CYCLING PERFORMANCE TIPS - Risks Of Overhydration With Exercise

CYCLING PERFORMANCE TIPS

Risks Of Overhydration With Exercise

(hyponatremia)

Failure to rehydrate with exercise is a an often unrecognized factor in poor performance during endurance events, particularly with hot and humid conditions. However fluid replacement requirements have been emphasized to the point that overcompensation, drinking excessive amounts of electrolyte (read sodium or salt here) free water, has occasionally led to the even more serious and life threatening condition of dilutional hyponatremia (low blood sodium concentration). In fact, a recent study found that overhydration was a far more frequent finding in collapsed runners than dehydration.

The problem of dilutional hyponatremia was aggravated by the fact that up to the mid 1980s it was generally accepted that sodium chloride losses during prolonged exercise were minimal and posed less of a threat to health and athletic performance than overall dehydration. Thus many coaches emphasized any type of fluid replacement, including pure water alone. This misunderstanding continues to come up, so the challenge to all endurance athletes is to understand the basic physiology and "thread the needle" so to speak, replacing both the fluids and electrolytes lost during the event. Sweat contains 15 - 50 meq Na per liter (about 1 gram), and you can lose anywhere from 2 to 4 liters (1 liter =~ 1 quart =~ 2 pounds of body weight) of sweat per hour while exercising, so in prolonged exercise, salt losses can be significant (up to 4 grams)and beyond the normal compensatory physiologic mechanisms available (see also handling heat).

The magnitude of the problem of hyponatremia was emphasized by a study of the 64 finishers of the 1984 Hawaiian Ironman triathlon which 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 in fact more of a risk to poor performance than dehydration.

Generally, hyponatremia occurs in events of five hours or more where it may actually be a bigger problem in slower athletes who:

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

In high-heat and humidity conditions, sodium depletion can occur in just a couple of hours. Beginners especially, who spend longer than average on the course, need to be particularly sensitive to this condition and especially if they overdrink salt-poor fluids - which describes many sports drinks.

There are several possible mechanisms for hyponatremia with prolonged exercise.

Sodium losses in sweat are are moderate but total body sodium content is near normal and replacement of fluid losses with a LARGE volume of sodium free water dilutes out the blood sodium concentration. (The most likely.)
Both sweat volume (water) and sweat sodium losses are large and the sodium losses are not adequately replaced by electrolytes in snacks or commercial drinks. Again, any water ingested dilutes out the blood sodium concentration - but in this scenario total body sodium content is also 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 above two scenarios.)

What about salt (sodium chloride) replacement? It goes without saying that some sodium chloride is needed to replace sweat losses. But probably not much unless one is consciously avoiding salt in snacks and fluids (and of course any deficit will be accentuated in longer events). Many athletes, observing general healthy-diet guidelines, watch their sodium intake and keep it low. This is not necessarily a good strategy for most endurance athletes, especially if the event will be in high temperature/humidity conditions. Sodium chloride also facilitates carbohydrate absorption (optimal concentrations have not been defined) from liquid carbohydrate drinks. And finally, there has been good evidence that the body will "defend" the sodium concentration of the blood over the circulating blood volume so that if you are slightly behind in salt replacement in addition to being volume depleted, the kidneys will preferentially maintain a normal sodium concentration and allow you to remain volume depleted (with its detrimental effect on physical performance) even when adequate water is taken in.

What is the answer to the quandry of under versus overhydration? First, fluid intake in the order of 500 ml/hr are appropriate for the majority of athletes during prolonged exercise. Only heavier athletes, extreme levels of exercise (prolonged periods at high % of VO@max), and severe environmental conditions may require higher rates. And, except for the unusual case of pure inappropriate secretion of vasopressin, hyponatremia is usually limited to periods of endurance exercise lasting more than 5 to 7 hours where it is can be identified by a WEIGHT GAIN from the extra fluid ingested compared to that lost in the urine or sweat. So the athlete:

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

For aerobic-endurance athletes (especially those exercising under high heat and humidity conditions), it is reasonable to plan on an intake of up to a maximum of one gram (1,000 milligrams) of sodium per liter (quart) of fluid loss. This is about one-half teaspoon of salt. It is preferable to eat salty foods or drinks rather than ingest salt tablets. Studies have shown that salty foods and drinks appear to stimulate thirst, and thus prevent the unintentional intake of dangerously high amounts of sodium.

It should be noted that athletes who have suffered from hyponatremia (with symptoms including seizures), and have continued to compete after modifying their fluid intake based on the above principles, 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 excess electrolyte free water replacement ie hyponatremia as described above, other causes such as:

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

need to be considered.

Although mild edema is common in women, it is recommended that recurrent problems of more than a minor nature be evaluated by your 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 your physician.

For those of you interested, here is another article:

Drinking Too Much.
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