If you ask some bike riders why they ride, the common answer is so they can eat whatever they want. To a certain extent this is true if you consider weight control, but not so true if you are talking about overall health. One of the most important functions of food is its use as fuel. In fact at the top echelon of cycling where athletes have to sustain a large amount of work over a prolonged period (e.g. grand tour), food starts to be viewed as a fuel rather than something pleasing to the palate. But, which fuels are most important?
Of the three macronutrients, carbohydrates and fatty acids are predominantly used to fuel our cycling. Protein, the third macronutrient, also contains energy but is usually used for maintaining structures in the body and is only used as a significant energy source when others are in shortage. Carbohydrates are of course what we commonly refer to as sugars or glucose, and fatty acids are fat (duh). Glycogen is the storage form of carbohydrate in the body, and adipose tissue is the storage form of fat.
Glycogen is stored in the liver and in the muscle. Both of these stores have distinct functions. Glycogen in the liver provides glucose to the blood and glycogen in the muscle provides glucose to the muscle. Important to nutrition is the fact that glycogen stores in the body are relatively small. In fact, if you only used glycogen to fuel cycling you would run out in about an hour. Fat, on the other hand, is stored quite well and there are many sites of storage in the body. There are sites under the skin (subcutaneous), within abdominal organs (intrabdominal), and in the muscle (intramuscular). As opposed to glycogen, the amount of adipose in the body, even in the leanest cyclist, can be thought of as endless. Fat consumed in the diet is preferentially packed away to storage while carbohydrate is used preferentially as a fuel. Contrary to what many people will tell you, an excess of carbohydrate is not made in to fat. However, excess calories do result in fat deposition for other reasons.
The factors that control fuel use is still a very active area of research mostly due to the alarming rate of storage of energy going on in society right now (read: obesity). But, the pattern of fuel use in healthy people during exercise is pretty well sussed. At rest and during low intensity exercise, fat is the predominant fuel source. At moderate to high intensity exercise, carbohydrates are the predominant fuel source. The included figure shows how fuel uses change over progressively higher exercise intensity. What you will notice about the figure is that when you are at a given percentage of your max exercise capacity, there is a given percentage of each fuel being used. For illustration, look at 40% VO2 max and you will see that there is an approximately even mix (50 % each) of fat and carbohydrate. More importantly, at higher intensities than 40% VO2 max, the intensities that we train and race at, carbohydrates are the predominant energy source. No matter how much you train, the % of each fuel used is determined by how hard you are riding. In other words, the lines on the graph do not shift by any meaningful degree after training.
The knowledge that fat is predominantly burned at lower intensity exercise has lead some to expound the idea of the "fat burning zone". But, this idea is another example of a little knowledge being a dangerous thing. Consider a 2 hr ride performed at 40% of VO2max to stay in the "fat burning zone", and lets say that it took 1150 calories to complete this ride. At a 50/50 mix of carbohydrate to fat burned for energy (as shown in the figure), 575 carbohydrate calories were used and 575 fat calories were used. Now consider a 2 hr ride done at 75% VO2max. Because of the higher intensity of this ride it takes 2300 calories to complete. According to the figure approximately 75% of the energy will come from carbohydrate and only 25% will come from fat. A simple calculation shows that 25% (fat) of 2300 calories is 575 calories. So, did you really burn more fat in the "fat burning zone"? In reality this is all a bit too simplistic and what we will likely discuss in the future is that for the purpose of weight control it does not really matter where your calories are coming from, but how many total calories are burned.
There is one other important point to this figure and our discussion. As stated, carbohydrates are the predominant energy source at the intensities we cycle at. I also explained that carbohydrate stores (glycogen) in the body are quite limited. It is therefore apparent why cycling for any length of time (greater than an hour) or for performance, must be supplemented with carbohydrate.
To summarize, fat and carbohydrate fuel activity. Our body has an endless supply of fat, but carbohydrate stores are limited. However, carbohydrates are a preferred fuel in moderate to high intensity exercise making it necessary to supplement the limited stores we carry.
Finally, there is a corollary. Lots of you have been told that training increases your ability to use fat. But, above I said that the lines on the figure stay the same no matter how much you train. Some of the more astute among you will catch on to the fact that even though the lines in the figure do not shift with training, the amount of work you are doing at a given percentage of VO2max does change. Training increases your ability to do work, so the amount of work you can do at 75% VO2max may increase from 250 watts before training to 300 watts after training. The lines on the figure at 75% VO2max do not change (75% CHO, 25% Fat), but the absolute amount of work, thus calories burned, does increase. The end result is that the overall amount of fat (and carbohydrate) used does increase. So, when we speak of training, it does not increase the % of fat used at a given intensity, but it does increase the overall amount.
Finally, a true/false question: glucose is the only carbohydrate used for energy during activity.
False, although many will only talk about the contribution of glucose (from the blood or muscle glycogen) to carbohydrate metabolism, lactate is actually another carbohydrate energy source that can be derived from glycogen in one muscle and transported to another muscle for use. At times, the amount of lactate being used for energy production exceeds the amount of blood glucose.
Ben Miller is Senior Lecturer at the University of Auckland. Ben did a PhD at the University of California – Berkeley and a Post-Doc at the Institute for Sports Medicine, Copenhagen before arriving in Auckland. As a departure from his life in a closed scientific box safe from the realities of the world, he is a cyclist regularly taking his life in his own hands on the streets of Auckland and in the local club racing and criteriums. Ben's wife is much more successful at cycling having competed full-time in Europe and the US for the last 4 years.