Tips & News



By Stephen Born

How your cooling system works

When we exercise, we burn molecular fuel (mostly glycogen) but also some protein, fat, and blood glucose from ingested nutrients. The breakdown of these energy providers releases heat that builds up and raises our core temperature. The body must rid itself of this heat and maintain a core temperature within a few degrees of the well-known 37° C. An active person needs a reliable cooling mechanism. Actually, you have several. You lose some heat through your skin. Blood carries heat to the capillaries near the skin's surface, removing heat from the body core. You breathe harder to get more oxygen, expelling heat when you exhale. But by far the most important part of the cooling system, accounting on average for about 75% of all cooling, is your ability to produce and excrete sweat.




What happens when the coolant runs low?

Just like a car, your body must dissipate the excess heat generated from burning fuel. Unlike a car, your body's coolant isn't in a sealed internal system; you use it once and then it's gone and needs to be replaced. Unfortunately, we don't come with built-in gauges or indicators that tell us just how much coolant we have left in our system. We do have some physiological signs, but they function at the Warning-Danger! Level - too late to maintain optimal performance. For instance, by the time you feel thirsty, you could have a 2% body weight water loss, already into the impairment zone.

The chart below shows what happens to human performance at each percent of weight loss. By weight loss, we mean the percentage of your body weight at the start of exercise that you have lost at the end via sweat. If you go out for a run at approx 72.5 kg and weigh in 32 km later at approx 70 kg, you've lost almost 4% of your body weight. That's too much to maintain your pace to the end, let alone expect to kick.

Symptoms by percent body weight water loss:

  • 6% --- weakness, severe loss of thermoregulation
  • 7% --- collapse likely unless exercise stops
  • 10% -- comatose
  • 11% -- death likely

How much is that?

As you can see from the chart above, sweat loss can easily escalate from an athletic performance issue to an acute medical issue. Clearly, we need to have some quantifiable idea of our intake and output. Let's start with converting the data on the chart to recognizable amounts. Perhaps you remember the saying, a pint's a pound, the world 'round. Now that's a convenient conversion for endurance athletes. Here's another: one pint = 475ml = one water bottle. Some bottles hold 600 ml, but consider a regular water bottle as a pint 475 ml. Two pints 950ml, which is equivalent to almost a liter - not quite, but almost. So when you read liter, think two water bottles. Losing slightly less than half a kilogram means a one-pint loss. One liter is about one kilogram.

Can you drink enough?


Research suggests that while electrolyte needs for individual athletes may vary up to 1000% (tenfold), fluid loss remains fairly constant. Also, we can measure fluid loss more easily than electrolyte loss; we don't need sophisticated lab equipment, just a scale. Thus, we can come pretty close in calculating fluid loss and replacement.

The numbers

On average, you lose about one litre of fluid per hour of exercise. Extreme heat and humidity can raise that amount to three litres in one hour. A trained athlete will store enough muscle glycogen to provide energy for approximately 90 minutes of aerobic exercise. As your muscles burn glycogen, water is released as a metabolic by-product and excreted as sweat. Researchers found that during a marathon (42km), runners released an average of two liters of sweat from muscle glycogen stores. This is in addition to sweat from other body liquids.

You can control or lessen these sweat rates by acclimatization and training for the event. Acclimatized athletes can reduce electrolyte and fluid loss up to 50%, but note that those losses cannot be fully replaced during the event. Remember the words of Dr. Bill Misner, The endurance exercise outcome is to postpone fatigue, not replace all the fuel, fluids and electrolytes lost during the event. It can't be done, though many of us have tried. In other words, our hydration goal is not to replace water pint-for-pint, but to support natural stores by consuming as much as we can adequately process during exercise.

At the most, you can absorb about one liter of water per hour, but only in the most extreme heat and humidity. Most of the time you can only absorb about half or not too much over half that amount, even though it won't fully replace your losses. Repeated intake of one litre per hour will ultimately do you more harm than good.

Can you drink too much?


Problems with too much or too little

Tragic consequences



So how much, how often?


Noakes believes intake of hypotonic fluids of one litre/hr will likely cause water intoxication and dilutional hyponatremia. He suggests that athletes may do better on 500 ml/hr fluid intake for ultra-events performed in hot weather conditions. In THE TOP 10 - The biggest mistakes endurance athletes make* article, Dr. Ian Rogers suggests that between 500-750 ml/hr will fulfill most athletes' hydration requirements under most conditions. According to Dr. Rogers, like most things in life, balance is the key and the balance is likely to be at a fluid intake not much above 500 ml per hour in most situations, unless predicted losses are very substantial. Other research suggests a similar consumption of 33-207 ml of water every 15 to 20 minutes of exercise.

How much fluid should you drink?
Average Athlete, average temps Lighter athletes or cooler temps Heavier athletes or hotter temps
600-750 ml/hr is an appropriate fluid intake for most athletes under most conditions. For lighter weight athletes, or those exercising in cooler temperatures, approx 473-532 ml may be perfect. Heavier athletes or athletes competing in hotter conditions may consider intakes upwards of 830 ml/hr.
Based on the available research, along with the thousands of athletes we have monitored, we have found that 600-750 ml/hr is an appropriate fluid intake for most athletes under most conditions. For lighter weight athletes, or those exercising in cooler temperatures, 473-532 ml may be perfect. Heavier athletes or athletes competing in hotter conditions may consider intakes upwards of 830 ml/hr. We also suggest that to avoid dilutional hyponatremia, fluid intake should not routinely exceed 830 ml/hr. The exceptions are heavier athletes, athletes exercising at extreme levels (prolonged periods at a high percentage of VO2Max), and athletes competing in severe environmental conditions.

600-750 ml is the equivalent of the typical regular-to-large size water bottle, and that’s an excellent gauge to work within.

Remember your electrolytes and calories!

We noted at the beginning of this article that besides cooling, water also plays an important role in nutrient transport. Water consumption bears directly on electrolyte and caloric uptake. You must consider the electrolyte content of your fluid intake, especially if you exceed about 710 ml/hr. If temperature and humidity rise above 21° C and/or 70% humidity, we recommend that you take electrolytes before and during every hour of exercise.




Beat the Heat
Tips to keep cool


Each scoop of Sustained Energy and Perpetuem that you put in a bottle reduces the water volume by about 44 ml. For example, if you add two scoops of Perpetuem to a 620 ml water bottle, you won't end up with that same amount of actual fluid; it will be approximately 500ml, perhaps even slightly less. For some athletes, 500ml/hr is sufficient, but for many athletes that's not enough; oftentimes upwards of 740-830 ml of fluid are required hourly. As a result, you'll have to drink your entire fuel bottle plus plain water from another source. After a while it can be difficult to keep precise track of your fluid intake because you're fulfilling your needs from two separate sources.

To make things easier when doing a three-hour or longer workout, we suggest making concentrated, multi-hour bottles of fuel. For example, if you're going to be exercising for four hours and you know that you need two scoops of Perpetuem to satisfy an hours’ worth of fueling, make an 8-scoop bottle 620 ml water bottle. Now you have four hours of fuel in one bottle and that provides a number of benefits:
Yes, there is some actual fluid left in that 8-scoop/4-hour bottle of Perpetuem, but the amount is small, yielding less than approx 118 ml hourly over the course of four hours. Does that small amount of fluid count towards fulfilling your overall hydration needs? Yes, but it's a small enough amount to not have to think about if you're keeping your overall fluid intake within our suggested guidelines (approximately 600-750 ml hourly). Plus, those hourly guidelines do have some flexibility built in (+/- 9-118 ml).

With that in mind, a concentrated bottle of Perpetuem can thus be thought of as a calories only bottle and you'll fulfill your hydration needs with plain water from another source. The beauty of this, among the other benefits mentioned earlier, is that because you're fulfilling your calorie and fluid needs from sources independent of each other, you're able to gauge your intake with greater precision.

So when your workouts are greater than three hours in length, give the multi-hour bottle of Sustained Energy or Perpetuem a try and you'll find that it'll be a lot easier to keep track of both your calorie and fluid intake . . . it's been a winning strategy for thousands of endurance athletes.

Other ways to cool yourself in extreme heat


Fluid intake suggestions apart from the workout or race

Now that you have a good guide for your fluid intake during exercise, we can turn to two other considerations: how much you should drink overall during the day and how you should hydrate just prior to racing or exercise. For your regular daily hydration needs (that is, in addition to your exercise-induced needs), no research has conclusively arrived at an RDA for fluids, but about 33-39 ml/kg makes a more accurate standard than the eight glasses a day commonly recommended for everyone.

  • One liter of fluid in the two hours prior to the start (500ml per hour), ceasing consumption about 20-30 minutes before you begin the workout or race.
  • 295-355ml of fluid each hour up to 30 minutes prior to the start. Keep in mind that even though these are our recommendations, you need to determine what works best for your system and the particular logistics of the race or training session ahead.


Personalized data is the key to hydration efficiency


If you've spent money on a heart rate monitor, a multi-function watch, or a body fat measuring device and if you use them properly, you already have some serious training tools. We suggest that a good scale (preferably one that can measure less than one kilogram increments, such as a balance scale) may well prove to be your most valuable fitness investment. Weigh yourself before and after each outing, carefully noting the time, exertion level, kilometres, weather and fluid, fuel, and electrolyte consumption. Another low-tech hint: make sure you know the capacity of your water bottles and hydration packs. Once you begin to log your fluid consumption and weight fluctuations, you'll have the data to accurately calculate your personal needs in this absolutely vital area.
Final checklist, some quick tips, and summary

  2. For very long events, such as a century bike ride, the average rider will also lose a 0,454 kilograms or more in energy stores (glycogen, fat, and muscle tissue) in addition to the water, so figure that into your weight difference.
  6. In general, keep fluid intake between approx 590-740 ml/hr. For lighter weight athletes, or those exercising in cooler temperatures, approx 473-532 ml/hr may be perfect. Heavier athletes or athletes competing in hotter conditions may consider intakes upwards of approx 830 ml/hr. If you feel you need more fluids, experiment with it in training, keeping in mind that you will require additional electrolytes. Regular fluid intake over approx 890 ml/hr increases the possibility of dilutional hyponatremia.
    deplete fluid stores more rapidly.
