Notes on Fluid Intake for Smooth
Summer Running
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Gender can influence susceptibility to hyponatremia; it is important to remember that women need less fluid than men, even after body weight is taken into account. Water makes up a greater percentage of male body weight, and therefore a greater percentage of water in relation to body weight needs to be replaced during exercise. The best way to monitor your fluid consumption is to weigh yourself before and after your long runs. You should never gain weight after a run. To perform an accurate test of your sweat rate, weigh yourself and then run for one hour in similar conditions and at a similar pace as the race you are planning for, refraining from drinking during the run. Weigh in again afterward. The resulting discrepancy in weight represents the amount of fluid consumption in ounces that you should not exceed during each hour of your race.

Finally, a word about caffeine. Caffeine consumption has been under scrutiny for years as to its possible role in dehydration. Because caffeine is a known diuretic, its effects on body fluid balance and heat storage are potentially negative. Yet it is also a readily available, legal performance enhancer. Recently, some clarity has been shed on this topic.

At rest, caffeine increases urine production, as well as the excreted amounts of sodium, chloride, and potassium within a given amount of urine or sweat. Prior research has shown that the volume of urine and sweat is not affected by caffeine during exercise—but what about the amount of electrolyte loss within these substances?

In one recent study, seven endurance-trained males completed six experimental trials each, consisting of pedaling for 120 minutes at 65% VO2max. The subjects received during exercise: 1.) no fluid; 2.) water replacing 97% sweat losses; 3.) Gatorade sports drink; and then each of these treatments plus caffeine in 6 mg doses per kilogram of body weight. The trials were conducted in a hot, dry environment of 97 degrees Fahrenheit.

The results indicated that caffeine does not produce more heat or have a negative effect on heat dissipation, even in such a warm exercise environment. This coincides with a 2001 study published in Clinical Physiology. The new research did find, however, that combining caffeine with water increased urine production, but this did not exacerbate dehydration—due to a low contribution of urine to total fluid loss. The drug did increase the concentration of electrolytes in the sweat, but blood sodium levels remained unaltered.

It looks as though the effects of caffeine at rest simply do not match those found during submaximal exercise, thereby making caffeine an acceptable choice for runners, without increased dehydration or heat-exhaustion risk.

MSSE, 2009, Vol. 41, No. 1, pp. 164-173

AMAA/ARRMS, “Optimal Hydration: Establishing a Hydration Plan for Marathons”

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