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:

• may experience less nausea and thus drink more,
• cover the event distance over a longer time interval and thus have the opportunity to take in greater amounts of fluid.

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

• Sodium losses in sweat are moderate and total body sodium content is near normal but replacement of fluid losses with a LARGE volume of sodium free water dilutes out the blood sodium concentration. (The most likely today as so many athletes, sensitized to fluid replacement, overcompensate in their drinking.)
• Both sweat water volume and sweat sodium losses are large. Water, but not sodium losses are replaced by electrolyte poor snacks or commercial drinks. Again, any water ingested dilutes out the blood sodium concentration. In this scenario total body sodium content is diminished. (More common in undertrained individuals or those competing in unusually hot and humid conditions.)
• An inappropriate release of the hormone vasopressin with exercise leads to a decreased urine volume but, unaware of this, the athlete continues water replacement at normal rates. Total body sodium concentration is normal, total body water content is above normal or baseline. (Felt to be uncommon alone, but may play some role in combination with either of the first two scenarios.)

• should not assume that he or she can drink unlimited amounts of water fluid during exercise expecting that the excess will be lost either in sweat or through the kidneys.
• should realistically attempt to estimate his or her OWN real sweat and urine losses and replace them accordingly.
• should use a scale during training rides or during the event itself to answer the question as to whether water loss and replacement have been balanced correctly.

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:

• excess salt replacement
• venous or lymphatic obstruction from positioning on the bicycle

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.

For those of you interested, here is another article:

• Drinking Too Much.
  

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