A question that is asked and answered frequently.
The short answer is simple. Yes, that can happen. There is no absolute guarantee that you will burn all or even mostly fat when you do cardio. In fact you don't burn all fat, and the choice of energy substrate is based on the envirnment you subject yourself to.
Let me just make up some ballpark numbers for you to think about. If everything is average to excellent as far as the environment for fat burning...Steady state burns lets say 6 calories per minute and lets say 85% of those are from fat at the 15 minute point. The other 15% would then be from glucose/muscle. Now, HIIT might burn 12 calories per minute, but lets say 65% is from fat. So 20 minutes of either could burn roughly 100 calories of fat for steady state, to 150 calories from fat for HIIT. At the same time you would be burning glycogen and eventually, muscle to make up the remaining calories burned.
Burning fat optimally requires a few things to be optimal. If they are not, muscle is a fine energy source and the body will instruct itself to resort to it.
If your diet, rest, recovery, and other factors are off, then fat burning can be interrupted and muscle burning can happen when the glycogen runs out.
There are neurotransmitters and enzymes that we can improve the production/reception of, and these tell the body to burn fat. The more practice you have, the better your body gets at it.
I just wrote an argument in response to a similar question and I will share it here. Otherwise, skim the 13 pages for some of my other replies and it should fill out about 90% of the answer to your questions.
I have more I would add... but the basics are covered in the first post!
Diet has to be optimal and not deficient. Water is ESSENTIAL, rest and relaxation are massive, timing of nutrition matters, tracking results helps, but progression and variance are foundational.
If you have more questions about this and want a more in depth answer to something specific, ask away! I realize that my answer here has been fairly general.
The Article....
Don’t look to cardiovascular exercise to lose body fat
For decades, when someone wanted to lose body fat, the universal instruction that they have received has been that they must do cardiovascular exercise. Magazines of every kind have touted the newest fad cardio system for “burning off that annoying fat.” More recently, the focus has been on special lifestyle strategies for losing fat. However, underlying each new approach is usually a plan to increase cardiovascular exercise. The mistaken belief has been that a plan of steady-state cardiovascular exercise will, by itself, lead to a long-term reduction in body fat. This has not proven to be an effective approach and it is not supported by research.
Research has shown that the results from undertaking steady-state cardiovascular exercise alone are often the reverse of what has been touted. Instead, it has been found that solely pursuing a long-term practice of steady-state cardiovascular exercise, rather than leading to overall reductions in body fat, has frequently caused a reduction in lean tissue (muscle and other tissue that is the place in the body where fat is burned.) This can easily result in an overall decrease in resting metabolic rate (the rate at which the body burns calories during periods of non-exercise) (Hansen, Dendale, Berger, van Loon, & Meeusen, 2007). In the long term, this act of reducing the amount of muscle tissue, and thus slowing the overall rate at which calories are burned in the body, introduces the all-to-often realized risk of long-term maintenance of or even increase in body fat stores (Ibid).
While cardiovascular exercise does impart many benefits to the cardiovascular and respiratory systems, and while it may increase caloric expenditure during a bout of cardiovascular exercise, the overall physiological effect of solely pursuing a long term practice of steady-state cardiovascular exercise has not been found to lead to long term reductions in body fat (Donnelly et al., 2009). While body fat may play a role in energy expenditure during a bout of steady-state cardiovascular exercise, the long term outlook is that the same steady-state cardiovascular exercise practiced over time will result in lower and lower energy expenditures as the body adapts and becomes more efficient at responding to the demands of training (Gardner, Poehlman, & Corrigan, 1989). In response to this, people who practice steady-state cardiovascular exercise have tended to increase the duration and frequency of their training. This has lead to a faster rate of adaptation to training and an overall lowering of long-term caloric expenditure from exercise (Ibid). Additionally, some individuals, in order to continue to see benefits from their steady-state cardiovascular exercise, have increased their training time to periods in excess of 90 minutes per day. One result of this is the loss of additional lean mass through the conversion of lean muscle in to glucose to meet energy needs: a process called gluconeogenisis (Hansen, Dendale, Berger, van Loon, & Meeusen, 2007).
The long held belief has been that cardiovascular exercise burns fat and that resistance exercise builds muscle. These two statements refer to two different processes that are not mutually exclusive, and this confusion has contributed to the formation of the mistaken belief about the relationship between cardiovascular exercise and fat burning. Closer to accurate but still misleading is when cardiovascular exercise is touted as aerobic training, meaning training that occurs in the presence of oxygen, while progressive resistance training is touted as anaerobic training, or training that occurs without the need for oxygen. Part of this misunderstanding comes from a misuse of terms and lack of clarity concerning the underlying concepts. Sadly, in response to this mistaken use of concepts nearly everyone, including, gyms, physicians, and trainers, has promoted cardiovascular exercise as the ideal way to lose fat.
The choices in terminology that have been used to describe the various types of exercise are based on the underlying chemistry in the energy systems of the human body. In simple terms, fat is an energy source that requires oxygen, while blood sugar, or glycogen, can be converted to energy without vast inputs of oxygen. Muscle that performs the brunt of repetitive, cardiovascular type exercise is known as Type I, or slow-twitch, Oxidative muscle fiber. Type I muscle fiber has a very high capillary density, which, among other things, provides the muscle with large amounts of oxygen (accounting for the term aerobic) and removes fatigue byproducts rapidly. Muscle that is primarily used in progressive resistance training is known as Type II (this can be A, B, or A/B), or fast-twitch, Glycolytic muscle fiber. This muscle fiber has a low capillary density, which provides only a small supply of oxygen (accounting for the term anaerobic) and removes fatigue byproducts slowly (Seeley, Stephens, & Tate, 2008).
Contrary to the belief that cardiovascular type exercise burns fat, the human body has two primary sources of energy that it can utilize during cardiovascular type exercise: fat and/or glucose, while the primary energy source during resistance training is glucose. It should be noted that the energy expended during a bout of cardiovascular exercise compared to a bout of resistance training is about the same, roughly eight calories per minute (American Council on Exercise, 2001). However, the rate at which the body burns calories in the hours following cooling down from progressive resistance training is considerably elevated, while for most bouts of steady-state cardiovascular exercise, the body returns to it’s normal resting metabolic rate rapidly after cooling down (Burleson, O'Bryant, Stone, Collins, & Triplett-McBride, 1998). Another point to recall is that muscle is the place in the body where fat is burned, and any increase in the amount of this lean tissue will increase the body’s overall capacity for fat burning on a day to day basis. As has already been pointed out, steady-state cardiovascular exercise can have a muscle depleting effect and thus the reverse effect on the body’s overall capacity for fat burning.
To illuminate a more accurate and effective approach to losing body fat, we can again look to findings in published research. Research has been done comparing different modalities for fat loss, and the most consistent finding is that when one compares long-term programs following steady-state cardiovascular exercise with either a) long-term programs following combined steady-state cardiovascular exercise and progressive resistance training, or b) long-term programs following stand alone progressive resistance training, one finds that the latter two are seen to result in maintenance of lean mass, increase in total energy expenditure, and drops in body fat, while stand alone steady-state cardiovascular exercise is seen to have a drop in lean mass and no drop or an increase in body fat (Geliebter et al., 1997).
These findings should have realigned and had a significant effect upon the thinking of everyone involved in fitness and fat loss. One might reasonably have hoped for a complete re-visioning of the fat loss exercise paradigm. Combined with the current findings on the additive benefits of manipulation of macronutrients and calories, there should not be any trouble in helping individuals achieve their goals when those goals include fat loss.
Worth noting, are the consistent findings showing that, for people who are deconditioned and overweight, beginning a program of cardiovascular exercise can lead to a short term loss of fat and an increase in overall fitness and other major health measures. For these populations, the same benefit is derived from beginning a progressive resistance-training program (Donnelly et al., 2009). Additionally, there are cardiovascular regimens that follow a program of long term, non-linear, intensity progressions that have all the benefits of cardiovascular exercise and have been shown to attenuate muscle loss (Ibid).
What is now known is that, if all other factors are equal, stand alone progressive resistance training programs and programs that combine progressive cardiovascular exercise with progressive resistance training can lead to reasonable sustainable fat loss, maintenance of lean tissue, and the improvement in overall health measures. In fact, it has been noted by many researchers that the addition of progressive resistance training to fat loss regimens leads to greater fitness program adherence (Pollock et al., 1998). Conversely, the age-old practice of doing long-term steady-state cardiovascular exercise has repeatedly been shown to lead to lean tissue loss, which can lead to sarcopenia and a reduction in overall health measures (Vella, Kravitz, ND). The confidence in long-term steady-state cardiovascular training programs for fat loss is not reasonable. With the volume of research available demonstrating successful methods for accomplishing fat loss goals, there should no longer be the mistaken reliance on long-term steady-state cardiovascular training programs for fat loss.
References:
1. American Council on Exercise. (2001). Calorie Burners: Activities That Turn Up The Heat. Fit Facts. San Diego, CA. (NA).
2. Burleson, M., O'Bryant, H., Stone, M., Collins, M., Triplett-McBride, T. (1998). Effect of weight training exercise and treadmill exercise on elevated post-exercise oxygen consumption. Medicine & Science in Sports & Exercise, 30, 518-22.
3. Donnelly, J., Blair, S., Jakicic, J., Manore, M., Rankin, J., Smith, B. (2009). Appropriate Physical Activity Intervention Strategies for Weight Loss and Prevention of Weight Regain for Adults. Medicine & Science in Sports & Exercise, 41(2), 459-471.
4. Gardner, A., Poehlman, E., Corrigan, D. (1989). Effect of endurance training on gross energy expenditure during exercise. Human Biology, 61(4), 559-69.
5. Geliebter, A., Maher, M., Gerace, L., Gutin, B., Heymsfield, S., Hashim, S. (1997). Effects of strength or aerobic training on body composition, resting metabolic rate, and peak oxygen consumption in obese dieting subjects. American Journal of Clinical Nutrition, 66, 557-563.
6. Hansen, D., Dendale, P., Berger, J., van Loon, L., Meeusen, R. (2007). The Effects of Exercise Training on Fat-Mass Loss in Obese Patients During Energy Intake Restriction. Sports Medicine, 37 (1), 31-46.
7. Pollock, M., Gaesser, G., Butcher, J., Després, J., Dishman, R., Franklin, B., Garber, C. (1998). The Recommended Quantity and Quality of Exercise for Developing and Maintaining Cardiorespiratory and Muscular Fitness, and Flexibility in Healthy Adults. Medicine and Science in Sports and Exercise, 30(6), 975-91.
8. Seeley, R., Stephens, T., Tate, P. (2008). Anatomy and Physiology (8th Edition). Columbus, OH: McGraw-Hill College
9. Vella, C., Kravitz, L. (ND). Sarcopenia: The Mystery of Muscle Loss. IDEA Personal Trainer, 13(4), 30-35. Retrieved from:
http://www.unm.edu/~lkravitz/Article%20folder/sarcopenia.html