If you have any physiology, performance, or nutrition related questions, email Ben at ben@yourgroupride.com.
For this column I decided to not directly answer a question, but rather set the stage for some important questions. The goal of this column is to lay out, in a rather simplified way, the fuels you use during exercise.
ATP is the energetic currency of the human body, however, it is not stored to any large degree, but rather made on an as needed basis. The body uses nutrients to convert the potential energy contained in those nutrients to ATP. The main nutrient fuels available to the body are carbohydrates (CHO), fats, and to a lesser degree protein. This column will only focus on CHO and fats since they are quantitatively the most important fuels (see Figure 1 for energy storage sites). Within carbohydrate the main energy sources are glucose and lactate. Fats start as triglycerides, which are broken down into free fatty acids (FFA). CHO are stored as glycogen in the muscle and the liver, FFA are stored as adipose tissue (fat) in a variety of locations. The amount of CHO that can be stored as glycogen is limited while the amount of fat that can be stored as adipose is nearly unlimited. Consequently during a bout of exercise the amount CHO is limited while the amount of fat is not. However, even though fat is unlimited and CHO is not, CHO is the preferred fuel at the intensities that cyclists race at.
There is a simple laboratory method to estimate the proportion of fat and CHO being used at any given time in a person. The method is based on the fact that the burning of fat and CHO require different amounts of oxygen and release different amounts of carbon dioxide. By measuring the ratio of carbon dioxide expired to oxygen consumed in the breath, the proportion of fuels burned in the body can be calculated. The ratio is called the respiratory exchange ratio (RER for short). An RER of 0.7 is indicative of only fat being used and an RER of 1.0 represents all CHO. Any number between 0.7 and 1.0 represents a combination of fuels. If a person exercising is producing 28 molecules of carbon dioxide and using 30 molecules of oxygen, the RER is 0.93 (=28/30). The number 0.93 is closer to 1.0 than 0.7, and actually calculates to 77% CHO and 23% Fat. Because of these relatively easy laboratory measurements, there is a great understanding of what fuels are being used during exercise under different conditions.
Every person has a certain maximal ability to use oxygen to do work (VO2max – a topic for another column). When you are exercising, you are at some percentage of that maximum. When monitoring heart rate with a monitor you are essentially loosely estimating what percentage of your VO2 max you are exercising at. Now, take a hypothetical athlete working at 65% of their VO2max. For this hypothetical person 65% VO2max corresponds to a heart rate of 130 beats per minute (of a 195 beat per minute heart rate maximum). Under most conditions, when exercising at 65% VO2max, RER is around 0.93. An RER of 0.93 calculates to a fuel mix of 76% CHO and 24% Fat. From this simple example, it is easy to see that anything above an easy intensity (130 beats per minute) is predominantly driven by CHO
An idea propagated in popular magazines and columns is that exercise training increases the ability to use fat. This is only half true. As an example, I can set up another scenario from lab testing. Imagine that we tested a hypothetical untrained person during a 1-hour exercise bout at 65% VO2max. For this person 65% of his/her max happens to be 150 watts and resulted in an RER of 0.93. If the person trained for 12 weeks the normal response would be for his/her VO2max to increase. If he/she repeated a 1-hour exercise bout at 150 watts, the likely response would be that the RER would now decrease to something closer to 0.88, which meant that he/she was using a higher percentage of fat for that work. Some interpret this as the ability to use fat during exercise increased. However, since his/her VO2max increased, an exercise bout of 150 watts may be only 50% of his/her new VO2 max (a lower relative intensity). If the he/she was tested at 65% of the new VO2 max, which may equate to being able to do 180 watts, RER would remain very close to 0.93. Another way to think of this is if you and Bradley Wiggins both rode at 65% of your max, you would both be using the same percentage of fat and carbohydrate, although Sir Wiggins would be doing a much higher amount of work (e.g. Bradley 300 watts, you 200 watts). If you and Bradley both rode at 150 watts, a much lower percentage of Bradley’s max than yours, he would be using a higher percentage of fat compared to you to do the same amount of work. Your ability to use fat only goes up at the same workload (termed absolute intensity) but not at the same percentage of max (termed relative intensity). Finally, before you commit to training at a lower intensity because a higher percentage of the energy comes from fat, keep in mind that at the lower intensity you will be burning fewer calories overall.
There are some very good reasons that CHO are preferred during exercise (e.g. more oxygen efficient). Also, there are some important reasons that fats are not. To keep things simple, fat is a much better storage form of energy than carbohydrate. To illustrate, CHO contain 4.2 calories per gram, take energy to store as glycogen, and brings along 2.7 grams of water per gram of CHO when stored as glycogen. Fats contain 9.4 cal per gram, there is no energy cost for storage, and does not store appreciable water with it as adipose tissue. In total, the energy value of stored for CHO as glycogen is 1 cal per gram while stored fat has an energy content of approximately 8 kcal/gram, a very clear difference. Storing energy as fat is extremely efficient in that the same quantity of energy can be stored with considerably less weight. If a relatively lean person stored as glycogen what is normally stored as fat there would be an increase in body weight by more than 36 kg (79 lbs). Therefore, while this huge increase in weight would not be a problem for an oak tree (which may explain why plants generally store a great majority of their energy as carbohydrate in the form of starch), it would clearly be a huge disadvantage for humans. If considered from an evolutionary perspective, the need to move to catch dinner, or avoid becoming dinner, would be severely hindered by 36 extra kg that did not help you move. For future columns, the take home is that CHO are preferentially used and fat is a preferentially stored.
Figure 1. Energy content of an average 70 kg (154 lb) person with 15% body fat. Exercise time is how long that fuel store would last when performing exercise at 65% of VO2max. Table adapted from Sports Nutrition: An Introduction to Energy Production and Performance, by Asker Jeukendrup and Michael Gleeson 2004.