Endurance Training · Nutrition

Train your body to burn fat over carbohydrate

You may be familiar with the three sources of fuel your body uses to create energy; carbohydrates, protein, and fat. But are you familiar with how much of each of them are used when you exercise? Do you know how the foods you choose to put into your body affect the percentage of these fuels used during exercise? The focus of this article is to explain the role of these fuels at rest, during exercise, and how you can alter your nutrition and exercise strategy to optimize your performance and body composition.

Let’s briefly discuss the three primary sources of fuel and how they are used to create energy. Carbohydrates are found abundantly in fruits and certain vegetables such as sweet potatoes and squash. The differences in various types of carbohydrates (i.e. complex vs. simple, starches, fibers, etc.) will not be discussed at length in this article. You break down and store these carbohydrates in the body as glycogen (within the muscles and liver) and glucose (circulating in the bloodstream) (Kenney, 2015). The quantity of carbohydrates contained in food and how quickly they raise blood sugar levels (also known as glycemic index) varies greatly among the foods we eat. More information on glycemic index (GI) can be found here. Carbohydrates are the preferred source of fuel during high intensity bouts of exercise such as a one rep max back squat attempt or a 100-meter sprint. They also provide energy for exercise efforts of up to 2-3 minutes depending on fitness level (Kenney, 2015). Once that point has been reached, high exercise intensity can no longer be sustained. In general, if exercise continues, intensity must be reduced and the body will start to transition to fat as its preferred source of fuel. Carbohydrates will contribute to the production of energy for approximately 90-120 minutes (during endurance events). They will then be depleted and must be replaced through the diet (Kenney, 2015).

It is important to understand that both carbohydrates and fats are used for fuel at any point in time, however, the percentage of which source is used varies. Fats can be found in foods such as avocados, nuts, seeds, and coconut oil. As with carbohydrates, different types of fat do exist (i.e. saturated, unsaturated, polyunsaturated) but we will not discuss these in depth. Unlike carbohydrate, fat has the capacity to provide an unlimited amount of energy, even in a lean person (Kenney, 2015). We have the capability to train ourselves to be more efficient in the utilization of fat, which we will discuss later. Finally, the last source of fuel the body can use is protein. It does not resort to this source unless it is absolutely necessary (e.g. starvation, or extreme endurance events lasting longer than 10 hours). If the body must resort to this source of fuel, it will take protein from muscles and convert it to glucose in a process known as gluconeogenesis (Kenney, 2015).

Now that you have a basic understanding of these fuel sources and how they are used in the body, I will now discuss the concept of burning sugar versus burning fat. I was first introduced to the concept of “sugar burners” vs. “fat burners” (Cordain, 2012) during my time attending the University of Illinois at Urbana-Champaign. I didn’t think anything of it until I saw things from the “fat burner” perspective just a couple years ago when I drastically changed my diet from mostly pasta, grains, cereals, and little fat to lots of vegetables, some fruit, meats, nuts, and seeds. The concept is simple. “Sugar burners” use more carbohydrate as a percentage of their total calories used, both at rest and during exercise (Cordain, 2012). At rest, our bodies tend to burn primarily fat for energy. When we are not exercising and the intensity of physical activity is low, we turn to fat because it is a more sustainable form of energy. However, consider the following hypothetical situation: a person who goes about their day eating cereal or bagels for breakfast, a sandwich for lunch, and pasta for dinner will quickly raise their blood sugar during each one of those meals. The body will convert and store as much glycogen as it needs and the rest will be converted and stored as fat. Unless you are exercising enough and actually using those carbohydrates for high intensity efforts, you do not need them in excess. In my experience as a multi-sport athlete and personal trainer, I believe that most people do not need carbohydrates in those amounts, even those people who are regularly active.

Now, let’s transition to how this concept affects you during exercise and ultimately affects your performance and body composition. Allow me introduce you to a something called Respiratory Exchange Ratio (RER). RER is a measure of the ratio of carbohydrates to fats that you are burning during exercise. It is calculated by measuring the rate of carbon dioxide release and oxygen consumption during exercise (Kenney, 2015). Unfortunately you need certain expensive equipment to test it, but if you’re ever feeling up to finding your RER at a local testing lab, go for it! You can still use the following information to help you reach your goals without knowing your RER.

RER is measured on a scale from .7 to 1 (Cordain, 2012). At rest, you should be somewhere around .8, but here is where “sugar burners” want to pay attention. Guess what happens if you’re constantly eating foods with a high GI (above 55)…you guessed it! That RER will be elevated at around .9 or higher when you’re at rest (Cordain, 2012). What does that mean for you? You’re constantly burning sugar, not fat. The closer that RER gets to 1, you are burning more sugar for energy. This is particularly important for endurance athletes because they will run out of stored glycogen much faster than those who are more efficient in using fat as a fuel source. Have you ever wondered why you’re not losing that stubborn excess body fat even though you’re exercising regularly? RER and the foods you choose to put into your body have a very large role to play in it.

So, how can you use this information to help you increase your athletic performance and utilize fat more efficiently during exercise and at rest?

1) Reduce your consumption of pastas, grains, sweeteners, and anything other highly processed food with a high GI. Increase the amount of vegetables you eat. I will always recommend, “eating a rainbow”, and no, this doesn’t mean eating Skittles. It means eat as many different colors of vegetables that you can throughout the day. Although you should eat fruits, take it easy on them because they still do contain a lot of sugar.

2) Increase your healthy fat intake. This means avocados, brazil nuts, almonds, chia and flax seeds, and coconut and olive oils. Supplementing with Omega 3’s is a good idea also. The more fat you have in your diet, the lower your RER will be, which means your body will prefer to burn a higher percentage of fat at any point in time (Cordain, 2012).

3) Do High Intensity Interval Training (HIIT). High intensity efforts using cycling, swimming, running, and circuit training modalities have numerous health and fitness benefits. For example, doing 5-8 sets of a 30 second all-out effort with 2-3 minute active recovery periods will provide both anaerobic and aerobic training adaptations (Kenney, 2015), and they will keep your metabolism elevated for some time following the workout which will cause you to burn more fat at rest, provided you are eating an ideal diet to facilitate this.

4) If you are an endurance athlete, go slower than you think you need to on your Long Slow Distance (LSD) training sessions. Throughout my experience as a multi-sport athlete, a trend I noticed was that many of these athletes go too hard on their LSD training sessions. Just as there is lactate threshold (LT) where lactic acid begins to accumulate in the muscles faster than it is cleared, there is aerobic threshold (AT). During submaximal exercise, AT is the point at which our bodies start to transition to a higher ratio of carbohydrates used as fuel (Greenfield, 2014). I still find to this day that most endurance athletes do their LSDs slightly above this threshold. At that point, you are not training yourself to be more efficient at burning fat. You can get a good sense of this threshold by taking note of when you transition from a pace that you can easily carry out a conversation at all day to a pace that elicits slightly heavier breathing and needing to taking more frequent breathes between sentences when conversing. Athletes I have spoken to say they choose this exercise effort because it gives them the feeling that they’re working “hard enough” to get something out of it. The point of an LSD training session is to go slow and train your body to burn fat. It’s difficult to run slow, but just do it and save the quality high intensity efforts for your interval and tempo training sessions.

If you hold true and make a consistent effort in doing these things you will find that the excess fat will start disappearing and you will feel a lot more energetic throughout the day and during your workouts, which will lead to quality training sessions and greatly improved performance.


1) Ward, Colin. Glycogenolysis and glycogenesis [internet]. 2014 Aug 13; Diapedia 51040851111 rev. no. 13. Available from: http://dx.doi.org/10.14496/dia.51040851111.13

2) Kenney, W. L., Wilmore, J. H., Costill, D. L., & Wilmore, J. H. (2015). Physiology of sport and exercise. Champaign, IL: Human Kinetics.

3) Cordain, L., PhD, & Friel, J., MS. (2012). The Paleo Diet for Athletes (2nd ed.). Rodale Inc.

4) Greenfield, Ben (2014). Beyond Training: Mastering Endurance, Health, and Life. Victory Belt Publishing, Inc.

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