Here is some extracts from a paper by Craig S. Atwood, Richard L. Bowen that examined the metabolic performance of an élite cyclist, Lance Armstrong, before and after his treatment for testicular cancer. What the authors are saying is that the improvements in Armstrong’s performance were caused largely because the treatment allowed his body to use fat as a fuel more efficiently.
There are two primary sources of energy available to athletes: fat and carbohydrate. Muscle and liver store most of the body’s carbohydrate, enough fuel (400–600 g) for about 90– 120 min of high-intensity exercise. Fat stores on the other hand could supply energy needs for 60–100 h due to its higher energy content and abundance throughout the body compared with carbohydrates. Endurance athletes use a mixture of both fuels and when the carbohydrate becomes fatigued they ‘hit the wall’, which is characterised by a drop in speed as a direct result of decreased carbohydrate use which in turn is as a result of a fall in blood glucose levels due to depletion of muscle and liver glycogen stores and blood glucose stores.
Atwood and Bowen state:
Fatty acid utilization is unchanged during fatigue, indicating that lipid is the preferred fuel of muscles, but is rate limiting, and that carbohydrate utilization is required for optimal performance. Therefore, those athletes that can use a higher FFA/glucose ratio at any given speed (i.e. _V O2 ) for their overall energy needs will endure longer than those with a lower FFA/glucose ratio. Furthermore, athletes that do not utilize all their carbohydrate stores during an exercise period will have a greater chance of replenishing their carbohydrate stores to maximal levels compared to those that start with lower carbohydrate stores. This means exercise of a similar or greater intensity and duration can be achieved on subsequent days, and is perhaps the key to understanding the remarkable day-to-day endurance of Lance Armstrong compared with other cyclists1.
So whilst carbohydrate is necessary for high output, one way to improve performance is to make the racers use of fat for fuel more efficient and so slow down the rate that the precious carbohydrate is consumed during the race. In this way the time/distance a racer can keep up optimum performance is increase and at the same time because the carbohydrate stores in the muscles are less depleted at the end of a race, the recovery post race is quicker.
So how do you improve your body’s ability to metabolize fat? Certainly exercise has a big role to play:
Atwood and Bowen go on to say:
These changes indicate an adaptive response to endurance training that decreases glycogenolysis in muscles and spares glycogen reserves. Conversely, detraining leads to an increased reliance on carbohydrate metabolism during exercise, as shown by a higher exercise respiratory exchange ratio, and lowered lipase activity, GLUT-4 content, glycogen level and lactate threshold . Hence, well-trained individuals using a higher proportion of FFA for energy will spare more muscle and liver glycogen, and together with their higher basal glycogen reserves, can therefore maintain a similar level of intensity for a longer period of time compared with untrained individuals1.
But it’s not the only way. As part of their research they conducted the following experiment:
The advantages of increased fat utilization on performance are highlighted by the results of a chronic (4 week) eucaloric ketogenic diet (high fat) on submaximal exercise performance in trained cyclists. The mean ergometer endurance time for continuous exercise to exhaustion at 62–64% _V O2 max on this diet was 151 min compared to 147 min prior to the ketogenic diet . Despite a drop in RQ (from 0.83 to 0.72), a 3-fold drop in glycogen oxidation and a 4-fold reduction in muscle glycogen, the endurance of these well-trained cyclists was slightly better. These results indicate that aerobic endurance exercise by well-trained cyclists is not compromised by 4 weeks of ketosis. Thus, physiological adaptations to a high fat diet conserve limited carbohydrate stores (glucose and muscle glycogen) and make fat the predominant muscle substrate at submaximal exercise.
Now I’m not suggesting that élite endurance athletes adopt a ketogenic diet, but very high carbohydrate diets reduce the body’s ability to utilise fat for fuel so perhaps there are benefits to a low carbohydrate diet for endurance athletes.