By James Showery
Everyone wants to know the best way to burn fat. This is after all one of the greatest debates of our fitness generation. For years, athletes, fitness enthusiasts, crossfitters, and people of all kinds have pondered this question, often struggling to achieve the results they desire, only to end up frustrated and discouraged. Everyone from fitness gurus, personal trainers, to your family doctor seem to have the answer. Unfortunately, their advice is often unfounded, anecdotal, or flat-out wrong. Today, I want to open up the debate, present the results from hard-core scientific studies, and clear up some of this confusion.
First, I'll start with what we know. The human body's energy stores (i.e. fat, glycogen) are purely the result of lifetime energy input (food) minus lifetime energy output. That being said, there are many genetic and environmental factors that affect these two parameters. Some people absorb food more efficiently than others and some have faster metabolisms. Additionally, hormone levels, stress, and nutrition can affect how the body absorbs, stores, and utilizes food energy. None the less, diet and exercise are still the most important factors in sustainable fat loss.
BUT DON’T WORRY, THERE’S HOPE! With the proper diet and exercise regimen, you can tip the scales (pardon the pun) in your favor.
Now that we are on the same page, let’s start talking SCIENCE!
So you have your diet down, you have been weight training for a while, and you want to know the best way to burn fat without compromising those awesome #gains. You know that spring time is coming and you want everyone to be 'mirin your shredded physique when you peel off your fitted-T at the pool. The questions remains, what is the most efficient way to burn fat without tapping into the muscle you have worked so hard to develop.
In a classic study conducted by Romjin Et. Al (1) in 1995, the authors found that exercise at 25%VO2 max after a 10-12hr fast was fueled almost entirely by the breakdown of peripheral fat. Additionally, they found that the rate of fat breakdown was HIGHER at 25%VO2max than it was at 65%VO2 max and 85% VO2 max, even though the athletes in the study were burning 2-3x more energy at these higher exercise intensity levels.
Now to decode this geek talk a little bit. VO2 max is maximum amount of Oxygen your body can utilize at maximal exertion. Think of this as the rate at which your body will use oxygen at a full sprint for 60 seconds. So 25% of VO2 max roughly correlates to 25% of maximal exertion. Just to save y'all the math, most healthy and active people with a resting heart rate between 60-75 are exercising at 25% VO2 max when there heart rate is around 100-115. If you are really interested in calculating your heart rate at different exercise intensities, you can use the equations below:
Max HR= 220-age HRR= Max HR- Resting HR
HR at 25% VO2max= Resting HR + (0.25x HRR)
Let’s take a 30 year old female with a resting heart rate of 65. The max heart rate for this individual should be around 190 bpm and her HRR should be around 125 bpm. Thus, her heart rate at 25% VO2 max= 65+ (0.25x 125) = 96.25 bpm.
Diving a little deeper into the study, the authors measured the relative amounts that different body energy stores contributed to fueling exercise at varying levels of intensity. The study population consisted of 5 endurance trained cyclists that were instructed to fast for at least 10 hours prior to exercise trials. The trials were run for three consecutive days and took place at the Human Performance Laboratory at the University of Texas at Austin. Since the athletes were trained cyclists, exercise was conducted on stationary bicycles for 120 minutes at 25% VO2 max and 65% VO2 max and for 30 minutes at 85% VO2 max. Oxygen consumption and CO2 production were monitored continuously and blood was drawn at 5 minute intervals during exercise to measure blood glucose, plasma free fatty acids, plasma glycerol, and other markers for energy utilization.
Take this graph of some of the results from the original article where graph A represents substrate (energy type) utilization at 65% VO2 max and graph B represents substrate utilization at 25% VO2 max.
In these figures, it is clear that around 90% of the energy used for low intensity exercise is derived from plasma free fatty acids (peripheral fat) whereas only 40-50% of the energy used for intermediate intensity exercise is derived from plasma free fatty acids. Additionally, the remaining energy required for intermediate intensity exercise is derived mostly from muscle glycogen (stored muscle carbohydrate) and muscle triglyceride (intramuscular fat). From this information alone, we can determine one thing. A greater percentage of energy used in low intensity exercise is derived from peripheral fat compared to higher exercise intensities. However, this does not tell us what type of exercise burns MORE fat in terms of quantity. After all, higher intensity exercise will burn more calories in a given time interval.
In addition to the percentages of substrate utilization, the authors measured the quantity of substrates utilized at each level of exercise intensity. It demonstrated that progressively less fat is burned as exercise intensity increases. Notably, this is represented as a rate of fat breakdown. So, even though low intensity exercise is hardly enough to make one break a sweat, fat breakdown is actually occurring at faster rate than it would at higher exercise intensity. Additionally, it is important to note that almost all of the energy used in low intensity exercise is derived from peripheral fat. Thus, as long as the energy demands for low intensity exercise do not exceed the body’s fat burning capacity, the body will not breakdown muscle for amino acids to supply energy for the exercise.
So the next big question is WHY. After all, wouldn't it make sense for your body to burn more fat if you are burning more energy? That's the thing, if there is one consistent lesson I have learned studying medicine, it is that the human body doesn't make sense sometimes. Although no one really knows for sure, scientific studies seem to suggest that fat utilization is probably limited by the capacity of muscle to use it and not the ability of fat cells to release it (2,3). Additionally, the rate limiting step of fat oxidation (breakdown) seems to be the enzyme carnitine palmitoyltransferase, or CPT, responsible for shuttling free fatty acids into muscle cell mitochondria (the cellular power-plant) where they can be broken down and used for energy (4). Because this enzyme is limited in its ability to move fat into the place within cells where it can be broken down, the rate at which fat can be utilized is significantly limited.
Now you know that low intensity exercise at around 25% VO2 max is the best way to burn fat AND you know why. But we shouldn't forget about one key factor in this exercise equation. The athletes in this study performed this exercise after fasting for at least 10 hours overnight. This significantly affects energy metabolism in a variety of ways. First off, after a 10 hour fast, liver glycogen is nearly depleted. This reduces the ability of the liver to release extra glucose into the blood to fuel exercising muscles. By limiting the availability of blood glucose, the body is forced to utilize fat for energy. Additionally, consuming carbohydrates before a session of low intensity cardio will increase blood glucose and insulin production. When this extra glucose is picked up by muscle cells, some of it may be converted into fat if glucose availability exceeds the energy demands of exercise. If this occurs, then an intermediate in the conversion of glucose to fatty acids, malonyl-CoA, will actually inhibit CPT (the rate limiting enzyme required for fat breakdown) and stop the breakdown of fatty acids for energy (5,6,7). Thus, in order to maximize fat burning potential, low intensity exercise should be conducted in a fasting state.
Although fasting is important to maximize fat breakdown during low intensity cardio, supplementation with branched chain amino acids(BCAAs) before, during, and after exercise will protect muscle from breakdown without slowing fat utilization. Amino acids are the building blocks of protein, in the same way that glucose and fatty acids are the building blocks for complex carbohydrates and fat, respectively. Branched chain amino acids (leucine, isoleucine, and lysine) are a specific group of amino acids that contain multiple branches on their side chains, hence their name. BCAAs are important because they make up to 40% of the amino acids in muscle proteins, are used by the liver and muscle to make glucose and ketone bodies, and may increase muscle synthesis through various cellular signaling pathways. Since muscle will breakdown and release BCAAs to be used as an energy source, consuming 5-10g of BCAAs in workout supplements like Atomic Fallout before, during, or after low intensity cardio will keep muscle BCAA concentrations high. And like most biochemical processes, muscle breakdown is inhibited by its end product. Additionally, since BCAAs do not increase blood insulin or glucose concentrations, fat breakdown should continue to occur at the same rate. (9)
A note on high-intensity interval training or HIIT. HIIT has come into vogue in recent years as the optimal exercise for fat burning. And while HIIT style weight training is a fundamental component to a well-balanced training program, there is no evidence to show that HIIT burns peripheral fat faster than low intensity exercise. That being said, there may be some basis for this theory. During high intensity exercise, muscle glycogen and triglyceride (fat), are the major sources of muscle energy. During these intervals, energy use can be 3-4x greater than low intensity exercise and these energy sources are consumed very quickly. In theory, the rest period between intervals allows the body to restore intramuscular energy stores. In fact, plasma free fatty acids (from peripheral fat) are the major substrate utilized by resting muscle to resynthesize muscle triglycerides (intramuscular fat) (8). Additionally, the muscle breakdown that occurs during HIIT stimulates the body’s metabolism, increases resting energy demands in order to repair microscopic tears in muscle fibers, and acts as a stimulus for muscle growth.
Here are the problems I see with HIIT as an addition to a traditional weight training regimen. First off, we know that fat breakdown by muscle is intrinsically rate limited. Thus, it is physically impossible to breakdown fat faster than what would occur during low intensity cardio at the threshold of maximum fat metabolism. What's more is the additional energy demands exceeding the maximal rate of fat breakdown will require the utilization of other energy sources like blood glucose, amino acids, muscle triglycerides, and muscle glycogen. Once muscle glycogen and triglyceride stores are depleted from exercise, amino acids derived from muscle protein can be used by muscle to meet energy demands that exceed the threshold of maximal fat metabolism. And this means bad news for your gains. When performed in a fed state HIIT is an excellent way to exercise and improve your overall fitness, however it’s probably not the most effective way to burn fat.