During exercise you burn your limited stores of glycogen muscle fuel and you lose fluids and electrolytes through sweating. If you exercise long enough these deficits build and will compromise your ability to train or compete. So the challenge during endurance workouts and competitions is to maintain hydration and spare your limited muscle fuel reserves. Meeting these two challenges will help you to exercise longer and at a higher intensity.

Refueling during exercise
Carbohydrates are the fuel of choice during moderate-to-high-intensity endurance exercise. Glycogen is the storage form of carbohydrate fuel in muscles and the liver. However, glycogen stores are limited and 60-90 minutes of exercise can seriously deplete your reserves. The fatigue that results from depleting glycogen stores requires cutting way back on exercise intensity or even stopping altogether.

Fortunately, you can delay the onset of fatigue during endurance exercise by consuming carbs. A common mistake many athletes make is not consuming enough carbs. The amount of carbs you need is determined by the duration of your workout or event.

For exercise lasting 60 minutes or less
You don't necessarily need to consume additional carbs because your glycogen stores should be adequate to meet your fuel needs. Water or a sport drink is appropriate.

For exercise lasting 1 to 3 hours
Consuming 30 to 60 grams of carbs over the course of each hour of exercise will help sustain your performance and delay the onset of fatigue during endurance exercise. For example:

For exercise lasting 2 to 3 hours or longer
Consuming 45 to 90 grams of carbs over the course of each hour of exercise is recommended provided that the carbs are in a 2:1 ratio of glucose to fructose (eg, PowerBar® products with C2 MAX). For example:

For long-duration events it is important to strike a balance between fuel availability (45-90 grams of carbs per hour) and gastrointestinal comfort. The actual amount of carbohydrate you consume per hour should be determined by you during race pace training.

Practical carb options during exercise
It's one thing to define the optimum carbohydrate intake needed to delay the onset of fatigue, but it's altogether another to try to achieve that intake during training or competition. The logistics of different sports often dictate what is achievable and when.

Cyclists and triathletes typically utilize sport drinks, energy gels and bars, and sometimes solid foods to meet their carb needs.

On the other hand, runners generally restrict themselves to sport drinks and energy gels as carb sources during exercise.

For team and racquet sports, taking every opportunity to refuel during brief breaks in the action is recommended to help maintain blood glucose levels and to spare glycogen stores. Sport drinks, energy bars, and energy gels are good on-the-go options for carbohydrate delivery.

Hydration during exercise
During exercise, blood flows to the muscles at a high rate to deliver oxygen and other substrates needed to power performance. At the same time, a high rate of blood flow to the skin is needed to carry heat away from muscles to the body surface where it can be dissipated. When blood volume is decreased due to unreplaced sweat losses, blood flow to the muscles and the skin is compromised. As a result, your ability to perform athletically decreases. Compromised athletic performance can be avoided by hydrating during exercise.

Your hydration zone
You can perform at your best if you don't lose anymore than 2% of your body weight due fluid loss during exercise. This 2% rule is called your hydration zone. For example:

When you fall out of your hydration zone, especially in high temperature/humidity conditions, the perceived difficulty of exercise is much harder and your ability to think clearly and tactically is compromised.

Know your sweat rate
The right level of hydration for you is based on your personal sweat rate, not the sweat rate of your training partner. To measure your sweat rate, click on the Sweat Rate Calculator.

Calculating your sweat rate is fairly simple. In fact, you'll be guided through a one-hour test workout that will calculate your sweat rate for you. It's recommended that you conduct the test workout at the intensity and in climate conditions similar to the conditions you encounter during training and competing. Also, test yourself at different exercise intensities and in different climate conditions, and as your fitness level progresses. By repeating sweat rate testing under varying conditions, you will better understand how your body responds to different conditions. This will help you fine-tune your hydration plans for whatever you encounter during training and competing.

Balancing sweat rate and fluid intake
Knowing your sweat rate is only half the battle. The other half is actually consuming the fluids you need. Many athletes find it difficult and uncomfortable to consume fluids at a volume that matches their sweat rate. If that is the case for you, remember that your hydration zone is losing no more than 2% of your pre-exercise body weight due to fluid loss during exercise. So, you don't need to exactly match your sweat rate. Instead, you need to make sure that you don't drop out of your zone while you are exercising. For example:

The point is to know your sweat rate so that you know how much fluid you are losing and how much to consume to remain comfortable and within your hydration zone.

You may also be able to gradually train your gut to absorb more fluid during exercise. So, if you're having trouble consuming enough fluids to stay in your hydration zone, start at a volume you are comfortable with, and gradually increase the volume consumed per hour during training until you reach a more acceptable level of intake.

Electrolytes
Sweat contains a number of electrolytes, but the two in highest concentration by far, are sodium and chloride, which together form salt. Thus, consuming sodium in the form of salt during exercise helps to replace what you lose in sweat. The benefit of consuming some sodium during exercise is that it helps maintain the fluid-electrolyte balance in your body and stimulates thirst. This helps you hydrate more effectively. Sodium and chloride are included in well-designed sport drinks, energy gels, and energy bars.

Salt tablets aren't routinely recommended as they can cause gastrointestinal distress.

Important fluid features
Whenever possible when hydrating during exercise, drink beverages that are cool in temperature and have flavor. These attributes will help stimulate your thirst and increase the amount of fluid you consume.

Putting it into practice: Fueling and hydration strategies by duration of exercise

Exercise lasting 60 minutes or less
Plain water or a sport drink is fine. The benefits will be alleviating dry mouth and possibly reducing the perception of the difficulty of the exercise.

In high temperature/humidity conditions, a sport drink is a better option than water because of the electrolytes present.

For exercise lasting 1 to 3 hours
Definitely use a sport drink as you will benefit from the carbs and electrolytes. Begin ingesting fluids early in your exercise and at a rate that is both comfortable and practical to replace most of the fluid you will lose as sweat.

Plan to consume 30 to 60 grams of carbs per hour for exercise of this duration to help maintain blood glucose levels and spare glycogen stores. The intake of carbs can come from sport drinks and gels. Also, energy gels taken with water can be used as an alternative to a sport drink at any point during exercise.

Easily-digested solid foods may be consumed by some athletes during exercise depending on opportunities to do so, but remember that fluid requirements need separate attention.

For exercise lasting more than 3 hours
Definitely use a sport drink as you will benefit from the carbs and electrolytes. Begin ingesting fluids early in your exercise and at a rate that is both comfortable and practical to replace most of the fluid you will lose as sweat.

Plan to consume 45 to 90 grams of carbs per hour for events of this duration to help maintain blood glucose levels and spare glycogen stores, provided it is a 2:1 ratio of glucose to fructose (eg, PowerBar® products with C2 MAX). The intake of carbs can come from sport drinks and gels. Also, energy gels taken with water can be used as an alternative to a sport drink at any point during exercise.

Easily-digested solid foods may be consumed by some athletes during exercise depending on opportunities to do so. This may include small amounts of salty snacks to help replace electrolyte losses. Remember that fluid requirements need separate attention.

Hyponatremia or overhydration is more commonly seen in long-duration endurance events. The primary cause of hyponatremia is consuming fluids at a rate that exceeds the rate of sweat loss. Prevent hyponatremia by avoiding consuming fluids at volumes that cause you to gain weight.

Fine-tuning to meet your individual needs
Consuming carbs, fluids, and electrolytes during exercise can improve your endurance performance. The recommendations above are a starting point for you to implement during training. It's likely that your specific needs and preferences will vary from these general recommendations, so use your training workouts to adjust and fine-tune the recommendations to meet your specific needs.