At some point in time we've all pondered, how can I lose some weight? And if you drop that catch phrase of "weight loss" into a search bar you'll find 10,000+ options. This is not an article to confirm or refute any plan, rather it is a simple physiological understanding of how the body can and does use fat as a fuel source. In its most simple form of sugar, glucose is the body's go to fuel source. Glucose is the metabolic result of numerous metabolic processes in the human body. You eat it or drink it and the body is going to have to break it into glucose to use it for energy.
That brings us to the three macronutrients that the body needs: Carbohydrates, Fat, and Protein. The body needs all three to function and dangerous results can arise from too little or too much of any combination of the three. The scope of this article is the macronutrient of fat and it's elementary understanding.
The body stores excessive calories in adipose (fat) tissue, bloodstream, and muscle tissue throughout the human body. Think of fat storage in adipose tissue as being a reserve Army in civilian clothing. The fat storage in the bloodstream is an Army that is dressed, in formation, and awaiting deployment. Finally, the fat storage in the muscle is an Army that is dressed, standing behind the front lines without ammunition, and awaiting orders. So who is the front line Army? Those would be the glycogen stores in human tissue. Glycogen is how the majority of carbohydrates are stored and is one "order" away from executing an attack. Glycogen is capable of being broken into glucose in simple, quick fashion and as such it is the body's immediate fuel source and path of least resistance when we need to execute an "order" and exert force.
Our bodies are truly masters of energy efficiency by executing under the path of least resistance.
Because of this fact we will routinely use glycogen for the majority of our daily tasks. When you introduce physical demands above normal daily function you need to call on the reserves.
Calling all fat molecules - our reserve Army!
Remember we mentioned that fat is the Army reserve and the reserves can be massive. The only problem is that Army is not quite to the front line yet. It is going to require energy to acquire new energy from fat molecules. Remember those soldiers above that we said were in the muscle tissue? They are the first to the battle but first have to sprint to the trenches. Fat in the muscle tissue is stored as a fatty acid that isn't quite ready to go to battle. First the fatty acids need to be broken further down to jump into the trenches. The trench is actually going to be the mitochondria (powerhouse) of cells and in order to jump into the trench they are going to need to be broken down into that wonderful glucose molecule. Once broken down and into the mitochondria they are finally at war for you. Now onto those fatty acids sitting in your bloodstream. They have the same process to undergo once they march down the bloodstream highway and leap into the awaiting muscle tissue. Finally you have those undressed army reserves sitting back at home in the adipose tissue. They have the longest road to glory because they first have to be suited up (extracted from fat cells and broken into fatty acids) moved into formation on the highway (bloodstream), march to the target region (muscle tissue), leap into the fields (muscle), and run to the trenches (mitochondria). That's a long journey and needs a lot of energy to gain energy! By no means is this a quick or easy process. During all those "breakdown" steps for the adipose tissue and fatty acids there are byproducts that could be considered "waste" material that the body must remove or our body will rapidly fatigue, heat, ultimately tell us it can't go anymore, and demand rest. Hopefully, we're painting a picture here that using fat for fuel is no simple task. Because of the difficulty to first transport, deliver, and breakdown fat cells the body will openly use those glycogen stores from carbohydrates as the frontline defense for energy demands.
So how do we help the body use fat cells for energy?
That's a deep question with a lot of variables, but we're going to break it down into a simple understanding.
At the beginning of any form of exercise (a demand greater than normal daily breathing and functioning) blood flow is redistributed to focus on delivery to and from fat and muscle tissues. Those civilian dressed fat cells will quickly be dressed in combat gear, placed onto that highway, and directed to the target battlefield. Studies have shown this to occur equally among extremely fit individuals versus unfit individuals. So no matter your fitness level there is hope that this can and does occur! Depending on the intensity of exercise your body will be able to process fuel in very different methods. Lower intensity exercise will afford the body time to direct that army within the bloodstream into the trenches. Moderate exercise intensity will need a more quickly positioned army and will use the fatty acids in the muscle tissue. Heavy exercise intensity needs the front line soldiers and defaults to using those glycogen stores.
So what constitutes low, moderate, and heavy intensity of exercise?
There are two major ways of discussing: VO2 Max (your body's ability to take up and utilize oxygen) and Heart Rate Response (number of times your heart beats in one minute)
For the sake of this article we will leave out VO2 Max because the majority reading will not have immediate access or the desire to have their VO2 Max assessed. But for those that might possibly know their VO2 max, you're maximal fat oxidation (best utilization of fatty acids for energy) occur between 25-60% VO2 Max. OK, now with that out of the way let's move onto something that every single one of us can learn to measure and use to our advantage:
Heart Rate
The maximal fat oxidation occurs 60-75% of your age predicted maximum heart rate. What the heck is that? As you age your heart loses some capacity to control its rate of contraction. There are numerous scientific formulas to calculate this maximal rate. Below you'll find three of the common techniques.
220 - Age = Maximal Heart Rate
217 - (0.85 x Age) = Maximal Heart Rate
206.9 - (0.67 x Age) = Maximal Heart Rate
Play the math with your own age and see the variety of the results but you'll likely find minimal difference in the results. If you're not one for complicated math (or use of calculators) 220-Age is a safe bet. Now that you have your Maximal Heart Rate you can calculate the heart rate range:
Maximal Heart Rate x 0.60 = low heart rate range
Maximal Heart Rate x 0.75 = high heart rate range
This range will give you a target to shoot for during your workout.
How long do I need to exercise?
There is no absolute timing, but research suggests that around 40 minutes of exercise in this heart rate zone is the minimal amount of time required to maximize your fatty acid conversion to glucose. Exercise at 60-75% max HR less than 40 minutes will simply just use muscle glycogen stores for quicker energy.
Perform exercise between 60-75% of your age predicted maximum heart rate for more than 40 minutes to burn away fat.
Ways to burn fat after exercise
Remember we mentioned that heavy intensity exercise needs energy so rapidly that glycogen is used from carbohydrates for immediate energy. This intensity of exercise would be considered greater than 75% maximum heart rate. The human body can exert heavy efforts for much shorter duration because of such extreme cell breakdown and byproduct that is created during heavy intensity workouts. Some of these factors include a limited volume of muscle glycogen stores, a shunting of blood AWAY from fatty tissue (redirected primarily to muscle tissue), reduced enzymes availability for fatty acid breakdown, and a drop of body pH. All of these physiological changes ultimately lead to cell damage that limit the continuation of physical effort. Luckily for us, however, the cell damage requires repair and replenishment of muscle glycogen stores. To repair tissues the body craves post exercise oxygen guided metabolism. What is that you say, oxygen metabolism? That's right - fatty acid conversion into glucose yet again!! The point being driven here is
Heavy bouts of exercise will not burn fat during a workout, however, fat will be burned after discontinuing a workout during the repair/recovery phase.
Research has shown the greatest use of fatty acids for cellular repair occur following high intensity interval training (HIIT) type activities that emphasize eccentric loading. So while the use of HIIT has some limitations and requires guidance you can be rest assured that you will be burning fat for some time even AFTER your workout!!! Hooray to human physiology!! Workouts that include HIIT should be performed at intensities that offer frequent rest periods and should only last a total of 20-30 minutes of exercise. HIIT style training has a high frequency of short and long term tissue damage but severity of damage can be reduced with a guidance of a knowledgeable provider.
In conclusion you can burn fat by exercising at low to moderate intensities of exercise and should use your heart rate for guidance. Aim for a HR between 60-75% max HR and spend a minimum of 40 minutes in that zone to efficiently burn fat DURING exercise. To burn fat AFTER your workout, aim for an average HR greater than 75% for 20-30 minutes of activity. Both styles of exercise have multiple benefits and should not be overlooked. One should strive to participate in no more than 2 workouts per week at intensities greater than 75% Max HR and for 3-4 workouts at intensities 60-75% Max HR.
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