salt tablets or making salted boiled potatoes (the carbohydrates and high glycemic index of the potatoe offer a great energy source as well). Other strategies include pairing other foods that are high on the glycemic index with salt for that 1-2 punch of sodium and carbs that the body can quickly use.

So there you have it...a long winded guide to avoiding hyponatremia during your next ultra endurance training session or event. Good Luck and be sure to post questions, comments and feedback so we can continue the discussion.

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