When it comes to exercise, the idea of burning fat in place of carbohydrate is very appealing. Unfortunately, fat burning is an inefficient process above certain intensities of training, and our bodies are unable to convert fat directly to glucose. If we could convert fat directly to glucose, weight loss would be a piece of cake, literally. Your daily diet, pre-workout meal, training volume/intensity, and overall fitness are all variables in how your body burns fat.
The composition of your daily diet may influence what your body uses for fuel during exercise. One of the more popular concepts of manipulating your daily diet is called “Metabolic Efficiency” (ME). This phrase, coined by Bob Seebohar, MS, RD, CSSD, CSCS is based on the textbook definition of the ‘cross over’ point, a standard metabolic response to the physiologic demands of exercise. Implementation of a diet to ‘improve’ ME focuses on eating less processed foods, which by default will reduce intake of refined sugars and starches. Due to the drastic reduction in bagels, cookies and bread in most people’s diet, most people think this is a low-carb diet. However, fruit and vegetables are carbohydrate and are standard in an ME diet. An ME diet is utilized most often during the off-season or during periods of low training volume/intensity but depending upon where you are in your annual training plan, grains may be consumed.
By reducing refined sugars and process foods there is more control over insulin response. The release of insulin is part of the normal breakdown of carbohydrate; high levels of circulating insulin inhibit the breakdown of fat. Thus, controlling the insulin response may promote or enhance the breakdown of fat. Carbohydrate metabolism is the same regardless if you are eating a Kit-Kat or brown rice; however, the insulin response will be less if you eat brown rice. The insulin response can be used advantageously to promote recovery post workout.
To know what your body burns for fuel at rest and during exercise can be measured in a physiology laboratory by measuring the amount of carbon dioxide and oxygen contained in the air you exhale. The resulting number is called a Respiratory Exchange Ratio (RER). This number between 0.7 and 1.0 illustrates how much fat, carbohydrate and protein you are metabolizing for fuel at a specific moment in time. RER can be manipulated through diet (as explained above) based on the fact that you burn what you eat. If you eat a high carbohydrate diet, you will burn more carbs; and if you eat a high fat diet, you will burn more fat. But – it’s not that simple, and I never recommend a high fat diet. The point is that RER at rest and during exercise may be influenced by your daily diet, but it also varies during exercise depending upon intensity. Fat loss in response to exercise depends not only on exercise energy expenditure but also on exercise–induced changes in RER. This suggests that development of strategies to maximize the change in resting fat oxidation in response to an exercise training program may help individuals to maximize exercise-induced fat loss (1).
Exercise Intensity & Fitness
Fat and carbohydrate are always being used to some extent to fuel exercise but the ratio of each varies with intensity and fitness level. When exercise begins after eating an average meal, 90-95% of energy to fuel the exercise comes from fat and 5 – 10% is from carbohydrate (glucose). As exercise increases, our bodies begin to burn more carbs than fat (cross over point). For sedentary people, this point happens early after exercise is initiated. Athletes use fat at higher intensities of training compared to sedentary or untrained people due to physiological adaptations resulting from their training.
The percentage of VO2max at which an athlete burns fat will vary per sport. Depending upon the methodology used in research, maximal fat oxidation (MFO) in fit runners occurs near 50 – 54% VO2max, or between 60-80% of maximal heart rate (2). Trained cyclists had higher whole body fat expenditure at higher intensities (60% and 80% of VO2Max) than untrained subjects, with untrained subjects experiencing maximal whole body fat burning at 40% VO2max (4). The variability is largely due to differences in gender, age, and fitness. Less consistent exercisers will reach MFO sooner. As your fitness improves, you are able to train at higher levels of effort, while being at lower intensity relative to others. This means, that when you used to run 10 min/mile pace at 60% VO2max and now run 7:30 min/mile at the same VO2max, fat is contributing slightly more to your energy source than someone running a 7:30 min/mile pace but it is at 80% VO2max. This allows competitive athletes to perform at a lower perceived exertion while actually working harder (in relation to themselves), thus utilizing more fat than carbohydrate for fuel.
Pre Workout Meal
Manipulating what your body burns during exercise depends on the composition of your pre-workout (2-3 hrs) meal and your fitness level. The body preferentially uses fuel from foods recently consumed before digging into stored fuel. If your last meal was high carbohydrate, you will mostly burn carbohydrate; if it was high in fat, you will predominantly burn fat. However, consuming a meal high in fat prior to workout will not bode well for your gut. Regardless of what you eat, you will always burn some percentage of carbohydrate and fat. If an individual is on a low carbohydrate diet in addition to under-consuming calories, the body will be in ‘starvation’ mode oxidizing fat and protein for energy. When a body is in ‘starvation’ mode, if it is fed glucose, the glucose is not used for energy like it is when a person is on a normal diet. Instead, the body converts the glucose to glycogen for storage. I have worked with athletes who change their eating habits by reducing their grain consumption, to become “clean” eaters. Often, performance improves but can be then diminish if recovery nutrition isn’t implemented well.
The other side of this debate is to not eat at all and to train in a ‘fasted’ state. Implementing ‘fasted’ workouts into your training plan should be done on customized basis because current diet (Paleo, Low-Carb), average caloric intake, and training volume all need to be taken into consideration. Typically, training fasted is placed strategically into a microcycle with individualized recovery nutrition to make sure the next workout is not jeopardized. Training fasted has been shown to improve fat oxidation at varying exercise intensities.
The practical application of a fat burning zone is only intended for training, not racing. On race day, carb intake should be used according to the specific demands of the race. Different workouts throughout the week can be completed with specific dietary recommendations to achieve changes in RER while also attaining the goal of a particular workout. Separate from the enticing notion of fat burning in relation to weight loss, does this really matter in the big picture on race day? If you will be racing near aerobic threshold for more than two hours you will have to consume some form of carbohydrate. If you have a solid fueling plan you shouldn’t be worried about GI issues. Otherwise, you should focus on fueling for your training and adapting your gut to what you will need to consume for peak performance on race day.
- Barwell et al. (2009). Individual responsiveness to exercise-induced fat loss is associated with change in resting substrate utilization. Metabolism, September; 58(9): 1320–1328.
- Carey, D.G. (2009). Quantifying differences in the “fat burning” zone and the aerobic zone: implications for training. J Strength Cond Res.,Oct;23(7):2090-5. doi: 10.1519/JSC.0b013e3181bac5c5.
- Wee, Shiou-Liang et. al. (1999). Influence of high and low glycemic index meals on endurance running capacity. MSSE,31(3): 393-399.
- Achten, J., Gleeson, M., & Jeukendrup, A.E. (2002). Determination of the exercise intensity that elicits maximal fat oxidation. MSSE, 34(1); 92-97.