Endurance Fuel
The endurance athlete is fueled by 2 major fuels -carbohydrate and fat.
Carbohydrate is like "high octane" fuel - a steadysupply is necessary for high intensity exercise tocontinue. Fatigue during endurance exercise isassociated with depletion or reduction of the body'scarbohydrate stores. As carbohydrate supply declines,the athlete must slow down.
Fat is like "regular" fuel - it provides a good source ofenergy for sustained, lower intensity exercise. Welltrained endurance athletes can rely more on fat for fuelwhich helps to conserve their carbohydrate fuel anddelay exhaustion and the need to decrease exerciseintensity.
The carbohydrate "fuel tank" is limited in size. Mostendurance athletes can store between 2000 and 4000Calories worth of carbohydrate in a form calledglycogen. Most of this is stored right in the muscletissue. Some also is stored in the liver.
The fat "fuel tank" is usually thought to be ratherunlimited in size. However, this may not apply to allendurance athletes. Most of the fat is stored in fat cellsthroughout the body. However, some fat is stored rightin muscle tissue in a form called muscle triglycerides.
As the the intensity of exercise increases, the the bodyuses proportionately more carbohydrate and less fat forfuel.
During exercise, the muscles have 2 sources ofcarbohydrate. The muscles are using carbohydrate forfuel in the form of glucose. This glucose fuel comesfrom, 1) the breakdown of glycogen right in the muscletissue, and 2) glucose taken up from the blood into themuscle. Glucose in the blood comes from carbohydratethat is absorbed from food and drink and from glucoseproduced by the liver. The liver can produce glucosefrom the breakdown of glycogen in the liver and bymaking glucose from various other molecules.
During exercise, the muscles also have 2 sources offat. The muscles are using fat for fuel in the form offatty acids. This fatty acid fuel comes from 1)breakdown of triglycerides right in the muscle tissue,and 2) fatty acids taken up from the blood into themuscle. Fatty acids in the blood come primarily fromthe breakdown of tryglycerides in fat cells throughoutthe body. As the triglycerides are broken down in the fatcells, the fatty acids are released into the blood and theytravel to the muscle where they are taken up and usedfor fuel.
During long duration exercise, the body can depletethe muscle glycogen "fuel tank" within 3 to 4 hours. Byconsuming proper mixtures of water and carbohydrate,the athlete can provide more glucose to the musclethrough the blood to help decrease reliance on muscleglycogen and conserve it longer.
During long duration exercise, the body also candeplete muscle triglyceride (muscle "fat fuel tank")significantly. Thus, the muscles can become morereliant on fatty acids "shipped out" from fat cells andtaken up from the blood into the muscle.
Thus, limitations in fuel supply to the muscle can berelated to depleted carbohydrate (glycogen) anddepleted fat (triglycerides) right in the muscle as well asa decreased supply of these fuels from the blood.
How much fat is too little fat? For good nutrition?For endurance performance?
Dietary fat intake that is excessively low for anextended period of time would be expected to result indecreased fractional absorption of fat soluble vitamins,and deficient intake of essential fatty acids. It can takemany months or even years for fat soluble vitaminlevels and essential fatty acid levels to reach clear levelsof clinical deficiency.
The appropriate level of fat intake may vary from oneperson to another. Amount of body fat and overallenergy expenditure of training may influence theamount of fat that is appropriate for an athlete. Due togenetic differences, some athletes may perform better athigher levels of body fat than others. In other words, itis not always better to have less body fat. There is apoint of diminishing returns that likely varies greatlyfrom one person to another.
The optimal range for fat intake of an athlete isprobably 15 to 25% of energy intake. Athletes with highenergy expenditure may benefit most by consuming fatat the lower end of this range since 25% of a very highcalorie intake can be too many grams of total fat intakeand may not be conducive to long-term health in someindividuals.
C. Alan Titchenal, PhD, CNS
Department of Human Nutrition, Food & Animal Sciences
CTAHR, University of Hawaii
