I hear some statements quite often. "You need to do aerobics for twenty minutes to start burning fat. Before that, you are only burning sugar." "Walking is better than running because walking burns fat, running does not."
There are many misconceptions about fitness, but none greater than the misunderstandings that abound about "fat burning". Why is this so? It is because fat burning is a complicated physiological issue that is not easily explained to or understood by the public. Somewhere along the way, the message that was passed on from the research scientist to the fitness professional and the media to the exercise participant was garbled. In trying to explain the mechanics of fat burning to the public, much has gotten lost in the translation.
It started about ten years ago when exercise scientists discovered that the higher the intensity of the exercise, the less fat you burned. This led to a slew of exercise videos touting "fat-burning" workouts at very low intensities. Aerobic instructors started to tell their clients that they weren't burning any fat during the first twenty minutes of aerobics. Super-fit individuals who, previously, were happy doing high intensity workouts were now doubtful about whether they were burning fat or not.
All exercise burns fat.
Our body needs energy to function. This energy is found in the food we eat. All foods are made up of fat, carbohydrates, and protein. For the sake of our discussion, we will leave out protein because it is not a primary source of energy except in times of starvation and prolonged illness. We will focus strictly on fat and carbohydrates, the two primary sources of energy under normal conditions. A calorie is a unit of measurement of energy. There are four calories per gram of carbohydrate and nine calories per gram of fat.
The body is constantly using a mixture of both carbohydrates (stored in the liver and muscles as glycogen) and fat (stored in the fat cells as fatty acids). There is no such thing as only fat burning or only carbohydrate burning during exercise or that there are two types of exercise - that which burns fat and that which does not.
Actually, all exercise burns fat. For every calorie your body uses at rest, approximately 50% comes from fat and 50% comes from carbohydrates. Some studies have found that if you are a fit individual, 70% of that calorie can come from fat.
Therefore, technically speaking, resting in bed could be considered a "fat-burning" exercise! Of course, resting is not a very good way to burn a significant amount of fat because very few calories are used up lounging around in bed. Don't get me wrong, I am not, repeat, not advocating being sedentary as a way to lose fat!
How exercise intensity affects burning fat calories.
Having established that the body burns a mixture of carbohydrate and fat, let us now proceed to see how increasing exercise intensity affects this mixture. As you increase your exercise intensity, going from rest to, let's say, walking briskly, you will (don't faint!) burn less fat per each single calorie. As you increase the intensity even more by going from walking to running, you burn even less fat and more carbohydrate per each calorie your body uses. So it is true that the higher the intensity of the exercise, the less percentage of fat you burn. But that "truth" can be misleading because less percentage of fat does not automatically mean less total volume of fat burned! Read on.
A big pie versus a small pie.
When you run (exercise harder), you breathe in more oxygen than when you just walk. For every liter of oxygen that you consume, you burn approximately five calories. Therefore, at the end of a 30-minute run, you will have consumed a larger volume of oxygen (more calories burned) than if you had walked for the same length of time. High intensity exercise uses a smaller percentage of fat but a larger total number of calories burned.
An exercise physiologist named Owen Anderson, put it this way: a small part (smaller percentage of fat per calorie at a higher intensity) of a big pie (more total calories burned) can be just as big or bigger than a larger piece (bigger percentage of fat per calorie at a lower intensity) of a smaller pie (less total calories burned).
I don't like math much either but get out your calculator and let's look at a hypothetical case.
In case you were wondering, a machine called a metabolic gas analyzer that calculates how much oxygen a person consumes while exercising measured the percentages of fat. Don't expect to see this gadget any time soon on your favorite home shopping network. It's only available at exercise laboratories in large universities or research facilities.
Determining exercise intensity.
How exactly does one determine the intensity of an exercise? I won't get into the technical definition (40 - 50 % of your maximum heart rate etc.) but would like to give you a practical way to determine whether an exercise is low, moderate or high intensity. This method is called the "ratings of perceived exertion" or RPE. You rate the intensity of the exercise according to how it feels or how you perceive it to be.
Low intensity exercise feels very comfortable. You are breathing easily, you can carry on a conversation, and you feel like you could go on for hours. Walking at a leisurely pace would be a good example.
Moderate intensity would feel like you are breathing a little faster, you can still carry on a conversation but you are huffing and puffing a bit. An example would be brisk walking.
During high intensity exercise, you can feel your heart thumping, you are breathing hard and you can talk but only in very short sentences. Running, an advanced aerobics class, or a singles tennis match would be a good example.
Super-high intensity would be hardly being able to talk and gasping for air. The 100-meter dash would be an example. At this level, the body is burning almost all it's calories from carbohydrates.
Another important fact to remember about exercise intensity is that it depends on the fitness level of the individual. A very unfit individual may perceive brisk walking to be a high intensity exercise while an extremely fit individual may find the same activity to be very low in intensity.
Each intensity level has its advantages and disadvantages. Low intensity exercise is perfect for unfit people and obese individuals. The advantage is that low intensity exercise is something they can do without injuring themselves and putting too much of a strain on their cardiovascular system.
Also, it has been found that low intensity exercise is beneficial for those with Syndrome X, a cluster of risk factors that include insulin resistance, low HDL cholesterol (the "good" cholesterol), mild hypertension and elevated triglycerides (a type of fat found in the blood).
The disadvantage is that to burn a significant amount of calories, you have to do it for a much longer period than moderate or high intensity exercise.
If you are a fit individual, you have the advantage of choosing time-efficient moderate or high intensity exercises. You can challenge yourself to work out as long as possible at as high an intensity as possible to maximize calorie burning. This is assuming, of course, that you enjoy high intensity workouts (some people don't, no matter how fit they are) and that by doing so you will not ruin your knee, shoulders, and back joints! Your heart and lungs may be fit enough to run kilometers everyday but your knees may not. Compromise and choose exercises that challenge your fitness level but are still safe for the rest of your body.
Super-high intensity training is usually only done by elite athletes training for specific sports.
Total calories more important than fat calories.
Most experts feel that it is not important what percentage of the calories burned come from fat or from carbohydrates. What is important is how many calories are burned for any given activity and whether you enjoy that activity or not. Don't get obsessed about how much fat you are burning. Just pick the exercises you enjoy at the intensity you are comfortable with and "just do it".
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