I actually receive this question from clients quite often, but someone asked me about it again the other morning which inspired me to write about it. The question was, and this goes along with conventional "broscience," is fat burning more effective if you workout fasted?
It is common for individuals to workout first thing in the morning, which many end up doing in a fasted state (haven't eaten anything prior to exercising). Let me start off by saying that I commonly workout in a fasted state, and in fact, have done so for many years. Having said that, I have recently changed my daily routine based on my work schedule so my workouts tend to get pushed to later in the day now. This gives me ample opportunity to eat prior to working out, and although what I am about to say is based off of relatively short period of time and is anecdotal, I have felt like my strength and performance has been much better, but we can discuss more of that later.
Anyways, so the training when fasted theory goes something like this. If you are training fasted, then that means that your circulating blood sugar is low, priming your body to be more conducive to fat metabolism. This is a theory, however, and doesn't quite play out as simply as you would think. To discuss exactly what is going on here, let's break this down into 2 topics of interest.
1. Your body is never entirely using one source of energy over another.
So you have three energy systems in your body: the Phosphagen System, the Glycolytic System, and the Oxidative System. Without nerding out too far, the Phophagen System produces a lot of power, but can only last for a few seconds (think 3 or less repetitions of something heavy or a short sprint). The Glycolytic System can produce a little less power but can last a bit longer (higher rep resistance training or a longer run, up to about 2 minutes). The Oxidative System cannot generate a lot of force, but is much more lasting in nature (less intense activity such as walking, etc.). The Oxidative System is just that, oxygen-based, so that the amount of breathing you do keeps up with the energy demands of your activity. Oxygen, along with fat, produces energy in the Oxidative System. Your breathing may increase a bit during a brisk walk, for example, but you are never pushed to a point of exhaustion.
During regular, every day activity, our bodies are constantly transitioning between the three energy systems. Walking, talking, general daily activities tend to keep us in our Oxidative state. If we bend down to pick up something heavy, suddenly we transition to either the Glycolytic or Phosphagen system for that quick burst of energy. However, throughout the day, our bodies are constantly metabolizing energy from all 3 energy systems in some ratio depending on the activity.
So how does this change based on whether or not we exercise in a fasted state or not? It doesn't really. Our bodies are great at storing and utilizing energy based on our needs and demands. It doesn't matter when your last meal was, if you need a quick surge of energy to pick up something heavy then you have stored energy for that.
Sure, activity at low intensities when in a fasted state may illicit a greater promotion of fat oxidation, but I would like to think that most people now realize that training in a specific heart rate zone to maximize one energy system over the other is far less beneficial (for overall body composition and health) than exercising intensely.
Not only that, but research tells us that when you are in a fasted state, your body may increase adipose tissue breakdown in order to increase blood triglyceride levels, but the production far outweighs the demand. In other words, the Oxidative System is slow moving and only produces energy at a certain pace - a slow pace at that! So even if you're in a fasted state and your body is increasing blood triglyceride levels, that doesn't mean that those increased levels in the blood are going anywhere. In fact, once you're done exercising, they get shuttled back into your fat cells.
Also keep this in mind: the more carbohydrates you burn during a workout, the more fat you will burn after the workout is over.
Neither the Phosphagen or Glycolytic system require oxygen for their immediate energy output but the Glycolytic System does utilize oxygen in order to replenish spent energy stores. If oxygen demand is not met through respiration during exercise, our bodies go into a state called EPOC, or Excess Post-Exercise Oxygen Consumption. This is oftentimes also called the "afterburn" because calories are still being used to replenish spent energy stores through respiration. This is also why you are still breathing heavy long after an intense workout session.
So let's sum this up.
Low-intensity exercise primarily uses aerobic metabolism (fat oxidation) to generate enough energy for the activity demand. Going for a brisk walk, for example. There may even be some truth to the body metabolizing a higher ratio of fat oxidation when fasted, however this is such a relatively small amount of calories burned compared to much higher intensity exercise.
Performing high intensity exercise will favour more of a carbohydrate-burning metabolism, which sort of defeats the purpose of training fasted. The amount of EPOC you can create for yourself, or in other words, the more intense your workout is, therefore causing a substantial "afterburn" will yield greater calorie burn and fat oxidation.
On top of all of that, research shows that training in a glycogen-depleted state causes the body to burn far more tissue proteins for energy rather than stored energy sources. Muscle protein can account for nearly twice the amount of calories burned in a fasted state versus that of a fed state. In other words, eat before you workout, but train hard!
2. Who really cares how many calories you burn during a workout anyways?
There is a flip side to this coin, however.
I try and remind clients about this on a consistent basis. Keep this thought in your head from now on and it will save you a lot of time and aggravation: Exercise is for your brain and your body. Sure, it burns calories, but you are doing it to keep your machine (your body) running at optimal levels. Body composition is largely controlled by the diet that you eat and the overall net balance of calories in versus calories out.
Doing an hour of light exercise to attempt to stay in your "fat burning zone" will burn such a minuscule amount of calories (in the grand scheme of things) that there really isn't any point in even counting them. Similarly, the amount of calories you may or may not be burning in a fasted state is negligible.
There should be two main factors that your training should focus on if you wish to optimize the amount of adipose tissue you wish to eliminate. First, make sure you are including enough resistance training to work as much muscle during your training sessions as possible. Compound movements involving our primary movers and joints are great for this. Compound movements include such things as squats, deadlifts, presses, rows, etc.
Secondly, your training should be intense. As mentioned above, if you're training intensely, the net amount of calories burned from the exercise itself, followed by the burn long after your workout is over, is far more beneficial from a calorie burning and hormonal response standpoint than trying to target fat loss specifically at a lower heart rate.
Between carrying around more lean mass (muscle), and exercising intensely, your body will be primed to have a much higher resting or basal metabolic rate to maximize your calories burned on a day to day basis.