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  Last updated: 12/1/2015


Why do some riders sweat more than others (when riding at a similar pace in the same ambient temperature and humidity)? Is it because they are out of shape? Or perhaps because they have more body fat (and thus insulation)?

First let's look at why we sweat. Sweating is the way we lose the excess body heat produced as the muscles inefficiently convert food Calories into the mechanical energy of muscle contraction. Our muscles are only ~ 25 % mechanically efficient (the ratio of mechanical work output to the total glycogen Calories expended) thus the 75% of Calories that are now heat energy need to be lost by either radiation or evaporation. This is a similar situation to the inefficiency of a gasoline engine where heat energy generated with the inefficient combustion of gasoline as a fuel is eliminated through the radiator.

When you start your ride, the heat generated from inefficient energy conversion increases the temperature of the muscle tissue and then elevates your overall body temperature as the heat energy is redistributed throughout the 70% of your body that is water. Then, as your core temperature rises, you begin to sweat to get rid of this excess heat energy. The redistribution of heat energy works in reverse when you stop cycling and explains why you continue to sweat as the excess body heat built up (and stored) with exercise is slowly dissipated.

The amount of excess heat energy generated is related to the total amount of mechanical work being done (in watts/kg body weight). It has nothing to do with the %VO2max at which you are working but rather a percent of the total amount of energy being generated by the metabolic conversion of glycogen into mechanical energy. If you look at the article, there is the expected linear relationship between total Calories of mechanical energy developed and the amount of excess heat energy that must be eliminated from the body with sweating. There is no correlation between sweating and the percent body fat or body habitus. But if you are a heavy rider, either fat or muscle tissue, maintaining your place in a pace line will require that you generate more watts/kg BW/minute to move your extra weight on the bike compared to a thin rider, and thus you will generate more heat energy, and will sweat more, than a smaller rider in your riding group as your body eliminates this excess.

Bottom line is that the amount of sweating has nothing to do with "being in shape" or how much body fat you might have, but rather the amount of work you are doing as well as the ambient temperature and humidity.


I was told an article in Cyclist magazine indicated if you knew the amount of sodium you were losing in your sweat, you could more effectively plan a rehydration strategy (and by implication improve your performance.) I followed a string of comments on the internet that pointed to this website as the basis for the article. Is there any evidence that the analysis of sweat electrolyte concentration will help with rehydration strategies or improve performance? What does the literature have to say.

Background: When you sweat, your body loses water and sodium. The kidneys respond by varying the sodium loss in the urine, to compensate for sweat losses and keep the Na concentration in body fluids within a safe range (the "normal limits" on your blood work). If you have had a significant weight loss with your workout (any weight lost is due entirely to water weight) and replace these salty fluid losses with pure water alone (no salt), you can overwhelm the body's normal physiologic mechanisms and develop hyponatremia (a low sodium concentration), a potentially serious medical problem. This is the rational for adding electrolytes to sports drinks.

So as you replace sweat losses, does it matter if you know the exact amount of sodium lost? First, let's look at just fluid losses (assuming blood sodium concentration is maintained within a normal range). We know that as far as performance is concerned it is not necessary to replace all the fluid you have lost. Many world class athletes perform in a slightly dehydrated state. This study of ultramarathoners competing in high temperature conditions concluded "...that weight loss greater than 2% does not necessarily have adverse consequences on performance..."

In recognition of this fact, the ACSM revised its recommendations on fluid replacement in 2007 proposing 400-800 mL/h, varying depending on the athlete's size, environmental conditions, and exercise intensity, suggesting that the rehydration goal be shifted from maintaining complete rehydration to limiting fluid loss to less than 2% of body weight for optimal health and performance.

What if you underestimate the total amount of salt lost and replace either too much or too little? How about muscle cramping? Numerous papers have shown that is not a realistic concern:

So now we know that there are no adverse effects on either 1) performance or 2) cramping from fluid and sodium deficits as long as some Na (salt) is taken along with replacement fluid. Get in the ball park with the amount oral sodium replacement and your kidneys will do the rest.

Is there data to support this benign approach to fluid replacement?

Bottom line - If you are going to take oral fluids with exercising, you do need to replace salt as well. But with the exception of the need for rapid recovery in multiday competitive events, or with extreme weight loss (>5% of body weight) normal eating and drinking practices will suffice. There is no evidence that knowing the total amount of sodium in sweat changes that recommendation.

Questions on content or suggestions to improve this page are appreciated.

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