A Case Study

by John Hughes

John Hughes is a former director of the WUCA, has been certified by the NSCA as a personal trainer and by USA Cycling as a coach. Learn about Hughes’ coaching at www.coach-hughes.com.

The Problem

Rider A, who lives in Southern California writes:

I am 53 years old and this year I started having problems with heat exhaustion on rides.

This problem has caused me to bail on five rides. The problem has occurred at temperatures from 72 to 104 degrees.

I start to get a headache followed by disorientation, nausea then vomiting. I cool down with ice under my armpits, neck and groin area and recover perfectly within two to three hours — enough to continue the ride.

Another symptom I have noticed is that my heart rate starts to climb into the anaerobic region. Maintaining my cadence, I can drink water and actually watch my HR go back down to normal, and then watch the cycle repeat. It’s like having a leaky radiator.

It happens as early as six hours into the ride on warm to hot days.


John Hughes writes:

Heat exhaustion is a function of (at least) five factors:

  1. intake of water and electrolytes
  2. ambient temperature
  3. sweat rate
  4. heat adaptation
  5. aerobic conditioning

For more info see Heat Training and Conditioning


The dialog continues:

1) intake of water and electrolytes

JH: From what you’ve said you and your doctor are only looking at the first factor. Let’s start there. How has your doctor determined that you are drinking enough water?

RIDER: I have tried between 16 oz to 40 oz an hour with no difference other than cramping at 40 oz, which I have learned to compensate for with increased electrolyte intake. My average intake on a hot day is 25 oz per hour. I weigh myself before and after and typically have less than a 2% loss in body mass. I have never came back weighing the same or more than I left. Also on hot days I start with frozen water bottles and ice in my hydration pack and replenish with ice at the convenience stores and event check points.

JH: How has the doctor determined that you are taking in enough electrolytes?

RIDER: He has reviewed my supplements, i.e., Sustained Energy/Perpeteum and Endurolytes, to make sure I was not taking too few or too much. Also he has taken blood tests, i.e., for glucose and electrolytes, after my normal rides and everything is in normal ranges.

JH: What electrolytes are you taking / hour?

RIDER: Four to six Endurolytes per hour when it starts getting hot. The four to six is based on my water consumption. Usually one or two per hour when it’s cool.

JH: You said you can’t eat solid food during a ride, are you using some sort of liquid nutrition?

RIDER: Mostly Sustained Energy, 2.5 scoops per hour in a 24 oz water bottle, however I usually have a hydration pack with pure water to compensate for the water loss from the Sustained Energy mix. Sometimes I alternate with Perpeteum. I have also had the problem when I was using GU, Cliff Bars (which I couldn’t stomach) and Gatorade Endurance Formula and alternatively GU and GUO2. NOTE – If I use an electrolyte drink I do not take the Endurolytes during that time.

Regardless of the system I maintain 300 calories per hour and watch that I don’t over do the combined sodium from the products.

JH: You should also consider the other four factors. Heat exhaustion simply means that you are working harder than your body can cool off — so you’re exhausted.

2) ambient temperature

RIDER: Three of the five rides were well above the 90 degree heat index, i.e., one was 90 to 95 degrees, and the other two 101 to 135 degrees, i.e. hot and humid.

Two though were 72 degrees to 78 degrees with the same identical symptoms. I have now started using the Weather Channel to predict problem areas.

3) sweat rate

How do you measure this? I just weigh before and after to make sure I am not losing more than 2% of my body weight.

4) heat adaptation

I reviewed the article on the website on Heat Training and have pretty much been trying to acclimate since June 3rd, in particular in the areas that I have been riding. I don’t have air conditioning in my car and heat typically doesn’t bother me.

5) aerobic conditioning

I do aerobic training. Speed and hill workouts and use a HR monitor in all training to optimize my aerobic capacity. Are there other forms?

Sources of Sodium
If you are getting your water intake right, which means matching it to your sweat plus urine rate, then you want to ingest about 1,000 mg of sodium per liter water. So for instance, you drink a liter of sport drink. Figure you got about 450 mg sodium (and 135 mg potassium) from it. You need about 550 mg more of sodium. Possible food sources include:

  • a dill pickle
  • V-8 or tomato juice
  • sliced turkey
  • Cup O’Soup
  • Chips or crackers

Each food product lists the sodium content on the label. Alternatively, you can add table salt to already salty foods (for example, chips and crackers). 1 teaspoon table salt has about 2500 mg sodium. Or you may choose to take pills – just check the sodium content of whatever you are taking.


Hughes suggests:

Problems on the bike are usually the result of multiple causes. A happens, and then B kicks in, and then C occurs . . . and there’s a cascade effect. I suspect your heat exhaustion is a result of combination of factors:

1) Slight dehydration. You wrote that you never lose more than 2% of your body weight. 2% isn’t enough to be dangerous, or cause heat exhaustion by itself, but it does affect your performance.

To measure your sweat rate weigh yourself nude. Then ride for several hours. Keep careful track of exactly how much water you consume. Weigh yourself after the ride.

Every 2.2 lbs that you lose = 1 kilogram = 1 liter of water that you sweated out. (1 liter = 35 fl. oz.) Add to that how much you drank. Then divide by the duration of your ride in hours. That’s your hourly sweat rate.

Bear in mind that sweat rate varies with conditions and how hard you’re riding. So this’ll give you a baseline, but you’ll have to adjust.

2) Not consuming enough sodium. You’re using Endurolytes, which only contain 40 mg of sodium / capsule. A liter of sweat contains 1000 mg. So you’d need to take 25 Endurolytes / liter of water.

You say that you try to be careful not to overdo the sodium. You can’t overdo the sodium — if you consume too much you’ll just eliminate it in your urine. But you can under consume — which can lead to hyponatremia, a potentially dangerous condition. About low blood sodium

3) Riding a little too hard for the conditions. One way to deal with the heat is simply to slow down. You’ve been riding at a level of exertion where you can’t take in enough water and sweat enough to keep your body temperature under control. So you should back off a bit.

The Results

The rider reports:

Friday night I heard that record temperatures would be set on Saturday. I was excited to try out your suggestions on a century.

It was 112 degrees and the heat index was off the scale. I rode for almost eight hours in what most would be considered unbearable heat, however my core body temperature was cool the entire time. I sipped every five minutes on frozen/iced water bottles (which I was doing before) however this time taking one salt tab and a beef jerky stick per hour (almost 1000 mg sodium per hour) along with my usual two packs of gel per hour.

Yes, I was craving solid food again. This year I haven’t been able to stomach solid foods on the rides. I now believe it was because I wasn’t getting enough water and salt. My water needs at these temperatures was over two plus water bottles an hour. I was sweating like crazy, but I was so cool that I hardly noticed the heat. Four hours into the ride I was starved and put down a jumbo hot dog and Frappuccino at a convenience store.

I also bought extra water bottles at each stop to occasionally drench my head and shoulders during the ride. I am also learning some new tricks such as putting the iced water bottles under my armpits, back of my neck and eating while waiting in the check out line. By the time I get to the counter I only have empty wrappers for the cashiers to swipe.

This was the hottest and best ride I have ever done. After the ride I felt that I could have done 300 to 400 miles.

For further reading

  • The Exercise-Associated Hyponatremia (EAH) consensus panel statement (Hew-Butler et al., 2005) is available for free Consensus Statement. It is the second article in the July, 2005 issue. (Lulu Weschler is a co-author on this paper.)
  • Institute of Medicine, Food and Nutrition Board, Dietary reference intakes for water, potassium, sodium, chloride, and sulfate (2004) National Academies Press. Institute website
  • Barr, Susan Ph.D Nutrition Principles for Endurance Athletes
  • Weschler, Lulu Water and Salt Intake During Exercise
  • Setnes, Keven and Karl King Electrolyte and Fluid Replacement: Any Debate?