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  Last updated: 10/19/2020

Risks Of Overhydration With Exercise

Failure to rehydrate with exercise is a frequently unrecognized factor in poor performance with endurance events, particularly with hot and humid conditions. It can also be an issue at indoor centers and should be kept in mind while exercising on fitness equipment at home or at the gym.

With the frequent emphasis on "staying hydrates", the pendulum on fluid replacement requirements has swung to the opposite extreme where overcompensation, drinking excessive amounts of electrolyte free water (read sodium or salt here), has occasionally led to an even more serious and life threatening condition - dilutional hyponatremia (low blood sodium concentration). This is supported by a recent study which found that overhydration with hyponatremia was a far more frequent finding in runners who had collapsed than dehydration.

Prior to 1990, the problem of dilutional hyponatremia was aggravated by the false assumption that sodium chloride losses during prolonged exercise were minimal and posed less of a threat to health and athletic performance than overall dehydration. As a result, many coaches emphasized fluid replacement of any type, including pure water alone. All endurance athletes need to understand the basic physiology of water and electrolyte balance with exercise and "thread the needle" so to speak, replacing both the fluids as well as electrolytes lost during an event.

Sweat contains 15 - 50 meq Na per liter (about 1 gram), and you can lose from 2 to 4 liters (1 liter =~ 1 quart =~ 2 pounds of body weight) of sweat per hour while exercising. Thus with prolonged exercise, salt losses can be significant (up to 4 grams)and easily outstrip normal compensatory physiologic mechanisms (see also handling heat).

The magnitude of the problem of hyponatremia was emphasized by a study of 64 finishers of the 1984 Hawaiian Ironman triathlon. It demonstrated an abnormally low serum sodium concentration in 29%. As overhydration or "water intoxication" with a low blood sodium concentration can be accompanied by nausea, fatigue, confusion, and even seizures, it was common enough to be more of a factor in poor performance than dehydration.

As a rule, hyponatremia occurs in events of five hours or more, and in high-heat and humidity conditions, sodium depletion can occur in as little as 2 hours. Interestingly hyponatremia may be a more significant problem in slower athletes who:

Beginners especially, who spend longer than average on the course, need to be particularly sensitive to this condition, especially if they over drink salt-poor fluids - which describes many sports drinks.

There are several scenarios to consider in the development of hyponatremia with prolonged exercise.

What is an adequate strategy to properly hydrate? First, fluid intake of 500 ml/hr is a good goal for the majority of athletes during prolonged exercise. Heavier athletes, extreme levels of exercise (prolonged periods at high % of VO@max), and severe environmental conditions may require higher rates. With this approach, dilutional hyponatremia is usually limited to episodes of endurance exercise lasting more than 5 to 7 hours where it can be identified by post exercise WEIGHT GAIN from the extra fluid ingested over that lost in the urine or sweat. The athlete: For endurance athletes (especially those exercising under high heat and humidity conditions), it is reasonable to plan an intake of one gram (1,000 milligrams) of sodium per liter (quart) of fluid loss. (1,000 mg is about one-half teaspoon of salt.) It is preferable to eat salty foods or drinks rather than taking salt tablets to avoid overcompensation on the electrolyte side.

Athletes who have suffered from hyponatremia (with symptoms including seizures), have continued to compete after modifying their fluid intake (based on the above principles) and have gone on to complete subsequent endurance events without problems.

Peripheral edema (swelling of the ankles and feet) will occasionally occur with long distance cycling. As peripheral edema is uncommon with hyponatremia (excess electrolyte free water replacement) other causes such as:

are considerations.

Although mild edema is common in women, it is recommended that recurrent problems of more than a minor nature be evaluated by a family practitioner or internist as it might be the first indication of a kidney or heart problem. Assuming a negative examination, there are 2 treatments to consider - support stockings or a mild diuretic. Although the latter has few side effects for the casual cyclist, ongoing treatment should be monitored by a physician.

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