It is accepted wisdom that the best exercise for burning fat is aerobic activity; this is why you have legions of joggers hammering their knees into dust on the treadmill at any popular gym. What if I told you that this popular wisdom was wrong though and that crafty marketers had once again twisted scientific fact in order to sell workout equipment and gym membership? What if I told you that there were three ways your body produces energy all of which require carbohydrates for fuel and that relying on a low carb diet actually forces your body to cannibalize muscle tissue for energy not fat? Would you believe me if I told you that you actually burn more fat while going about the rest of the day then while you’re actually exercising? What if I told you that manic high intensity routines like Insanity and P90x actually burned mostly carbs during the workouts not fat?
There are three methods your body utilizes to make energy; The ATP/CP cycle, CP being Creatine Phosphate, the Glycolytic Pathway, and the Oxidative or Aerobic Pathway. In any movement you engage in, regardless of type, your body first begins by using the ATP/CP cycle which simply means it burns the ATP and converts the CP already stored in your cells for energy. However these stores are quickly exhausted most of the time in fewer than two seconds. From here, your body begins to breakdown glucose also stored in the cells to manufacture more ATP for energy by combining CP with ADP to make more ATP. I know there are a lot of abbreviations here but just focus on the fact that ATP or Adenosine Triphospate, is the source of energy for all human effort regardless of what diet you’re on.
Unfortunately all this ATP production fills the cells and bloodstream with Lactic Acid. How efficient your body is at removing lactic acid and getting it to your liver and kidneys where it can be broken back down into glucose and synthesized into more ATP determine how quickly exhaustion sets in. With continued training and conditioning the body becomes more efficient at flushing lactic acid from the blood and cells. No matter how good you are at removing this acid though the fact remains that after about 80 seconds of continual effort your body will still be out of energy, so now you must either stop what you’re doing or switch on the oxidative pathway to produce more ATP.
This final energy production cycle is where we get the term Aerobic. The body uses oxygen via the Krebs cycle and electron transport train to produce ATP. Many of you will vaguely remember this process from your college biology classes. The Krebs cycle is capable of producing much more ATP then the other two pathways but takes much more time to do it. What is important for our discussion is to know that the oxidative pathway is the only one that fat can be used for energy. Breaking down fatty acids though is a long process called beta oxidation and even when complete does not directly produce energy or ATP. What it does is provide the coenzymes needed in the Krebs cycle. Lesson one then is; no fat in your body then no Krebs cycle, no Krebs cycle no aerobic pathway for ATP.
Scientists have estimated that while at rest, not during physical activity, 70% of the energy consumed by the body comes from the Oxidative pathway and fat, not carbohydrates or protein. However, as activity increases especially during exercise, more carbohydrates must be used over fat. At the upper limits of the aerobic pathway, 100% of the energy comes from carbohydrates not fat and if carbohydrates are not available in the body, the body will begin consuming muscle tissue for energy not fat. Lesson two then is this; a no-carb diet is almost a certain guarantee of muscle tissue loss as your body struggles to find energy. Ketosis or the burning of fat directly for energy as prescribed by popular diets is actually a panic process of your body that shortcuts the Krebs cycle and only partially converts fatty acids, filling your bloodstream liver and kidneys with toxic waste products.
Let’s wrap this up. Any physical activity lasting longer than two minutes must rely on the oxidative pathway to provide energy. Potential energy sources for this pathway utilizing the Krebs cycle are; glucose, glycogen, fatty acids and amino acids broken down from lean muscle tissue. This energy cycle takes place in your mitochondria, the more of these you have the more energy you make. The way to increase mitochondria is with conditioning and exercise. Even though fat is broken down during this process the mechanics behind it require glucose. The more carbs or glucose available the more efficient the process is. If no glucose is available your body will consume its own muscle tissue to power the process to break down fat.
Aerobic cycles do indeed utilize fat to produce energy. Somehow this had been confused with Aerobic exercise burning fat for fuel itself, and is actually very misleading. Even though fatty acids are being broken down to create co-enzymes your body still burns far more carbohydrates in this process than fat and in fact if you do not have a ready supply of carbs in your kidneys and liver that can be easily broken down into glucose exhaustion and fatigue will quickly set in regardless of how efficient your aerobic cycle is. Glucose is fuel for everything your body does.
The goal here is to engage in an activity that stresses your aerobic pathway but not to the max. You want to hit a target heart rate at or below 80% of your max for an hour at the most. What this will do is trigger the body to make more mitochondria so that as you go about the rest of your day more mitochondria will break down more fatty acids to make coenzymes. The other side of this is that you need to build up your body’s ability to store ATP and CP to stave off exhaustion and fuel depletion. These two energy sources are stored in the muscle cells themselves and the only way to gain more storage is to make them bigger. Bigger muscles store more ATP and CP and also increase mitochondria production. Finally weight based workouts have been proven to engage the aerobic pathway more easily than long slow steady cardio. Make sure that your exercise routines utilize a combination of both for maximal fat burning and conditioning.