Metabolism and Your Fat Burning Zone by John Cardillo

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John Cardillo

When understanding how your body uses macronutrients for fuel its best to take a look back instead of taking a look forward.  Meaning, let’s say you could take a ride on a time machine back to the age of our hunter-gatherer brethren.  Once you stepped off the time machine and took a look at the specimen that was once your relative, you would be shocked to see how impressively built they were.

Your ancient relative stood tall, was lead and muscular, had broad shoulders, and could run down animals.  Most likely, your ancient relative was way more impressive looking than you are today in the 21st century, minus the long hair and loin cloth.  But how could this be?  This hunter-gatherer didn’t have access to a gym, grocery store, personal trainer, or a fancy workout plan.  The physique of your hunter-gatherer brethren was the result of the lifestyle of the time.  Days that were filled with lots of walking and moving across great distances of terrain, gathering whatever edible plants and fruits one could find, setting up shelter, and hunting dangerous game.  Now before understanding how this hunter gather lifestyle plays into how your body uses macronutrients for fuel, you must understand that your body and physiology still thinks that it is the age of hunter-gatherers.  We simply have not had enough genetic mutations over the last 10,000-15,000 years; our bodies still believe it is 15,000 BC.

The preferred energy source that your body wants to use is fat.  Fat yields 9kcal of energy per gram, opposed to 4kcal of energy for carbohydrates and protein.  When your hunter-gatherer brethren were trekking across the tundra for dozens of miles per day, they were surely burning fat for energy. However, when it came time to chase down that animal that would become dinner, carbohydrates in the form of muscle glycogen would be used.  They key differences here lie in intensity.  For lower intensity activity, such as walking across vast distances of land, the preferred fuel source is fat, because it will give a longer steady level of energy.  For higher intensity activities, such as running down and killing an animal, carbohydrates become the main fuel source.  Carbohydrates offer an easily available fuel source that will power humans during high intensities.  However, the carbohydrate fuel source is limited and will not last nearly as long as fat.  A 170lb male has 3,500 grams of carbohydrate stored versus 70,000 grams of fat. This takes us back to trekking across the tundra.  If we were burning carbohydrate we would run through our resources relatively quick and would have no fuel source for any high intensity activity, such fighting or hunting.  A hunter-gatherer’s day revolved around lots of low intensity activity with bouts of high intensity activity.  For fuel sources, we rely on fat to fuel our low intensity activities and carbohydrates for high intensity activities.

Unfortunately, today our lifestyles and diets do not resemble anything close to those of our hunter-gather brethren.  Today we are cooped up inside all day sitting in front of a computer screen and are wreaking havoc on our physiology.  Despite sitting down and not receiving anything close to normal daily activity, our diets consist of great amounts of carbohydrates that are nutrition-less and do not fuel any high intensity activity.  This paradox is what has led to a great prevalence of obesity, with 70% of the population being classified as overweight or obese.  What people need to do is take a page out of the book of our hunter-gather brethren and introduce lots of low intensity daily activity, in the form of walking, gardening, standing, using a standing work-desk, cleaning, riding a bike.  After incorporating considerable increases in daily activity, we need to mix in bouts of high intensity, such as weight lifting, sprinting, jumping, boxing, or wrestling. The bouts of high intensity exercise will justify the carbohydrates that you are in taking.

So what does this mean for you in regards to your training?  It means that you need to be more cognizant of how your physiology relies on primal human traits that are the result of hundreds of thousands of years of evolution. If you want to get your health back on track, shed excess body fat, look better, and feel better, then you need take steps to change your lifestyle, not just try to exercise a little differently. Once you have gained a basic understanding of your physiology and understand the lifestyle changes that you need to make, you are ready to become a fat burning beast.  First, you must keep your blood glucose levels down, otherwise your body will prefer this fuel source because of its easy access, and will store excess glucose as adipose fat (the unsightly fat under your skin that you don’t want!).  When keeping your glycemic load down by eating less sugary foods and fewer carbohydrates, your body will fuel itself with fats. Fat uses different metabolic pathways than carbohydrates and are broken down slowly in the gut by enzymes known as lipases.

Dietary fat is broken down into free fatty acids for energy.  These fats provide a powerful
fuel source that can power your activity for long periods of time, depending upon the intensities of the activities you are doing.  But what about adipose fat? The fat that is under your skin and makes you look unsightly. Your body will use adipose fat for energy as well in a similar manor. Adipose fat is stored as readily available energy in the form of triglycerides.  When you are doing activity and have burned through dietary fat and carbohydrates, your body will now use a process called lipolysis to use adipose fat for energy.  Again, this is not the preferred fuel source during high intensity activity, but it is a fuel source that can provide seemingly endless supplies of energy.  The same energy that was needed 15,000 years ago for our hunter-gatherer brethren that may not have been able to eat for days at a time before they landed a big hunt.  Once you have reached this state of energy metabolism, you are becoming a fat burning beast.  This however, does not mean that you should constantly starve yourself to train, carbohydrates and protein play vital roles in health. One needs to understand how to properly use carbohydrates and protein to fuel and recover so they do not go into a circle of weight-loss and weight-gain.

So as a science, how exactly do you get into your fat burning zone?  It still depends on intensity, and the substrate source that is readily available.  To understand intensities, there are two simple measuring points you should know, VO2max and heart rate.  VO2max is simply the maximum amount of oxygen that you can consume to keep up with metabolic demands, the harder you work, the more oxygen you are going to need.  Gauging hear rate is also fairly simple, the harder you work the faster your heart is going to pump in order to keep up with demands, and you max heart rate is defined as 220-age. In a fasted stated, fat is going to be the predominate fuel source.  During rest to very low intensity activity (25% VO2max) your body uses primarily (85%) FFAs (free fatty acids) that are found in the blood stream.  As intensities increase (65%VO2max), your body will use about half FFAs in the blood stream and half intramuscular triglycerides.  This 65% Vo2 max sweet spot is the zone where you will be specifically burning fat.  You are now consuming oxygen at fairly high rate, meaning you are breathing heavy but not gasping for air. What you should know is that your body wants to hold on to stored muscle glycogen for the absolute most important times.  Times where you need to run away from that sabertooth tiger or sprint down an elk to go in for the kill.  That is why your body is not using glycogen all day long to fuel you for average tasks.  With increasing intensities (85% VO2max or higher), your body is now going to use about 1/3 fat for energy and 2/3 stored arbohydrate for energy.  The fat that is going to be used during these high intensities is intramuscular triglycerides, and the carbohydrates are going to be stored muscle glycogen.  Typically, this is where you see a decrease in performance with prolonged activity (>30 mins), because your carbohydrate stores are limited and fat is not a great fuel source for high intensity activity.  The exact term is called beta oxidation (slow glycolysis), as part of the aerobic energy system.  The process takes place in the cell mitochondria after fat has been broken down into acetyl CoA. Here, oxidized fat (acetyl CoA) goes through the krebs cycle and then the electron transport chain to yield ATP to fuel your body.

What’s an easy way to know you are in the fat burning zone? Well there is an equation for that; it’s called the 180 formula.  One simply subtracts their age from 180.  This is not to be confused with the max heart rate formula, which is 220 minus your age.  Now you subtract your age from 180 giving you a much lower number.  This is your aerobic fat burning zone.  There are a few modifications that must be taken into consideration.  If you have any health problems such as injury, obesity, allergies, asthma, two or more bouts of the flu per year, or have been inconsistent with training, you add an additional 5 to the number that you subtract.  If you have been training consistently for four days a week for over two years you will only subtract 5 from your age.  An example being a man who is 32 years old who has trained consistently for over two years; (180-27=153).  For this person, their fat burning heart rate zone is 153.  An easy way to know you are in this zone is if you can have a conversation with someone while you are doing cardio and you do not feel out of breath.  If you want to use V02 max as a gauge, there is a simple way to do so, you can run a mile and a half as hard as you can, then measure time to your sex/age/bodyweight. This means that you’re pushing enough oxygen through your body that you are just burning fat. At first, this heart rate might seem way too low to effectively burn fat, however, you have not become an effective fat burner at this stage.  You may only be able to run 12-minute miles during this period. Over time, your body will become efficient at training in this zone and your time could drop to as low as 7-minute miles all at the same heart rate.  When this is accomplished, you have become an effective fat burner and your body will become an engine with a seemingly endless fuel tank.

This guest post was submitted by John Cardillo.

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