So, does exercise really actually burn or melt body fat? Well, a very plain and simple answer would be no, not really. The answer is certainly more complicated than that, and I plan on explaining myself further, but this is certainly a topic that most people get wrong, or are greatly misinformed.
Sure, a lot of you may read this blog and think, "Your argument is just semantics. Exercise (in a roundabout way) burns fat!" Well, maybe. Maybe this could be considered semantics, but I personally believe this plays a crucial role in how people perceive not only the role of exercise, but the role of food and their diet as well!
Heavy science jargon and content ahead. I have done my absolute best to explain what is going on here. You've been warned. If you're still here, let's dive in.
I came across this interesting review the other day:
Abdominal fat reducing outcome of exercise training: fat burning or hydrocarbon source redistribution?
Fat burning, defined by fatty acid oxidation into carbon dioxide, is the most described hypothesis to explain the actual abdominal fat reducing outcome of exercise training. This hypothesis is strengthened by evidence of increased whole-body lipolysis during exercise. As a result, aerobic training is widely recommended for obesity management. This intuition raises several paradoxes: first, both aerobic and resistance exercise training do not actually elevate 24 h fat oxidation, according to data from chamber-based indirect calorimetry. Second, anaerobic high-intensity intermittent training produces greater abdominal fat reduction than continuous aerobic training at similar amounts of energy expenditure. Third, significant body fat reduction in athletes occurs when oxygen supply decreases to inhibit fat burning during altitude-induced hypoxia exposure at the same training volume. Lack of oxygen increases post-meal blood distribution to human skeletal muscle, suggesting that shifting the postprandial hydrocarbons towards skeletal muscle away from adipose tissue might be more important than fat burning in decreasing abdominal fat. Creating a negative energy balance in fat cells due to competition of skeletal muscle for circulating hydrocarbon sources may be a better model to explain the abdominal fat reducing outcome of exercise than the fat-burning model.
Lots of science talk, let's break things down and give some thoughts as to what is being discussed here.
Fat burning, defined by fatty acid oxidation into carbon dioxide, is the most described hypothesis to explain the actual abdominal fat reducing outcome of exercise training.
This is one of these popular "facts" making its way around the internet lately. The idea that as you exercise and burn fat, the fat then just starts melting and you magically breathe it out as carbon dioxide. Sure, carbon dioxide is a by product of metabolism and respiration, and you certainly burn some fat during exercise, but it isn't really that simple.
This hypothesis is strengthened by evidence of increased whole-body lipolysis during exercise. As a result, aerobic training is widely recommended for obesity management.
Right. This has been heard for years. This is actually one point that seems to be at least somewhat well-known to be a mistruth now. That just because adipose tissue (body fat) is only burned in the presence of oxygen (oxidation), then low-level exercise must be best for burning fat. Right? Go for a nice long, easy run on the treadmill and you will get thin and sexy. Well, not exactly. My readers should know that intense exercise is better suited for reducing body fat by now so lets move on.
anaerobic high-intensity intermittent training produces greater abdominal fat reduction than continuous aerobic training at similar amounts of energy expenditure.
Study after study after study has shown just this - high intensity interval training is more effective for reducing body fat than steady state cardiovascular exercise.
Significant body fat reduction in athletes occurs when oxygen supply decreases to inhibit fat burning during altitude-induced hypoxia exposure at the same training volume
Ah good, now things get interesting. So what this states is that body fat is reduced more in individuals that have decreased oxygen supply. Doesn't oxygen need to be present to burn body fat? Well, as the previous statement pointed out to us, high intensity exercise - you know, the type that has you gasping for air (oxygen deprived), is actually best at obtaining or maintaining an optimal body fat percentage.
Lack of oxygen increases post-meal blood distribution to human skeletal muscle, suggesting that shifting the postprandial hydrocarbons towards skeletal muscle away from adipose tissue might be more important than fat burning in decreasing abdominal fat.
This gets into the meat of this paper's argument, and one that I will elaborate on below. People need to stop thinking of exercise as a fat burner, and instead consider exercise (both resistance training and "cardio") as a means to make your body a better fat-burning machine.
Creating a negative energy balance in fat cells due to competition of skeletal muscle for circulating hydrocarbon sources may be a better model to explain the abdominal fat reducing outcome of exercise than the fat-burning model.
Once I dove deeper into this paper, I got a better sense of what point the authors were trying to prove. Your muscle cells and fat cells both have the ability and goal in mind to store energy. In fact, there seems to be a competition between the two. Your body is constantly varying its sources of energy based on your level of activity. When you are exercising intensely, your body is primarily using glucose as a fuel source, for example. Sure, there is some fat being oxidized, but the primary fuel source is glucose.
Compare that to the amount of fat being burned between aerobic exercise, resistance exercise, and non-exercise. Sorry resistance trainee camp, not even you can argue that resistance training is "better" than aerobic exercise for burning fat - at least not directly.
This one is telling for the "exercise until you puke" camp. The notion that the harder you exercise, the more fat you burn is total b.s. as well. Do I think intense exercise is important? Absolutely. Do I think intense exercise is necessary for weight loss and body fat reduction? Not really, or at least not primarily. Some is good, but only to a certain level.
So what is the point to all of this? Well this is where the semantics comes in.
The current understanding is that when you are exercising, your body is literally burning away those love handles as you crank through all those burpees or squats. As we saw in figure A above, this is simply not the case. Yes, intense exercise promotes lower body fat percentages, but not because the fat is literally being burned and exhaled as carbon dioxide. Ok, then it must be the post-workout "burn" where metabolism is revved up. That is a common theme, correct? Again, not the case. Because oxygen must be present in order to burn fatty acids as a fuel source, by exercising intensely, you are specifically forcing your body to turn to glucose as a primary energy source.
So low-level exercise is better for burning fat, right?
Well, no. Research has proven time and time again that shorter, intense exercise is not only more efficient and effective than low-level, steady-state exercise to improve cardiovascular health and a healthy body-fat percentage.
So what gives?
As this review points out, the mindset as to what exercise actually does to your body and how body fat is reduced is the most important part. Exercise, and more specifically, intense exercise (ideally with external resistance, i.e. weights) not only builds strong muscles, but it turns your muscles into energy consuming machines. This causes a domino effect.
Body fat (adipose tissue) and lean tissue (muscle) are constantly competing over consuming incoming calories. The body seems to give preferential treatment to muscles the harder they work.
Energy that is not consumed and stored in muscles goes to body fat.
Carbohydrates are broken down into glucose. When your body is not active, glucose is not being burned as readily by muscles, so there is more glucose present and glucose becomes the primary energy source even during low-level activity (most of the day).
Fatty acids from adipose tissue are the primary energy source for majority of your day (i.e. the time you aren't working out intensely). But if glucose is present, blood sugar (glucose) becomes the energy source of choice.
If your body is using blood glucose as an energy source, body fat deposits are not reducing.
If your muscles are consuming large amounts of energy, especially carbohydrates, then your body primarily uses fatty acids (adipose tissue) as the energy source.
So although this isn't necessarily different than what most people should already know - intense exercise makes you thin and keeps you healthy, the mindset for how this works should change. Resistance training is used to not only strengthen the connective tissues of the body, but to make your muscles greater calorie-burning machines.
Carbohydrates should be consumed almost entirely just prior to, and/or immediately following a workout in order to reduce the amount that is stored as body fat.
Although intense activity is great and very important for overall health, the more active you are the rest of the day during "low-level activity" (walking, working, playing, etc.), the more effective your body will be at reducing your body fat percentage.
Tyler Robbins B.Sc. CSCS
Director of Fitness
Head of CrossFit