Just Google “fat burning zone” and the handy dandy search engine will pull up enough conflicting opinions to confuse a rocket scientist. There has been much debate as to whether more fat loss occurs at low intensity or high intensity training. Unfortunately, many people and fitness companies (and even personal trainers!) cite 24 Hour Fitness and the like as their “sources” of information, which are often opinions of their instructors and not necessarily based on real science literature. Such sources seem to think that since you burn more calories in general at higher intensities that you will then lose more weight. This is technically true, if you don’t care what kind of weight you lose. The fact is, most people would prefer to burn fat rather than burn muscle. Why? Because fat is unhealthy and unsightly, whereas muscle not only adds to our strength, but also requires much more energy to function during day to day activities. Which means that even when you’re not working out, your muscles are burning through calories, which is great for weight loss! Fat, on the other hand, is “in storage”, so think of it as your reserve fuel. Your body doesn’t want to burn through its reserve fuel if it doesn’t have to (the body is operating on survival instincts even though most of us don’t have to worry about whether or not we are going to get our next meal!), so you have to force your body to use that fat. It takes longer, lower intensity workouts to makes your body stick to fat burning. That’s why at the gym, you may notice that the cardio machines (i.e. elipticals and bikes) that have the heart rate monitors will give you a fat burning range that is much lower than cardio training range. When you’re working out and your body needs energy, it pulls it from wherever it can get it, whether that is carbs, fat, or muscle. Obviously, you want to maintain and/or build your muscle mass, keep your carbs for energy, and burn off your fat, right?? In order to do this, it has been scientifically shown that you should stay at lower levels of cardio intensity to target fat metabolism.

Be warned: what is about to follow is relatively lengthy and has direct summaries from scientific literature. If you don’t want to read all that, you can skip what’s in italics and go right to what is in bold, which is my plain-English summary of the “scienc-ese” summary. You can trust my interpretation because I have been reading scientific papers since I started my degrees (B.S. and M.S. both in Biology) years ago, and also because I am currently a Department of Defense contractor working in a Navy Physiology Lab! So I know my science, but feel free to read as much or as little of either my interpretation or the scientific abstract as you want.

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1. Carey, DG. Quantifying differences in the “fat burning” zone and the aerobic zone: implications for training. Journal of Strength and Conditioning Research. 23(7): 2090-2095, 2009.

The primary objective of this study was to examine the relationship of the “fat burning” and aerobic zones. Subjects consisted of 36 relatively fit runners (20 male, 16 female) who completed a maximal exercise test to exhaustion on a motor-driven treadmill. The lower and upper limit of the “fat burning” zone was visually assessed by examining each individual graph. Maximal fat oxidation (MFO) was determined to be that point during the test at which fat metabolism in fat calories per minute peaked. The lower limit of the aerobic zone was assessed as 50% of heart rate reserve, whereas the upper limit was set at anaerobic threshold. Although the lower and upper limits of the “fat burning” zone (67.6-87.1% maximal heart rate) were significantly lower (p < 0.05) than their counterparts in the aerobic zone (58.9-76.2%), the considerable overlap of the 2 zones would indicate that training for fat oxidation and training for aerobic fitness are not mutually exclusive and may be accomplished with the same training program. Furthermore, it was determined that this training program could simultaneously meet the requirements of the American College of Sports Medicine for both aerobic fitness and weight control. Maximal fat oxidation occurred at 54.2% maximal oxygen uptake (VO2max). However, the great variability in response between individuals would preclude the prediction of both the “fat burning” zone and MFO, indicating a need for measurement in the laboratory. If laboratory testing is not possible, the practitioner or subject can be reasonably confident MFO lies between 60.2% and 80.0% of the maximal heart rate.

So basically this study showed that while the upper and lower limits of fat loss and aerobic capacity do overlap, the maximal fat loss is at 50% of your VO2max! If you’ve ever been tested for your VO2max, you’ll know that 50% is not very intense (I have done the test here at the Navy Physiology Laboratory where I work). In fact, it’s estimated to be between only 60 and 80% of your maximal heart rate!

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 2. Després, J.-P. and LAMARCHE, B. (1994), Low-intensity endurance exercise training, plasma lipoproteins and the      risk of coronary heart disease. Journal of Internal Medicine, 236: 7–22.

Physically active individuals generally show a reduced risk of coronary heart disease (CHD) compared to the sedentary population. However, whether such reduction in CHD risk mainly results from the concomitant improvement in cardiorespiratory fitness or from the alterations in CHD risk factors has yet to be clearly established. Furthermore, there is still some controversy regarding the potential associations between endurance training-induced changes in metabolic variables considered as CHD risk factors (plasma glucose, insulin and lipoprotein levels) and the magnitude of improvement in cardiorespiratory fitness. From the results of several studies discussed in this article, it is proposed that prolonged endurance exercise of low intensity (˜ 50% V̊o2max), performed on an almost daily basis, seems to significantly improve metabolic variables considered as CHD risk factors through mechanisms that are likely to be independent from the training-related changes in cardiorespiratory fitness. The notion of ‘metabolic fitness’ is introduced and can be defined as the state of a set of metabolic variables relevant to CHD risk and affected by the level of physical activity. Evidence available suggests that these metabolic variables are not closely related to the adaptation of cardio-respiratory fitness in response to exercise training. The concept of metabolic fitness has several implications for the prescription of exercise and for the primary and secondary prevention of CHD. Indeed, emphasis should not be placed on aiming at increasing V̊o2max through high-intensity exercise, but rather on producing a substantial increase in daily energy expenditure that will eventually lead to weight loss and related improvements in carbohydrate and lipid metabolism. Therefore, from a practical standpoint, although a 1 h daily walk may not have marked effects on cardiorespiratory fitness, it probably represents an exercise prescription that is likely to substantially improve ‘metabolic fitness’, thereby reducing the risk of CHD.

This one is slightly different because it is looking primarily at combatting coronary heart disease. However, they do show that metabolic fitness, defined as “substantial increase in daily energy expenditure that will eventually lead to weight loss and related improvements in carbohydrate and lipid (lipids = fats) metabolism”, is achieved at 50% of the VO2max as well. So this confirms what the other study found as well in terms of the maximal fat metabolism threshold.

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3. Achten, J., Gleeson M., and Jeukendrup A.E. Determination of the exercise intensity that elicits maximal fat oxidation. Medicine and Science in Sports Exercise, Vol. 34, No. 1, 2002, pp. 92-97.

Purpose: The aim of this study was to develop a test protocol to determine the exercise intensity at which fat oxidation rate is maximal (Fatmax).

Method: Eighteen moderately trained cyclists performed a graded exercise test to exhaustion, with 5-min stages and 35-W increments (GE 351 5). In addition, four to six continuous prolonged exercise tests (CE) at constant work rates, corresponding to the work rates of the GE test, were performed on separate days. The duration of each test was chosen so that all trials would result in an equal energy expenditure. Seven other subjects performed three different GE tests to exhaustion. The test protocols differed in stage duration and in increment size. Fat oxidation was measured using indirect calorimetry.

Results: No significant differences were found in Fatmax determined with the GE35 /5, the average fat oxidation of the CE tests, or fat oxidation measured during the first 5 min of the CE tests (56 + 3, 64 + 3, 58 + 3%VO2max respectively). Results of the GE35 65 protocol were used to construct an exercise intensity versus fat oxidation curve for each individual. Fatmax was equivalent to 64 + 4%VO2max and 74 ± 3%HRmax. The Fatmax zone (range of intensities with fat oxidation rates within 10% of the peak rate) was located between 55 ± 3 and 72 ± 4%VO2max. The contribution of fat oxidation to energy expenditure became negligible above 89 ± 3%VO2max (92 + 1%HRmax). When stage duration was reduced from 5 to 3 min or when increment size was reduced from 35 to 20W, no significant differences were found in Fatmax, Fatmin, or the Fatmax zone. Conclusion: It is concluded that a protocol with 3-min stages and 35-W increments in work rate can be used to determine Fatmax- Fat oxidation rates are high over a large range of intensities; however, at exercise intensities above Fatmax, fat oxidation rates drop markedly.

This study shows slightly higher values of the maximum fat utilization, though their exercise time is much shorter than other studies. They found a maximal fat burning zone between 55 and 72% of VO2max (with a maximum at 64% VO2max or 74% maximal heart rate). They also showed that fat contributes negligible amounts of energy over 89% VO2max or 92% maximal heart rate, which is often where high intensity workouts keep you.

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4. Romijn JA, Coyle EF, Sidossis LS, et al. Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. American Journal of Physiology. 265(3):E380-E391.

Stable isotope tracers and indirect calorimetry were used to evaluate the regulation of endogenous fat and glucose metabolism in relation to exercise intensity and duration. Five trained subjects were studied during exercise intensities of 25, 65, and 85% of maximal oxygen consumption (VO2max). Plasma glucose tissue uptake and muscle glycogen oxidation increased in relation to exercise intensity. In contrast, peripheral lipolysis was stimulated maximally at the lowest exercise intensity, and fatty acid release into plasma decreased with increasing exercise intensity. Muscle triglyceride lipolysis was stimulated only at higher intensities. During 2 h of exercise at 65% VO2max plasma-derived substrate oxidation progressively increased over time, whereas muscle glycogen and triglyceride oxidation decreased. In recovery from high-intensity exercise, although the rate of lipolysis immediately decreased, the rate of release of fatty acids into plasma increased, indicating release of fatty acids from previously hydrolyzed triglycerides. We conclude that, whereas carbohydrate availability is regulated directly in relation to exercise intensity, the regulation of lipid metabolism seems to be more complex.

This one shows that as you increase intensity, more muscle breakdown occurs. They also found that utilization of fatty acids for energy decreased as intensity increased, plus fatty acids continued to be used at a higher rate even after exercise was complete! That’s good news, huh?

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As you can see, there isn’t much debate as to whether or not there is a fat burning zone, and that this zone is at relatively low intensities. All you have to do is read the back of a protein shake like Muscle Milk to see that the whole point of its existence is to repair muscles after high intensity training like weightlifting. If your body didn’t break down muscle during high intensity workouts and continue to do so even after the workout, there would be no need for these formulas! So what if you want to not only burn fat but also improve your cardiovascular fitness? Well, as Article #1 states, they are not mutually exclusive. In order to maintain a good balance between cardio training and fat burning, it may be beneficial for you to have a couple days that you do long cardio workouts at lower intensity to burn fat, while other days are dedicated to high intensity cardio training for shorter periods of time. As one of the articles pointed out (#2), even walking was sufficient to reach a fat burning heart rate! This means that even if you can’t make it to a gym every day of the week, you can still do something to keep your metabolism up and running. So no excuses! Now that you have all the info, get to work on building your better body in 2012!