Background: For years there have been many people telling people in the gym, for training or for general weight loss that you need to be working in your “fat burning zone.” A great deal of the time this is more for the weight loss crowd that they have been feeding this line but unfortunately this has been a little misconstrued and has lead to a great deal of people being mis-informed about what they can or should be doing from weight loss.
So, you might be wondering, “how did this idea get into so many peoples minds.” Well it all comes down to one simple little graph. This graph is known as several different names, whether you call it the “cross-over theory” or the “fat oxidation chart, it doesn’t really matter, it is the same thing. What this simply shows is a rough estimate of where your body begins to burn a great percentage of carbohydrates in relation to fats in relationship to your VO²max. These two fuel sources are chosen because during exercise those are the predominent fuel sources used by the body and therefore they chose to compare them. Sounds simple, right?
Study: “Determinants of fat oxidation during exercise in healthy men and women”
This study was designed to determine whether the changes in fat as a fuel source associated with increases in exercise previously observed in healthy trained men would also occur in a large group of healthy trained and untrained men and women. An incremental step test was performed by each athlete on a treadmill and was then followed by a VO²max test.
The results show, without any of the additional data recorded (dependent variales) being taken into consideration, that their Carbohydrate (CHO) oxidation was significantly higher in males than females and that the males Maximal Fat Oxidation (MFO) per kilogram of Free Fatty Mass (FFM) was lower. In other words… men tend to burn a greater percentage of carbohydrates at a lower intensity than women
With all of the dependent variables (variable being measured) taken into consideration they account for over 12% of the variance. In this study they found that the cross-over happened between 48 and 53% VO²max which is comparable to previous findings (Brooks and colleagues). Both VO²max and Self-reported physical activity (SRPAL) can increase fat oxidation by having an increased FFM. Or… Your activity level can have a direct impact on your fat mass which alters fuel metabolism percentages.
Practical Application: This is not telling you that you need to spend all of your time in the “fat burning zone.” It is simply stating that individuals with a higher FFM(musculature) will have a higher overall calorie expenditure with weight being equal. The biggest thing to keep in mind, however, is just because you are burning a higher percentage of fat does not mean that you are burning a higher total number of calories. Yes there is a sweet spot where you are burning more calories of fat but it will be different for all. If weight loss is your ultimate goal there is one thing to remember, “Calories out is greater than Calories in,” and to burn a high number of calories you can optimize your available time by doing a higher intensity workout.