The day is sunny, hot and humid. You’re four hours into a late-summer century. As you pull away from the halfway rest stop you reach for your best MTB hydration pack and grab your bottle to take a drink.

You expect the first swallow to be refreshing, but as the Super-Buster-Thirst hits your belly you begin to feel nauseated. You take a few more pulls on the bottle, and with each swig, you feel more and more as if you’re going to perform the dreaded Technicolor yawn. Two hours ago you felt fine, so what happened?

Cyclists are generally advised to drink a bottle of water or sports drink for each hour they exercise. After getting home the conventional wisdom has suggested you weigh yourself and for each pound lost during exercise you should drink another eight ounces of water.

While this approach may work for shorter rides (three hours or less), it is insufficient to meet the fluid requirements for most riders exercising in hot weather and for longer rides. As your body becomes more and more dehydrated your sports drink begins to taste sweeter and sweeter–a prime indicator that you aren’t drinking enough.

Dehydration as a result of sweat loss happens at different rates for different people, but while it does vary, it doesn’t vary greatly. I was given an opportunity to learn just how much sweat can be lost by a cyclist in a single hour.

Last winter when I visited Gatorade’s Sports Science Institute, John Stofan, one of the company’s project scientists, had a little fun with me. To illustrate how much fluid is lost in a single hour of riding, Stofan weighed me then had me ride for one hour on a bicycle ergometer and weighed me again after the hour was over.

During the hour of exercise I rode at a fairly hard pace (heart rate from 148 to 160 beats per minute) in a controlled environment (85 degrees and 15 percent relative humidity) and was offered as much lemon-lime Gatorade as I wanted to drink. During the test Stofan took samples of my perspiration to check the concentration of sodium and potassium, often referred to as sweat electrolytes.

Over the course of the hour I consumed one 20-ounce bottle of Gatorade. What I learned should have an effect on the way I hydrate for the rest of my riding days. In one hour of hard riding I lost 1.75 liters (50.8 ounces) of sweat, or .9 percent of my gross body weight.

Studies have shown that a 1 percent level of dehydration can adversely affect performance, causing heart rate to rise. Stofan estimated that I was losing .438 liters (14.7 ounces) of fluid every 15 minutes; in order to keep up with that level of fluid loss I’d have to consume 16 ounces of electrolyte solution every 15 minutes to avoid the worst effects of dehydration.

This could prevent a marked drop in my bloodstream’s sodium levels, but he said that consuming Gatorade wasn’t quite enough. His suggestion was to eat a half ounce of salted, fat-free pretzels every half hour. According to many I’ve talked to, sodium loss is much more harmful than potassium loss.

Ahead of the Curve 

It is unlikely that any of us drink enough during exercise to avoid a fluid deficit by the end of a ride. For every liter (33.6 ounces) lost, heart rate is elevated 8 beats per minute and fluid loss can exceed two liters (67.2 ounces) per hour.

In other words, it is possible to lose enough fluid in a single hour of riding to severely hamper performance. Whether or not we have a fluid deficit at the end of a training session isn’t really the point. How we handle that fluid deficit is.

Studies have shown that the best way to stay hydrated is to drink something that does not shut off the osmotic drive (the drive to drink) and minimizes urine output.

Research scientists actually went to the trouble to perform clinical trials to tell us that we’ll drink more if the drink tastes good, doesn’t upset the stomach (isn’t too strong), provides energy to muscles in the form of carbohydrates (about a 6 to 8 percent solution) and replenishes lost sodium.

While selecting easily digestible, good-tasting fluids may seem obvious, this has a big effect on energy and sodium levels.

Surprisingly, water is too good a thirst quencher to be effective in endurance exercise. Water shuts down the osmotic drive prematurely so that your body believes it has been rehydrated adequately. This happens because the osmotic drive is more easily switched off than on and it is possible to lose fluid faster than your body can sense.

Water also moves through your body very quickly, arriving at the bladder in a relatively short period of time. By adding sodium to a drink in addition to a sweetening agent, the body decides that you are still thirsty. And because a sports drink contains carbohydrates it can’t be sent almost directly to the bladder so your body has a better opportunity to use the fluid to rehydrate.

The balancing act a cyclist is forced to perform is a complicated one. To enjoy optimal hydration on the bike, we must consume a drink that tastes good and won’t upset our stomachs (as if we didn’t know that). The drink must also contain some carbs (here’s where it gets difficult) and it must still be strong enough that we receive enough energy from the drink to prevent the dreaded bonk.

Most of us can’t (or won’t) drink more than two bottles in an hour which is why scientists and nutritionists talk about that magic 6 to 8 percent solution of carbohydrate in a drink.

If the solution is much stronger, gastric emptying (the rate at which anything leaves your stomach) slows and you not only don’t get enough energy, but your stomach can also become upset, something that tends to manifest first with gas and second with nausea.

If, on the other hand, you don’t mix the drink strong enough, you risk undernourishing yourself and bringing on the bonk. Drink mixes like Champion Nutrition’s Cytomax, SmartFUEL’s WarpADE, Gatorade and Shaklee’s Performance remain some of the best options for fueling on the bike, but it is important to read the labels in order to establish how strong to mix the drink in question.

The number you see under nutrition facts for the best endurance supplements for cyclists refers to the percentage of the U.S. RDA based on serving size, not how strong the mix is. To find that out, you have to do a little math. Take the number of grams for a given serving, multiply it by .035 and divide by the number of ounces your water bottle holds.

It’ll look a little like this: 25 grams x .035 / 16 = .055 or a 5.5 percent carbohydrate solution. Of course, the final determinant in how you mix a drink will be your belly.

Finding a sports drink concentration level that works for you can take some time and experimentation. The magic 6 to 8 percent solution the lab coats recommend is a range you won’t want to deviate from too far.

There’s substantial clinical evidence showing that at concentrations above 8 percent most anyone exercising vigorously will experience stomach upset. It’s not necessary to reinvent the wheel on this one. Bear in mind that if you mix your drink below the 6 percent solution recommendation, you run the risk of bonking. If you do screw up on a ride, there are a few things you can do to make it home without self-destructing.

Eat something as soon as possible and back off on the intensity. You might want to plan ahead and keep a gel or energy bar with you just in case.

A gel or energy bar of most any sort is better than stopping at the local doughnut shop. With a few tries, though, you should be able to find that miracle strength for the drink that allows you to ride as hard as you want. And provided you put that bottle to your lips often enough, you should be able to ride all day.