What's the "Fat Burning" Zone

As with many concepts related to exercise and it's benefits, the effects and the rate of change can be quite specific to each individual, especially when talking about weight loss or body fat changes. However, science does give us some suggestions that can be helpful when trying to maximize your ability to burn fat without jeopardizing your health or your endurance potential.
Specifically, related to the question "should I workout slower to optimize "fat burning?" Let me suggest that there is indeed a specific zone of intensity where you will burn the highest amount of fat calories per minute. Unfortunately, depending on the individual, those fat calories may not add up very quickly. For example, during metabolic exercise testing and through a method call indirect calorimetry, scientists can evaluate the concentration of fat and carbohydrates that are utilized for energy at different intensities. Usually, during rest and at the lower exercise intensities we see the highest percentage of fat utilized. Keep in mind though, at rest and low intensity the overall calorie burn is quite low. So, if you are exercising in zone 1 you may be burning mostly fat, but your calories expended will take a very long time to add up to weight lossed. Additionally, if you are not at a high fitness level you may be burning even less fat because you are doing even less work. For example, an elite level triathlete may be able to run an 8-minute mile pace and still maintain his or her heart rate in zone one (i.e., very low relative intensity). Running at that speed will require a very generous amount of calories to sustain and most of it will be derived from fat. Alternatively, If I were running an 8-minute mile pace my level of intensity would be much higher and closer to, if not exceeding, the lactate threshold. That would put me at a zone 3 or 4 intensity level (i.e., hard sustainable), which would require utilizing mostly carbohydrates since I'm already exceeding my fat burning zones.

NOW KEEP IN MIND if I weighed the same as the elite athlete we would be burning close to the same amount of total calories to run that 8-minute mile pace. If we both ran an hour the elite athlete will have burned most of his or her calories from fat while I will have depleted much of my carbohydrate stores and very little fat due to "harder training".

QUICK REVIEW: As exercise intensity goes up so does the overall calorie expenditure, however the fat burn will usually increase only to a limited expenditure rate as other energy systems need to take over. Often, as your intensity approaches and surpasses the lactate threshold, metabolic testing reveals little to no fat calories expended and energy is derived from nearly 100% carbohydrates.
So, according to the above analogy you might surmise that I need to slow down if I am going to burn fat and ultimately lose body fat. However, things are not always how they seem. In reality my goal should be to become fitter so that when I run in zone 1 or 2 I am not doing a shuffle step or walk to keep my heart rate down. I do this by training at levels that stimulate continued improvement in my lactate threshold (LT). Training to increase my LT or sometimes referred to as the anaerobic threshold has been shown to be most effective when combining a variety of training levels. Specifically, sometimes I should train at or slightly above the lactate threshold (zones 3-4) to stimulate LT overload adaptations and other times during longer bouts (zone 1-2)to enhance aerobic kinetics and fat burning adaptations. So there are different adaptations at different intensities (i.e., specificity of training).

Special Note: Recovery after the harder workouts is a key to seeing threshold improvements. As my threshold improves so does my sustainable pace and so does my ability to burn more fat calories and the total calories during exercise.

Also, keep in mind the fact that burning off the extra body fat is dependent on how many total calories you expend at the end of the day as compared to how many you take in from food. So the additive effect of all exercise and activity will promote body fat loss.

Let me introduce a couple more energy/training related concepts that might help you further understand the mechanisms involved with burning calories and losing body fat.

1. In the lab exercise scientists have discovered that exercise that is intense but sustainable (at or around your LT) for bouts up to an hour can have significant effects on how many calories you burn post exercise. There is a term for this called EPOC or excess post oxygen consumption. This concept suggests that you not only can burn calories during exercise but when sufficient work levels are sustained we can see additional calories, specifically fat, burned for several hours after exercise. However, usually individuals who are not very fit will have limited benefit from extra fat burned post exercise because they are generally not able to sustain work rates high enough to accumulate much of an additive effect from the fat burned. SPECIAL NOTE: hard sustained exercise creates higher EPOC which also requires a longer recovery period because of the stress on the system and depleted energy stores.

2. There are studies that show elite endurance athletes who burn well over 4,000 kcals per day often may only consume 2500-3000 kcals. These athletes continued to function and train for months on end and didn't necessarily continue to lose body fat (note: this low calorie intake is quite common among endurance athletes). Studies observed that the athlete’s metabolism adjusted to adapt to the limited energy intake by holding on to the stored fat. It was suggested that the training effect was also compromised due to the lower fuel intake. The point I am making here is that how much and when you fuel your body can have an effect on your ability to use and burn fat. High stress loads with limited fuel restoration and recovery can effect metabolism for the worse.

In summary, how to maximize your body fat burn is often a comprehensive and somewhat complicated approach, not usually accomplished by quick and easy methods. We've all heard that there are no good quick solutions like pills and starvation diets. Similarly, even the "fat burn zone" by itself is not as easy an answer as we might like.

Good luck and keep challenging your training - you will win in the end if you train smart and continue to learn as much as you can.

These exercises or lifestyle changes should not be attempted if you experience any discomfort, pain or dizziness. If you have a history of knee or back pain, high blood pressure or musculoskeletal injuries, you should seek medical advice and the expert services of a trained fitness professional.