At some point I am certain most of you have used a modern piece of cardio training equipment, and doubtlessly somewhere on it could be found a sticker that showed the ‘fat burning zone,’ or at least something similar. This is a heart rate range where cardio equipment sales persons and marketing staff has stated that your body burns off the highest percentage of fat out of the calories it uses. This gets people to exercise at a certain moderate intensity range, which certainly hosts many cardiorespiratory and metabolic potential benefits, but the entire premise of it is a lie, or at best it is wrong, and can lead people away from methods that will truly achieve the particular goals they are going after.

So first of all let’s approach the mathematical side of it. If we accept the numbers these machines tend to give, then yes fat is burned at a higher percentage in the ranges they give than sugars. However…that does not mean you would burn more fat or quicker than say, at a higher intensity range that uses a lower percentage of fat. If you burn (for arbitrary example) 50% fat at intensity level A, but burn say, 200 calories an hour, you burn 100 calories of fat in an hour. If say (again for arbitrary example, these have no tethering to real biology) you exercise at higher intensity (intensity level B), you may only burn 25% fat, but you may burn 800 calories an hour! This means you’ve burned twice as many calories of fat at this higher intensity level in the same amount of time, even at a lower percentage of fat burned. So again, these numerical examples are fictional, however the relationships depicted are completely real. If you exercise at a higher intensity level you will burn more fat quicker, even were we to concede that you burned a higher overall percentage of fat at a lower intensity.

However, I do not concede such a point! Let’s take a step into a simple lesson in how your body uses energy, especially in exercise. The average adult body holds around one hundred grams of sugar when the ‘tanks are full,’ for example shortly after eating and replenishing stores of sugar. This is only about 400 calories of energy, so about 15-20% of our daily caloric needs with no exercise. At the same time we may have tens of thousands of calories of fat stored, depending on our body’s composition. Because our species spent around two million years trying to survive on a daily basis, with frequent bouts of fasting, or even starvation, our bodies have learned to hold hard onto this fat. That means that even though we have this MASSIVE abundance of calories available from fat, even in a very athletic body, we use our sugars first.

Our body gets energy from four basic pathways, three that use carbohydrates (sugar), and one that uses fat. Some of these can use protein, but that isn’t a normal significant part of energy supply (protein mostly being used for construction in our cells) so we’ll ignore that for this discussion. The first two of the sugar pathways are very quick, but inefficient and don’t last long. The last sugar source and the fat energy pathway are considerably slow but very efficient. On a constant basis our body needs energy for running our organs, pumping blood, breathing, and so on, and all four of these energy pathways are used, but to varying degrees. As I said before, our body wants to use sugar first before it dips into its valuable fat stores, which is our insurance policy against a famine and potential death from starvation. This means that when we have eaten recently (in the last few hours), we get most of our energy from the slowest but most efficient sugar system, with lesser use of the other three. When we get up in the morning or haven’t eaten for long stretches of the day, our body is mostly using the energy pathway that uses fats.

When we exercise at any given intensity, the same process happens. Our body wants to go through its sugar first and only uses fat as a last resort. This is actually why endurance athletes go through what’s called a ‘second wind’ during longer bouts of exercise, because they have gone through their sugar stores and the body has to start primarily using fat for energy, and because this cuts into important survival energy reserves, the body sends unpleasant signals to the mind trying to get it to stop, something like biological/mental ‘demons,’ as it has been called before. You generally feel overly exhausted, sometimes even sick to a degree, but as your body realizes that you aren’t going to stop, it finds its ‘stride’ and breaks more fully into using fat for energy.

So from this alone, we can see that the premise of the fat burning zone is complete rubbish. Your body always uses a degree of fat at any given time, but unless it has to it will not burn any substantial amount of fat unless it has to. The notion of calories in exercise is not completely useless, however. When you start exercising, the different energy pathways will start churning to create energy (basically they are all processes used to create or recreate a chemical called ‘ATP’ [adenosine tri-phosphate for those geekily curious]), with each of the four pathways working at different speeds as mentioned before. They are different speeds because they basically have different numbers of reactions that they have to do to give you energy, with the fastest process only really having one reaction, so it is extremely quick! However under any real exercise it gives it’s ‘all’ in just a few seconds, and then the next system takes over as the biggest supplier of energy for up to about one to two minutes before it can’t supply energy fast enough either. So basically the more intense the exercise, the higher ‘energy demand’ there is and while your body will burn energy using its fastest system first to make sure any activity you are trying to do it can give you as much energy you are asking for for as long as possible, especially if you need it quickly and possibly for a while because you are being chased by a bear! Eventually, however, the energy demand simply can’t be met fast enough if you are exercising intensely (regardless of whether it’s running from a bear or sprinting to a goal line) and your performance will go down until you rest some to replenish these different pathways ability to supply energy.

So to bring it more ‘basic’ once more, the higher intensity exercises, if carried on longer than the body would like will force more fat being burned because to supply the energy demand of the body it has to run all your energy systems at full speed, including the fat pathway. As you run out of sugar stores (which happens pretty quick at high intensities) then your body has absolutely no choice but to either stop working (which it may try to convince you to do!) or it has to start getting most (or nearly all after a while) from burning fat. The more intense and longer you exercise for the higher percentage of fat you will burn, and the more fat you will burn. I think the notion of “the fat burning zone” has been thoroughly torched through this discussion.

A few final notes, though. You can definitely burn more actual fat during exercise by exercising longer and harder, but the largest portion of fat is still usually burned while you are sleeping, during that many hour stretch when your body has run out of sugar and has to burn fat. Your body also is very active when it sleeps, believe it or not, rebuilding cells, building more muscle tissue if you have been exercising enough in a way that stimulates muscle growth, it is cleaning out your systems, and restocking it’s stores of chemicals. The fat you burn during exercise is still important, though the more and better quality exercise you do while you’re awake increases the amount of fat your body will burn while you are asleep on a permanent basis as long as you keep it up. Also I want to note, not eating enough calories each day, going long stretches without eating without professional guidance, or not eating before/after exercise in a proper fashion will all kill your metabolism and stop any fat loss you were hoping to cause! The true cause of fat loss is building a healthy functioning metabolism through being active, exercising, keeping each one of your muscles from getting too ‘dusty’ from misuse! Lower intensity cardio has its uses for heart and body health, don’t think I’m knocking it, just know that it is not the fat killing tool that it has been claimed to be for so many years.

That being said, use this knowledge as motivation and justification to exercise harder and for longer, always working smarter towards a healthier you!