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Fat + Carbohydrate are the Principal Fuel of Aerobic Exercise

Protein Needs Increase for Endurance >

What factors determine what muscle uses for fuel during aerobic exercise?

The primary fuel for aerobic and endurance activity depends on both the intensity and duration of the effort. The relative contribution of the different energy nutrients fueling the working muscle will vary depending upon whether exercise lasts for 15 minutes, 30 minutes, 1 hour, or 2 hours. Also, the mixture of fuel will be different at these time intervals when they are performed at different intensities.


Another important factor is when and what a person last ate and whether or not they are using sport drinks during activity. Whether it is exercise intensity or timing and composition of last meal, hormones will direct the fuel use during exercise. Nutrient availability from sport drinks or pre-exercise food and beverages will also influence the relative amounts of fuel used.


What neurotransmitter and hormones are involved in aerobic exercise fuel use?

At low intensities such as walking, the brain sends a signal through nerves to fat tissue to breakdown fat. The fat can then circulate to muscle and be used as fuel. As the intensity of an aerobic effort increases the level of epinephrine will increase, while the level of insulin will decrease. The brain is mostly responsible for doing this by sending signals to the adrenal glands to release epinephrine and to the pancreas to limit the release of insulin. This is important since epinephrine will promote the breakdown of glycogen in muscle and liver as well as fat in fat cells. Meanwhile, insulin promotes the building of these stores, which is opposite of what you want. Therefore, as exercise intensity increases, more glucose is available in muscle cells and more fatty acids are circulating to and available within muscle cells.


What is the relative breakdown of fuel during aerobic exercise?

A general rule is that for durations longer than 20 minutes the percentage of fat utilized climbs as the intensity level decreases. For instance, an unfed person performing lower intensity activities, such as brisk walking, bicycling (13 to 15 mph), jogging, and light roller blading, will burn a higher percentage of fat (60 to 70 percent). However, at more moderate intensity activities, such as bicycling (16 to 20 mph) or running (8 to 9 mph), the reliance upon fat for fuel decreases to about 50 percent. Further, as even higher levels of intensity are performed, such as by professional marathon runners and endurance cyclists, carbohydrate is the primary fuel followed by fat and then amino acids.



















How do we burn the most fat during cardiovascular exercise?

The amount of fat used as an energy source is greatest at a moderate intensity. So even though fat accounts for a lesser percentage of total energy expended, there is more total energy used at the moderate intensity, which leads to greater amounts of fat used. Think of it this way. Which would you rather have—60 percent of $100.00 or 40 percent of $200.00? This is one reason why cardiovascular exercise equipment often has a graphic on the display indicating the “target heart rate” or “fat burning zone.” Here the target heart rate is associated with the moderate level of intensity or the level of intensity in which you are burning the greatest amount of fat.



Another important consideration is duration. Cardiovascular exercise is always encouraged to last at least 20 minutes and preferably 30 to 45 minutes for most people. The reason for this is that it seems to take a little time for all the needed events for optimal fat utilization to come on line. This includes everything from mobilizing fatty acids from fat stores to increasing the delivery of O2 to working muscle. There are a few other biochemical reasons for this as well, but they are beyond the scope of this text. The important thing is that it takes a while, often 12 to 20 minutes, to reach optimal fat burning efficiency. So be patient and include a period of lower intensity warm-up as well.


What causes muscle exhaustion in endurance activities?

A principal factor associated with exhaustion during endurance exercise is the availability of carbohydrates to working muscle. Quite simply, when muscle glycogen stores are depleted, muscle exhaustion ensues shortly thereafter. The depletion of muscle glycogen along with dehydration are the most significant contributors to exhaustion or what endurance athletes call “hitting the wall” or “bonking.” From this it is easy to see why sport drinks such as PowerAde and Gatorade are so popular. Electrolyte imbalances may also lead to fatigue, but this might occur only during very long efforts in which water is provided. Today, with the popularity of sport drinks and endurance foods the risk of an electrolyte imbalance is often reduced.


Can diet affect the onset of exhaustion?

Stored carbohydrate in the form of muscle and liver glycogen reflects dietary carbohydrate intake. During training or competition, researchers have shown that athletes can significantly increase their training time or time till exhaustion by eating a high carbohydrate diet. For instance, one athlete on a low carbohydrate diet will reach muscle exhaustion long before another athlete on a high carbohydrate diet (> 60 percent carbohydrate).


A high carbohydrate diet allows the body to replenish glycogen stores in between training sessions. Contrary to what many people think, it actually takes a while to rebuild muscle glycogen stores that have been used during exercise. In fact, if an endurance athlete reduces his or her muscle glycogen to nadir levels during training or competition; it can take nearly a day to rebuild them. This means that the athlete should eat carbohydrates immediately after completing the session and throughout that day to provide both the needed glucose and insulin to rebuild those stores.


What is carbo-loading?

Some athletes preparing for a big event will attempt to carbohydrate load or carbo load. These events include marathons, triathlons, bicycle centuries or longer, and long-distance swimming. The desired outcome is achieving the highest possible level of muscle glycogen just prior to the onset of the competition by coordinating a high carbohydrate intake (> 60 percent) for a couple days prior to competition while at the same time tapering or eliminating training sessions.


Carbo-loading would not be beneficial for shorter endurance efforts or sports involving only brief efforts (e.g., power lifting, velodrome cycling, or most track and field events). However, intermittent sport athletes such as soccer, football, and field and ice hockey players might benefit, however, the practice and game schedule would make carbo-loading unrealistic during season for some sports.




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