Sweating by the Numbers

If, as Edison said, genius is 99 per cent perspiration, runners might be smart to find out their sweat rate. Senior editor Alex Hutchinson performed two sweat tests to see if they could help evaluate his electrolyte needs.

In the name of science, I dabbed some shaving cream on the back of my leg and scraped clear a fist-sized area of bare skin. I was about to undertake a new home sweat test to find out exactly how much salt my sweat contains, and I needed to make sure the absorbent patches would stay glued to my skin once the fluid started to flow. After cleaning the area with an alcohol wipe provided in the test kit, I pasted on the patches and headed out for a run.

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The science:

When we sweat, we lose electrolytes. Sodium, for example, is a key electrolyte that we get from table salt, among other places. Our bodies require sodium for a range of crucial functions, such as transmitting electrical potentials through the nervous system and signalling muscles to contract. Sodium levels are also linked to the maintenance of appropriate fluid levels in the body. For athletes, losing too much sodium is linked to decreased performance, and possibly to muscle cramps (though evidence about cramps is still unclear).

More seriously, low sodium levels can lead to hyponatremia (which means, literally, “low sodium condition”), a condition that has led to several highly publicized deaths at major marathons in the past decade. In these cases, the victims are thought to have been drinking too much water without replacing electrolytes, causing the concentration of sodium in their bodies to fall to dangerously low levels.

The challenge in maintaining appropriate sodium levels is that everyone is different. Some people sweat far more than others, and “salty” sweaters can have 10 times as much sodium in each drop of sweat as “low-salt” sweaters. That’s why Medion Corporation, the Toronto-based makers of eLoad and other sports nutrition products, has started offering a $250 home sweat test.

The home test:

Medion’s test kit is simple and self-explanatory. You paste absorbent patches on your forearm, your chest and the back of your leg, then you exercise vigorously for as long as it takes to completely soak the patches with sweat – half an hour was enough for me. Peel off the patches, seal them in tiny zip-lock bags, pop them in the included courier pack within 48 hours and wait for the lab to analyze the sodium content with a mass spectrometer.

According to Medion, my sweat has a sodium concentration of 1,102 milligrams per litre. The range found in human sweat is 300 to 3,500 mg/L, so I felt pretty normal. A series of handy online calculators at www.medioncorp.com allowed me to determine what type of sports drink I should be using for different durations of exercise.

The lab test:

I also had the chance to perform a more rigorous sweat test under laboratory conditions, thanks to Dr. Lawrence Spriet of the University of Guelph and Gatorade Canada’s Sports Science Institute. In this test, I cycled for an hour at 65 per cent of my VO2 Max, with multiple patches on my forehead, back and chest. Thanks to careful pre- and post-exercise weighings, they determined that I sweated 1.20 L/hour.

Surprisingly, they measured 2,069 mg/L of sodium from my forehead patch, 1,908 mg/L from my back, and 1,563 mg/L from my chest. This, they said, makes me a “high salt sweater.” My forehead sweat, for instance, was almost double the levels Dr. Spriet’s team measured in hockey players at the national junior training camp. No wonder my eyes start to sting on long, humid runs.

The comparison:

Why the discrepancy between the two tests? For one thing, they were measuring different parts of the body. Also, I was already dehydrated before the lab test, which can cause elevated sodium levels in sweat. (I know for a fact that I was dehydrated, because they measured the “specific gravity” of my urine and confirmed it.)

Ultimately, both tests tell me the same thing: that my sweat has more salt in it than average. For my typical runs these days, which seldom exceed an hour, that doesn’t require any big changes. But it does tell me that, if I ever manage to follow through on the goal of tackling a marathon, I’ll have to be very careful to consume sports drinks with enough sodium, rather than just water.


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