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!

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Comments
  1. dustin says:

    http://www.menshealth.co.uk/fitness/cardio-exercise/calculate-your-v02-max

    Slide 2 puts the optimal fat burning range at 80% and cites Journal of Sports Sciences. Seems too high.

  2. ckimp says:

    Hi Erin, so if I wanted to get stronger and lose fat I should also do LSD 3+ times/week with my lifting program. Should I do these runs on the same day and right after my lifts?

  3. Lacey says:

    My main goal right now is to lose fat (without losing muscle), with that said what would you recommend cardio wise? I lift anywhere from 3-5 days a week, depending on my schedule, but my cardio is all over the place. I would like to develop a cardio schedule (length and type) that would best suite my goals, any suggestions?

  4. Heather Sims says:

    LOVED this article! Really having a hard time trying to balance it all. My main goal is to burn fat (I’m 5’5 and 120), but I want to maintain my muscle, and I love all types of workouts. But due to the plethora of internet information, I’m trying to do so much: Yoga, Pilates, Kickboxing, Low Intensity Cardio, High Intensity Cardio, Lifting, Metabolic Resistance, Plyo, and all in a week?! Yikes!! I really do enjoy it, but I’m having such a hard time organizing it all, which makes it more of a chore rather than a positive experience. Would love to hear your thoughts/advice.

  5. Chris says:

    Hello, just read this and found it to be the best that the web had to offer – in other words – it was solid! Thanks for compiling and I appreciated your summaries as well. My question is this – how do you recommend I practically obtain my fat burning zone? I’m not sure if your 60% – 80% above is based on the Karvonen formula or other (i.e.220 minus age or other method)? I know the Karvonen formula tends to calculate higher than most. I just got a heart rate monitor from Polar and the 60% – 80% rates they recommend based on my age, height and weight seem too low especially in comparison to the much higher rates that the Karvonen formula gives. Any guidance would be greatly appreciated. Thanks so much!!

    • Hi Chris! Glad you liked my article! As far as the formulas, I’m honestly not sure what is the best way to estimate your rates, since these papers were doing actual laboratory tests to determine those ranges. I looked up some commonly used formulas, and would probably recommend the results from Robergs and Landwehr (2002), who studied 43 different formulas and came up with HRmax = 205.8 − (0.685 × age). Getting your max with this formula would give you a number to calculate your 60-80% range. You could also use the Karvonen formula to get straight to the range. Obviously, these are all approximations, so to truly know your HRmax, you’d have to get hooked up to an ECG and do a treadmill test. Some gyms offer a version of this test (I know Lifetime Fitness does), so you may want to check into that if you want to get a better estimate for yourself! Good luck!

    • Randall says:

      Are you using the simplified Karvonen where target HR is calculated by finding a percentage of HRmax where HRmax equals 220 – age? Or by calculating a percentage of HR reserve and adding resting heart rate to find the target heart rate?

  6. Philipp says:

    I love your blog.. very nice colors & theme. Did you design this website yourself or did
    you hire someone to do it for you? Plz answer back as I’m looking to construct my own blog and would like to know where u got this from. appreciate it

  7. […] the kind of cardio that I do. And while I have posted before about the fat burning benefits of low intensity cardio, there are also lots of benefits to high intensity cardio as well. These cardio workouts are […]

  8. […] This stance is backed by science, as I’ve outlined in my article: The Science of Burning Fat (https://erinsimmonsfitness.wordpress.com/2012/01/20/science-of-burning-fat/). I’m not sayin HIIT won’t burn the fat, it certainly will, but low intensity cardio burns the […]

  9. Joe says:

    So basically, for optimal fat burning the ‘traditional’ jogging is just fine, right? 50pct of VOmax — you could achieve that with an easy jog.

    What I’m wondering is where/what the motivation for HIIT stuff came from?

    Seems like alot of bodybuilders don’t enjoy ‘traditional’ cardio and so HIIT is seen as a quick short and more “exciting” version of cardio.

    What do you think?

    • Yes, jogging will do it! HIIT will burn a lot of calories, and in the process burn fat. The thing about low intensity is that it more specifically targets fat loss, whereas HIIT breaks down muscle as well. I’m sure the motivation for it came from not wanting to do long workouts and people would rather sacrifice some muscle breakdown to burn a lot of calories quick. I think bodybuilders especially should be conscious of this, since it is counterproductive to break down muscle. I believe everything should be in balance, personally. But in order to specifically target fat loss, low intensity long workouts are scientifically shown to be the way to go.

  10. Audrina jacob says:

    You definitely keep me motivated at the gym! I’m kinda stuck at the moment because it seems like when I go to the gym I do the same stuff. My overall goal is to tone my muscle but I haven’t really found a decent workout that I see results with! So I’m wondering if you could help me with that? And is there a good diet to go on I know some can cause health issues if they lack nutritional value.

    • Did you look at some of the different workouts I’ve posted? Those should give you some different ideas of things to do. As far as diet, I don’t recommend any certain diet (“fad” diets), just a balanced, healthy diet that meets nutritional requirements and reduces overly processed, unnatural foods. See the Mayo Clinic Pyramids that I posted. Also, check out my sample meal plan.

  11. Andy Johnson says:

    Great article, Erin. My approach has been to do 30 minutes of LISS in the morning (3.5mph walk at 4% incline) and weightlifting after work. I’d love to see you do a future article about whether “body recomposition” is actually possible. Most of what I’ve read indicates that the body is either cutting or bulking, depending on caloric intake, but it’s almost impossible to cut fat and build muscle simultaneously. On my workout plan, I THINK I’m seeing results in terms of fat loss, but the scale isn’t moving at all. (I’m doing bodyfat comp. tests every 2 months or so to check, but I’m about 3 weeks away from the next one and getting antsy). If there’s some hard evidence that recomposition either IS or ISN’T possible, I would change up the regimen accordingly.

    • Andy, I think what you should take from this article is that it is definitely possible to cut fat and build muscle at the same time! I have never personally seen anything to the contrary. What have you read that says you can’t? If you are doing low intensity cardio, then you won’t be burning through muscle for energy, you’ll be targeting fat. Then if you mix that with weightlifting (and I would also suggest proper supplementation of some sort of protein before and after your workout to combat the breaking down of muscle and helping it rebuild) then you are building muscle at the same time. I can try to find studies that have specifically looked at this, but like I said, if you build muscle then even when you’re not working out it will be burning calories that your body will take from fat as long as you are keeping isocaloric (not taking in more calories than you’re burning). Athletes build muscle while keeping body fat low all the time! But I’ll look and see what I can find.

  12. Jim says:

    If I haven’t had a VO2max test but have access to a heart monitoring device, what would be the optimal heart rate for a low intensity 5k (or 30 min) run to burn the most fat?

    • Hi Jim! Thanks so much for asking that question, I meant to include that part but forgot! The way we do get VO2max here is called the Balke Method (check out http://www.brianmac.co.uk/balketread.htm for detailed instructions). Basically, you start on the treadmill at a lower speed, bring it to a slightly higher speed after a minute, and then every two minutes after that you increase the incline of the treadmill (we use 2.5% every 2 minutes, but as the website says, you can do less than that). If you have a polar monitor or watch, you can do this test to monitor your heart rate during the test and get your maximum heart rate.

      Basically you can do this test as it’s described on this website and instead of monitoring your VO2max, monitor your heart rate. When you get to the end of the test, which means you reach ultimate fatigue where you can not go anymore (but preferable without flying off the treadmill!), then stop. This would be easiest with a polar monitor that you can then plug into your computer and see your heart rate throughout the test. Obviously, the highest value will be your maximal heart rate.

      Note: The website I gave you has a VO2max calculator, but I don’t like it. For instance, when I tested here at work, I ran for over 12 minutes if I remember correctly and had a VO2 max of 50 (this was using a mask that actually measures your oxygen intake and carbon dioxide output). This calculator says mine would only be 21! So I’m not sure how that’s calculating it, so I would just use your heart rate. Especially since you can’t monitor your VO2 max on any sort of instrument during normal training.

      • Jim Shaki says:

        I know cyclist that sleep with their heart monitors on at night and measure their resting heart rate first thing in the morning. Once I establish my resting and maximum what would be the optimal “fat burning zone” based on the research above? I’m guessing around 70%… is that correct?

        • Jim, the articles don’t mention anything about resting HR, though it would be good to know your range. But based on these articles, 60-80% of your maximum HR would be the fat burning zone! So yes, aiming for the middle of that zone around 70% would likely give you the best results.

  13. ksmahoney says:

    While it’s true that the body derives a higher percentage of energy from fat at lower intensities, none of these studies cite total calories burned. At higher intensities, you may have a lower percentage fat burned, but a large number of total calories. If you walk briskly for 30 minutes and burn 200 calories, 50% of those may be from fat (100 calories). However, if you ran hard for an hour and burned 800 calories, while the percent from fat is lower (perhaps 25-30), you would still burned a larger amount of calories from fat (200-250).
    Sara from http://www.losingtogether.com

    • True, however, the point is that if you were to do a lower intensity walk or run for a longer period of time, you are still burning a large number of calories that come from a greater percentage of fat. Your comparison doesn’t hold because you aren’t comparing equal amounts of time. If you were to just run hard for 30 minutes while burning a very low percentage of fat (and from these studies, that percentage is likely less than the 30% you say here) then you may only be burning 60-80 calories as opposed to walking for the same amount of time and burning 100 fat calories. See? You have to compare them over the same time intervals.

  14. Jed says:

    I go to the gym daily, and I alternate days of weight lifting with short jogs (usually 15-20 minutes), and days of long runs (45 minutes to an hour). Does this sound like a good way to burn fat?

    • Absolutely! Just keep in mind that it is probably your longer runs (assuming your longer runs are lower intensity, like mine are) that will be burning more fat, so if that’s your goal, you may want to have 3 days of those long runs and adjust the rest of your schedule accordingly.

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