Myths About the Fat Zone

We all want to be healthy. But let’s admit it – losing fat can be the major motivation behind our dietary and exercise habits. This month we’ll discuss the science behind exercising in the “fat zone”.

Have you ever noticed on the aerobic machines at the gym the bar graphs that depict different heart rate zones for your age? Although the calculations are crude, they’re based on different percentages of your maximum heart rate (often estimated at 220 minus your age). At 70% of your maximum rate you are burning mostly fat (aerobic “fat-burning” zone). At 85% of your maximum rate you are burning mostly carbohydrate (anaerobic zone).

This is all old news to you, right? But did you know that you might actually lose more fat if you exercise in the 85% range? First of all, at a higher heart rate, you are burning more calories per minute. The more calories you burn, the more pounds of fat you will lose. Even more importantly, your body tends to burn more calories for hours following a high intensity work out (the “after-burn”). After all, have you ever seen a fat sprinter?

So how does our body choose which type of fuel – fat or carbohydrate – to burn? Our body always wants to burn fat first. We only have approximately 2000 calories of energy readily available in our carbohydrate storage bins (muscles and liver). We store many tens of thousands of calories in our fat reserves.

Here is where it gets tricky. When we exercise at a high intensity, we are not capable of metabolizing fat rapidly enough to meet our immediate needs. Since we can metabolize carbohydrate quickly (think “sugar fix”), we will utilize carbohydrates to support that higher exertion level. However, several very important things happen after you are finished exercising. First, the calories you eat will go to replenish the carbohydrate stores that you lost, rather than to fat storage. As a result, we have indirectly created a “fat deficit.” Normally, when we have the mere 2000 calories of carbohydrate reserves available, our body will want to store anything we eat as fat. Since our body always prefers to burn fat when it can, it will pull calories from previously stored fat. The previously mentioned “after-burn” will also draw from your fat reserves.

Although exercising in the lower intensity fat zone will burn a higher percentage of fat while exercising, you will burn a higher total amount of fat for the day with higher intensity exercise. But don’t overdo it. A perfect mix would be higher intensity interval work 1-2 times per week, and lower intensity aerobic exercise 2-3 times per week.