The National Oceanic and Atmospheric Administration called the hottest June on record for the United States. More 100F+ days are making riding more challenging. Hydrating properly is one of the keys to riding in relative comfort. And it’s not as simple as “drink before you’re thirsty” and “drink early and drink often.” Here’s why.
Why Do You Get Hot?
It’s obvious – it’s hot outside! Actually, you don’t overheat just because the ambient temperature is high. You overheat in several different ways and understanding these will help you perform better in the heat:
- Energy production. The human body is only 20 to 40% efficient, which means that only 20 to 40% of the energy you get from eating is translated into forward motion. If you are a 160 lb. (73 kg) rider pedaling at 14 mph (22.5 km/h) for an hour, you are burning approximately 500 calories. A calorie is the amount of heat required to raise the temperature of one gram of water one degree Celsius. Only about 100 to 200 the calories you burn are producing forward motion. The remaining 300 to 400 calories are producing heat! You have to dissipate this heat or your core temperature will rise. Energy production and heat dissipation are the primary factors in overheating, whether you are riding on a 100 F (38 C) day or climbing hard when it’s only 60 F (15 C).
- Radiation. When the sun is out, you gain heat from direct rays of the sun, as well as from radiation reflected from the pavement. You may also gain heat from radiation through diffuse clouds. Your body also radiates heat even when it’s hot outside. The higher the sun is in the sky the more radiation heats you up.
- Respiration. You may dissipate heat when you exhale if your breath is warmer than the environment, and or if it’s really hot out you may actually gain heat through breathing. Hot air feels harder to breathe.
- Conduction. You also gain heat through hot parts of your bike. You lose heat when you apply an ice pack.
- Convection. Heat moves from your hotter body to the relatively cooler air; however, if the air is hotter than you are, the reverse flow occurs.
The fact that your energy metabolism is the primary source of heat has a very significant implication. Cooling your core is the most important.
Effects of Overheating
You know riding when you are hot is harder — harder to maintain a target pace, harder to put out more power to climb, and harder to ride farther. Interestingly, it doesn’t have to be very hot for performance to decline. In one experiment, trained cyclists rode at 70% of VO2 max in lab temperatures of 4, 10, 20 and 30C, (39, 50, 68, 86F). They were able to maintain that effort for the longest time at 10C (50F), and performance declined progressively after that. Thus, overheating can be an issue even if you ride in temperate conditions that most of us would consider ideal for cycling!
The normal temperature in your core surrounding your vital organs is 37C (98.6F). Your skin temperature is 34C (93.2F) if you aren’t gaining or dissipating a significant amount of heat.
As your core temperature starts to rise, your body circulates more blood through your core and to your skin to provide cooling and to protect your vital organs from overheating. As a result, less blood flows to the muscles to deliver oxygen and nutrients. As your core temperature rises, it’s also more difficult for your brain to recruit your muscles to contract forcefully, i.e., put out high power. These physiological changes partially explain why you can’t ride as hard when you are hot.
You also can’t ride as hard for psychological reasons — in short, it feels harder to ride when you are hotter. In another experiment, a group of trained cyclists were asked to ride at the same perceived effort they would expend if riding a 20 to 40K time trial. They repeated the trial rides at 15, 25 and 35C (59, 77, 95F). Based solely on perceived effort, their power output declined as the temperature rose.
Sweating—Your Body’s Primary Cooling Mechanism
Increasing your sweat rate and radiation from increased blood flow to the skin account for about 85% of your body’s cooling. The rest comes from conduction and convection.
The harder you exercise, the more sweating is the dominant mechanism for cooling. Because the primary source of heat is your working muscles, internal cooling to keep your core organs cool is much more important than external cooling. Think of your body like a car’s engine. Most of the cooling is provided internally by coolant, which flows through the engine and then through the radiator. Only a little cooling comes from heat given off directly to the air around the engine. Your blood is the coolant, your muscles are the engine, and your skin is the radiator. Thus, managing your internal coolant — hydration and electrolyte levels — is critical. Although it may feel good to dump water on your head while riding this only cools your skin. If you drink the water it helps to cool your core, which is much more important. Of course, if you have an ample supply of water, e.g., at a minimart then dumping on your head is fine as long as you also drink.
If you are riding in moderate conditions—that is, you aren’t gaining heat from the environment—each hour you’ll produce 600 to 800 ml (21 to 27 fl. oz.) of sweat! Each hour you sweat out almost as much fluid as contained in one standard (about 24 oz.) water bottle! If you push the pace a little, are climbing, are riding in hot weather or you are a larger rider, you could easily produce 32 fl. oz. (1 quart or liter) or more of sweat per hour. Heavy exercise can produce as much as 20 times the amount of heat produced at rest!
You’ve probably noticed riding with friends that some riders’ jerseys are soaked almost immediately while others’ remain relatively dry. A number of factors influence the variability in how much individuals sweat:
- Ambient temperature. The hotter it is, the more you sweat.
- Humidity. Sweat doesn’t evaporate as readily when it’s humid, so even more sweat must be produced to help cool your body.
- Intensity of exercise. The more energy you expend riding, the more you sweat. As noted above, your body isn’t very efficient at turning fuel into energy for the muscles. Any energy you expend that doesn’t move you forward increases your sweat rate, with no performance benefit—for example, standing to pedal or bobbing your shoulders.
- Physical size. If you are a big rider with a large surface area, you may produce and evaporate more sweat; however, you may also gain more heat from radiation from the sun.
- Gender. Women tend to have lower sweat rates and electrolyte losses than men, primarily because of their smaller body size and lower metabolic rate for a given workload.
- Fluid balance. The better hydrated you are, the more sweat you produce. As you get dehydrated, your sweat rate decreases to preserve fluid, so you don’t cool off as much.
- Clothing. Clothing that doesn’t breathe reduces evaporation.
As you become more acclimated to riding when you are hot, you start to sweat at a lower core temperature, and you sweat more, which keeps your core temperature lower. Your total blood supply increases so you can sweat more while still maintaining blood flow to the muscles.
When you sweat, initially the water (sweat) comes from your approximately five liters of blood. As you sweat and the amount of blood decreases, your heart has to beat faster to maintain the same blood flow, an effect known as cardiac drift. Even if you just maintain the same power, rather than producing more power, your heart rate will rise as you sweat. As you continue to sweat, your body pulls water from your cells into your blood to maintain blood volume. This impairs how well your muscles function.
You keep pedaling, and your muscles still need blood to supply oxygen and nutrients. Since blood volume is less, the blood supply to other parts of the body is reduced, with these consequences:
- Heart rate and core temperature increase.
- Skin blood flow diminishes, so you don’t get as much cooling.
- Higher core temperature is needed before you start to sweat, because your body is preserving fluid.
- Greater risk of heat exhaustion and heat stroke because your body can’t cool itself as well.
- Blood flow to the digestive system is reduced, making it harder for you to absorb needed fluid, electrolytes and calories.
- The brain may not get as much blood, which, combined with heat stress, contributes to diminished mental functioning and fatigue.
How Much Should You Drink?
When I started cycling in the 1970s I rode with one 16 fl. oz. (0.5 l) water bottle on the down tube and my pump on the seat tube. I finished a sub-5 hour century on this bike with only three quick stops at aid stations — riding hard despite dehydration. In that era, racers in the Tour de France were advised not to drink during a race, especially in hot weather.
In 1979, I rode the 1200 km (750-mile) Paris-Brest-Paris on a custom touring bike, which had an unusual design with cages for two 16 oz. bottles on the down tube! The French peloton all rode with only one bottle. Back then runners routinely finished marathons without worrying about hydration! Obviously, one could perform at a high level while getting progressively dehydrated.
Then scientists and coaches began to understand the effects of dehydration, and we were taught to “drink before you’re thirsty” and “drink early and drink often.” At the same time, producing and marketing sports drinks became a multi-million dollar industry. The CamelBak was invented in the early 1990s. Their motto was “Hydrate or Die.”
Thus, not worrying about hydration to the point of becoming dehydrated on a ride, and also drinking to the point of becoming bloated, nauseous and worse — both ends of the hydration spectrum — are both dangerous, and wrong.
We now know that appropriate hydration is more complex than either riding without drinking very much or drinking too much during a ride. Studies show varying effects on performance from dehydration. Dehydration appears to affect maximum power output only slightly. Pro sprinters ride so hard for many hours in the heat that, despite their best efforts, they become dehydrated — but look at them go! On the other hand, during lighter aerobic endurance exercise, dehydration results in increased core temperature which produces fatigue, affects endurance and increases perceived exertion. Chronic dehydration magnifies the effects and negates the effects of acclimatization.
Hydration status appears not to be a contributing factor to heat exhaustion or heat stroke. Researchers differ on whether dehydration may cause muscle cramps.
On the other hand, drinking too much fluid can result in dilutional hyponatremia, drinking so much fluid that you dilute the sodium concentration of your blood to a dangerously low level. Note that the sodium concentration in sports drinks is low relative to the concentration in your blood, so you can drink too much sports drink just as you can drink too much water. Hyponatremia may result in your body retaining fluid and swelling, which can be visible around a ring on your finger, your gloves around your wrists and your socks around your ankles. Your brain can’t swell because of your skull. The increasing fluid retained in your brain results in increase pressure so another symptom of hyponatremia is a headache. If your brain swells too much it can be fatal! If your body is swelling up in the heat, although it’s counter intuitive the remedy is to stop drinking until the swelling disappears.
Given the complexity of fluid replacement, scientists and medical experts do not have a consensus recommendation. The American College of Sports Medicine (ACSM) strikes a balance and recommends drinking enough during exercise to prevent both
- excessive dehydration (>2% body weight) and
- excessive losses in sodium balance.
Dehydration is expressed as a percentage of body weight. If the 160-lb (73 kg) rider above loses more than 3.2 lbs (1.5 kg) during a ride, the cyclist is more than 2% dehydrated.
Personal Hydration Strategy
You should take into account the following factors in developing your personal hydration strategy.
- Maintain performance. For your best performance, make sure that you are fully hydrated before throwing your leg over the top tube. You should need to go to the bathroom and produce a good stream of pale yellow urine before strapping on your helmet.
- Physiological limit. You can only absorb so much fluid per hour. Different studies report maximal rates of intestinal fluid absorption ranging from 600 ml (20 fl. oz.) to 1.6 liters (54 fl. oz.) per hour. The absorption rate varies by individual and by the composition, temperature and quantity that you drink (explained in detail below). You are drinking too much too quickly if you start to feel fluid sloshing in your stomach, get bloated or, even worse, develop nausea or diarrhea.
- What’s comfortable. When allowed to drink as much or little as they want, fully pre-hydrated marathon runners drink from 400 to 800 ml (14 to 27 fl. oz.) per hour. Faster, heavier runners racing in a warm environment drink toward the upper rate, while lighter, slower racers in a cool environment consume toward the lower rate.
- Satisfy thirst. Current researchers on hydration and hyponatremia recommend a simple rule of thumb: drink if you are thirsty, and otherwise don’t drink. However, by the time you feel thirsty, you may already have lost 1.5 to 2 liters (quarts) of fluid and be dehydrated. The need to drink is also a function of habit, ritual and the desire for a warming or cooling drink. Further, if you are age 65 or older, your thirst response is blunted, a dynamic that increases with age.
These factors influence gastric emptying; that is, how long it takes for fluid to move from your gut to your blood stream:
- Ride duration. Water can travel from your gut to your skin in as little as 9 to 18 minutes after drinking. You need to drink the water for cooling, so drink even on short rides.
- Palatability. If you like it, you’ll drink it. And if you don’t like it, then none of the other factors matter. What tastes good may change during a multi-hour ride, or as the ambient temperature changes, so be prepared to change drinks, if necessary.
- Carbohydrate concentration. Drinks that are more than 8% carbohydrate aren’t absorbed as quickly. Less concentrated fluids are absorbed at about the same rate as water.
- Amount consumed. If you drink a lot at once, then your gut will empty faster; however, this may cause your gut to bloat. If your gut bloats, then take small, frequent sips.
- Temperature. You are more likely to drink a cooler drink. Further, drinks that are cool rather than at body temperature or warmer are absorbed more quickly during exercise; however, the difference is slight.
- Carbonation. All other things being equal, carbonation doesn’t affect gastric emptying; however, a carbonated beverage will make you feel fuller, so you may not drink as much.
- Relative hydration. Progressive dehydration and higher core temperature increase the time it takes to absorb fluid, a good reason to stay relatively hydrated.
- Mental stress. If you are anxious, this will slow how long it takes to absorb fluid.
- Intensity of riding. High-intensity riding causes slightly slower gastric emptying than moderate riding.
- Caffeine. Small amounts of caffeine (< 180 mg / day) don’t increase daily urine output or dehydration. Depending on how it’s brewed, an 8 oz. cup of coffee has 100 – 200 mg of caffeine, and an 8 oz. cup tea has 15 – 60 mg. A 12 oz. glass of cola has 30 – 50 mg.
- Alcohol. Alcohol is a diuretic and will increase urine production and dehydration.
We’re each an experiment of one and fluid preferences vary by individual. Test drinks in hot conditions to find out what works for you.
- Allen, Hunter and Stephen S. Cheung Ph.D. (2012). Cutting-Edge Cycling: Advanced training for advanced cyclists. Human Kinetics. Champaign, IL.
- American College of Sports Medicine. (2007). Exercise and fluid replacement position stand. Medicine and Science in Sports & Exercise, 39, 377–390.
- Benardot, Dan, PhD, RD. (2006). Advanced Sports Nutrition, 2nd ed. Human Kinetics, Champaign, IL.
- Clark, Nancy and Hegmann, J. (2012). The Cyclist’s Food Guide, 2nd ed. Sports Nutrition Publishers, West Newton, MA
- Friel, Joe. (2009). The cyclist’s training bible (4th ed.). VeloPress, Boulder, CO.
- Hew-Butler, Tamara, et al. (2008). Practical management of exercise-associated hyponatremia encephalopathy, Clinical Journal of Sport Medicine, 18(4).
- 12 Myths about Hydration
- Bicycling in the Heat 101
- Heat Tolerance and Aging
- Cycling in Low and High Humidity
- How to Ride Safely in the Summer Heat
- What Is the Best Electrolyte Supplement
- Which Electrolytes do You Really Need?
Cycling in the Heat Bundle
You can learn more about the science of riding in the heat, and managing your fluids and electrolytes, in my two-part eArticle series:
- Cycling in the Heat, Part 1: Ride Management is 19 pages and covers how to acclimatize to hot conditions, how to train in hot months, what to wear, eat and drink, how to cool down if you overheat, and how to deal with heat-related problems.
- Cycling in the Heat, Part 2: Hydration Management is 21 pages and covers how to determine how much you should drink depending on your physiology and sweat rate, how best to replace your fluids and electrolytes, the contents of different sports drinks, how to make your own electrolyte replacement drinks, how to rehydrate after a ride, and how to deal with hydration-related problems.
The cost-saving bundled eArticles totaling 40 pages Cycling in the Heat Parts 1 and 2 are just $8.98 (a 10% savings).
- Cycling in the Heat, Part 1: Ride Management, 19 pages
- Cycling in the Heat, Part 2: Hydration Management, 21 pages
- Preventing and Treating Cramps, 10 pages Detailed look into the causes of cramps, prevention techniques, and tips (both on-bike and off-bike, including photos) for breaking and flushing cramps.
- Eating and Drinking Like the Pros, 15 pages
What pro riders consume before, during and after a stage and the benefits for cyclists at all levels. Eating and drinking like the pros offers recreational riders the same nutritional benefits, which you can customize to your own needs at a fraction of the cost of commercial sports food and drink, if you choose to make your own. I worked with a professor of nutrition and an expert on hydration and electrolytes (both experts are cyclists) in creating recipes for both sports drinks and food.
The cost-saving bundled 65 pages of eArticles Summer Riding are just $15.96 (a $4 savings).
Coach John Hughes earned coaching certifications from USA Cycling and the National Strength and Conditioning Association. John’s cycling career includes course records in the Boston-Montreal-Boston 1200-km randonnée and the Furnace Creek 508, a Race Across AMerica (RAAM) qualifier. He has ridden solo RAAM twice and is a 5-time finisher of the 1200-km Paris-Brest-Paris. He has written over 40 eBooks and eArticles on cycling training and nutrition, available in RBR’s eBookstore at Coach John Hughes. Click to read John’s full bio.
Bill Beck says
Good article. One small correction though. Coach Hughes notes that the 160 pound rider would burn 500 calories in an hour. But nutritional Calories (usually written with an upper case “C” are equal to 1000 physics calories. A nutritional Calorie is therefore the energy that it takes to raise a kilogram of water 1 degree Celcius.
Kerry Irons says
My own experience has been that electrolytes and hydration are the keys to cramp prevention and long rides in the heat. For long rides (100+ miles), I also add about a teaspoon of table salt to my feed bag (fig bars) to insure adequate electrolyte intake. Your check for hydration can be either weighing yourself as you leave and when you get back (retrospective), having to pee every 3-4 hours or so during the ride (concurrent), or keeping track of past rides and how much you drank and drinking appropriately (prospective). It never hurts to down a glass of water just before you get on the bike . NOTE: if you’re short on electrolytes, you’ll be peeing like a race horse and the water will just go right through you or it will slosh around in your stomach and you’ll want to barf.
Note in this article that the 500 calories at 14 mph is a very high estimate of calorie burn. Less than 300 is more accurate. And most sources say that a conditioned cyclists will be about 25% efficient (75 % of calories burned just result in heat).
Greg Shane says
I’m 160 lbs and wish I could burn 500 calories in an hour at 14 mph. My power meter indicates 375 calories for a 14 mph ride. Did I really burn 500 kcal or did it take 375 kcal to move me forward and the rest went to heat?. What am I missing?
Road Bike Rider says
Kilojoules of power generated on a ride are almost 1 to 1 with calories, so perhaps your riding efficiency is the reason why your calorie burn is lower. I think the power meter based data is the most accurate, so I would believe it over any estimation of the average person. https://trainright.com/3-ways-cyclists-are-using-power-meters-to-pursue-new-goals
Richard Handler says