By Sally Warner, PhD


The main differences in how to fuel for different duration events are the intensity and the total calories used for the effort.  Realistically, the duration of the effort will define the intensity or at least the average sustained level of effort.  Clearly nobody can sustain their 1 hour race pace for 8 hours.  For any of these you want to be sure to have as much fuel available before starting and then be smart about fuel replacement during the event.  The amount and kind of calories used depends on the intensity level of the sustained effort.  All of this technical information is helpful, but the ability of the body to utilize what is consumed is an important regulator of this whole topic.  Effective fueling requires knowledge, planning, and practice.

Available energy stores:

Certainly there is an advantage to making sure energy levels are “topped-off” prior to the exercise session, but there is a limit to how much glycogen we can store. Our maximal carbohydrate storage is about 1,800 calories for a 150 pound athlete including glycogen stored in blood, muscles and the liver.  After an hour of endurance exercise stored glycogen levels become compromised with significant depletion beyond 2 hours of sustained endurance exercise.  Glycogen is the source of energy most often used for exercise. It is needed for any short, intense bouts of exercise and supplies energy during the first few minutes of any sport. During long, slow endurance exercise, fat does help fuel activity, but glycogen is still needed to assist the breakdown of fat into something muscles can use for fuel.

Total energy required during exercise:

The total energy required for endurance exercise will depend on the type of exercise and associated exercise efficiency.  Results from a VO2 max test with oxygen consumption measured or standardized metabolic equivalents (METS) can be used to estimate the number of calories your body uses for a given exercise intensity.  Likely most of you are using somewhere between 600 and 1,200 calories per hour.  It is not realistic to assume that endurance athletes will be able to replace all calories used during exercise, but to maintain health, body weight, energy levels, and repeat performance, this caloric deficit will need to be filled within 2 hours post exercise.

Types of energy:

It is true that intense efforts (starts, sprints, hill climbs, etc.) will require anaerobic metabolism and nearly 100% carbohydrates.  However, a long duration sustained endurance effort will depend mostly on aerobic oxidative metabolism which requires a mix of carbohydrates and fats for fuel.  Assuming an athlete was averaging about 70% of maximal oxygen capacity, approximately 50% of their energy would be from carbohydrates and the rest from fats (oxidation of free fatty acids).  Endurance training can increase the proportion of energy derived from fat but not the total amount of energy used at a given workload.  As exercise intensity decreases, a larger proportion of energy comes from fat (mostly muscle triglycerides) and less from carbohydrates.

The primary goal for nutrient consumption during exercise is to maintain blood sugar (glucose) levels.  Not only does blood sugar provide readily available energy to working muscles, but it is also important for maintenance of clear thinking, coordination and responsiveness (brain and central nervous system). Current research supports the benefit of carbohydrate consumption at approximately 30–60 grams per hour (approximately 120-240 calories per hour).  This can be achieved by consumption of a sports beverage containing about 6%–8% carbohydrates.  Carbohydrate availability also helps prevent protein from being used to make glucose for energy. Protein is the main building block for muscles and other tissues, so it is important to spare its use as a fuel.  Additionally, the breakdown of protein puts stress on the kidneys and can put the body at risk for keto acidosis.

Another factor important to success of endurance performance is hydration.  No matter what macronutrients you provide the body, dehydration (water deficit in excess of 2–3% body mass) will decrease exercise performance.  Thus, adequate fluid intake before, during, and after exercise is important for health and optimal endurance performance.  This supports the use of a sports nutrition beverage to fulfill carbohydrate and fluid requirements together.

Regulators of energy availability:

There are many components that can regulate the use and availability of during exercise nutrition.  These start with the portability and palatability.  Most experienced endurance athletes have figured out their strategy to facilitate the portage of their supplements (hydration pack, water bottles, feed stations, etc.).  The next is making sure that you like the flavor and texture of the supplement and that this will be true for the whole duration of your event.  The mixed concentration and tolerability to your GI are things that also need to be worked out during training and not a surprise on race day.

Once you have figured out how to deliver calories to your body during exercise, the next ‘regulator’ of fuel availability is how well your body is able to absorb these from the gut.  Higher sodium concentration of a supplement encourages gastric absorption of carbohydrates which during ultra distance events is a win-win approach.  An optimal mix would include complex carbohydrates with high glycemic index carbohydrates to enable digestion and provide immediate energy during long and intense workouts.

Slow absorbing nutrients like fiber, fructose, gums found in gels, fats and whole food proteins can reduce nutrient absorption and cause GI distress.  Consuming products that have slow nutrients can be very detrimental to your absorption rate and can cause you to bonk prematurely.  Delays in gastric absorption shift your fuel dependency to stored glycogen while the gut tries to absorb.  As such, relying on fast absorbing nutrients like various high glycemic carbohydrates allows use of these calories immediately and spares your stored glycogen.

For ultra long endurance efforts, nutrition is a bigger determinant of success.  There is little difference in the type of fuel needed to go ultra long except for a greater need for electrolytes and amino acids.  The total caloric deficit and subsequent fatigue are certainly bigger risks with ultra endurance distances and hence appropriate pacing to assure a greater reliance on stored fat as a fuel source is key.  If you go too fast you quickly use up stored glycogen.  At a slower pace your body can efficiently break down your stored fats as a fuel source and spare your stored glycogen.   More time and careful planning should be considered.  Beyond fast carbohydrates, Ultra distance athletes should also focus on the addition of amino acids, electrolytes and replacing a more significant caloric deficit.

Timing of fuel consumption is also important.  Starting too late or not replenishing regularly enough can be detrimental to performance.  Using liquid sports nutrition products can help to encourage regular replenishment of tolerable amounts of calories.  For longer events at lower intensity, athletes should consider including solid supplements (bars, etc.) to add variety as well as caloric density required to complete long distances.

Fueling for endurance needs to start before you exercise.  Make sure your glycogen stores are optimized.  Prepare and bring enough of the right calories appropriate for the duration of exercise.  Make sure you choose your fuel wisely and practice your approach before any important events.