The “Fat Burning Zone” On Trial – The Low Intensity Fat Burning Zone
Origin of the myth: Dietary variables aside, the body’s proportional use of fat for fuel during exercise is dependent upon training intensity. The lower the intensity, the greater the proportion of stored fat is used for fuel. The higher the intensity, the greater proportional use of glycogen and/or the phosphagen system. But this is where the misunderstanding begins. Although I’m burning a greater proportion of stored fat typing this sentence, getting up and sprinting would have a greater impact on fat reduction despite its lesser proportional use of fat to power the increased intensity. Alas, sufficient investigation of the intensity threshold of maximal net fat oxidation has been done. In what’s perhaps the best designed trial of its kind, Achten and Jeukendrup found peak fat oxidation to occur during exercise at 63% VO2 max. This peak level got progressively less beyond that point, and was minimal at 82% VO2 max, near the lactate threshold of 87% .
Misunderstanding is perpetuated in fitness circles
It has been widely misconstrued that a greater net amount of fat is burned through lower to moderate intensity work, regardless of study duration and endpoints assessed. In addition the confusion of net fat oxidation with proportional fat oxidation, the postexercise period is critically overlooked. No distinction is ever made between during-exercise fat oxidation, recovery period fat oxidation, total fat oxidation by the end of a 24-hr period, and most importantly, a longer term of several weeks.. Thus, the superiority of lower intensity cardio continues to be touted over the more rigorous stuff that takes half the time to do. Fortunately, we have enough research data to gain a clear understanding. Let’s dig in.
Dissecting The Research
Mixed study protocols + mixed results = plenty of mixed-up trainees
As with all research involving applied physiology, the highly mixed set of results is due to a wide variation of study designs in terms subject profile, dietary manipulation, energetic balance, and actual intensities used. Nevertheless, the body of exercise-induced fat oxidation research can be easily deciphered by stratifying it into 3 subgroups: Acute effect (during exercise and immediately after), 24-hr effect, and chronic effect (results over several weeks).
Acute effects spawn ideas for further research
In addition to measuring fat oxidation during exercise, most acute effect trials look at fat oxidation at the 3 to 6 hr mark postexercise . Fat oxidation during exercise tends to be higher in low-intensity treatments, but postexercise fat oxidation tends to be higher in high-intensity treatments. For example, Phelain’s team compared fat oxidation in at 3hrs postexercise of 75% VO2 max versus the same kcals burned at 50% . Fat oxidation was insignificantly higher during exercise for the 50% group, but was significantly higher for the 75% group 3 hours postexercise. Lee’s team compared, in college males, the thermogenic and lipolytic effects of exercise pre-fueled with milk + glucose on high versus low-intensity training . Predictably, pre-exercise intake of the milk/glucose solution increased excess postexercise oxygen consumption (EPOC, aka residual thermogenesis) significantly more than the fasted control group in both cases. The high-intensity treatment had more fat oxidation during the recovery period than the low intensity treatment. This implicates pre-fueled high-intensity training’s potential role in optimizing fat reduction while simultaneously setting the stage for quicker recovery.
24-hr effects come closer to reality
You can call it Murphy’s Law, but the promise of greater fat oxidation seen during and in the early postexercise periods of lower intensity cardio disappears when the effects are measured over 24 hours. Melanson’s research team was perhaps the first to break the redundancy of studies that only compared effects within a few hours postexercise . In a design involving an even mix of lean, healthy men and women aged 20-45, identical caloric expenditures of 40% VO2 max was compared with 70% VO2 max. Result? No difference in net fat oxidation between the low and high-intensity groups at the 24 hr mark.
Saris and Schrauwen conducted a similar study on obese males using a high-intensity interval protocol versus a low-intensity linear one . There was no difference in fat oxidation between high and low intensity treatments at 24 hrs. In addition, the high-intensity group actually maintained a lower respiratory quotient in postexercise. This means that their fat oxidation was higher than the low-intensity group the rest of the day following the training bout, thus the evening out the end results at 24 hrs.
Chronic effects come even closer
Long-term/Chronic effect studies are the true tests of whatever hints and clues we might get from acute studies. The results of trials carried out over several weeks have obvious validity advantages over shorter ones. They also afford the opportunity to measure changes in body composition, versus mere substrate use proximal to exercise. The common thread running through these trials is that when caloric expenditure during exercise is matched, negligible fat loss differences are seen. The fact relevant to bodybuilding is that high-intensity groups either gain or maintain LBM, whereas the low-intensity groups tend to lose lean mass, hence the high intensity groups experience less net losses in weight [7-9].
The body of research strongly favors high-intensity interval training (HIIT) for both fat loss and lean mass gain/maintenance, even across a broad range of study populations [9-12]. A memorable example of this is work by Tremblay’s team, observing the effect of 20 weeks of HIIT versus endurance training (ET) on young adults . When energy expenditure between groups was corrected, HIIT group showed a whopping 9 times the fat loss as the ET group. In the HIIT group, biopsies showed an increase of glycolytic enzymes, as well as an increase of 3-hydroxyacyl coenzyme A dehydrogenase (HADH) activity, a marker of fat oxidation. Researchers concluded that the metabolic adaptations in muscle in response to HIIT favor the process of fat oxidation. The mechanisms for these results are still under investigation, but they’re centered around residual thermic and lipolytic effects mediated by enzymatic, morphologic, and beta-adrenergic adaptations in muscle. Linear/steady state comparisons of the 2 types tends to find no difference, except for better cardiovascular fitness gains in the high-intensity groups .
Summing Up the Research Findings
• In acute trials, fat oxidation during exercise tends to be higher in low-intensity treatments, but postexercise fat oxidation and/or energy expenditure tends to be higher in high-intensity treatments.
• Fed subjects consistently experience a greater thermic effect postexercise in both intensity ranges.
• In 24-hr trials, there is no difference in fat oxidation between the 2 types, pointing to a delayed rise in fat oxidation in the high-intensity groups which evens out the field.
• In long-term studies, both linear high-intensity and HIIT training is superior to lower intensities on the whole for maintaining and/or increasing cardiovascular fitness and lean mass, and are at least as effective, and according to some research, far better at reducing bodyfat.
Part 2: False Hopes for Fasted Cardio
The bandwagon is lead by blind horses
Many trainees pigeonhole weight training as an activity exclusively for building muscle, and cardio exclusively for burning fat. On the contrary, weight training can yield very similar results to cardio of similar intensity when 24-hr energy expenditure and macronutrient oxidation is measured . The obvious advantage of weight training is the higher potential for lean mass and strength gains. In the bodybuilding context, cardio should be viewed as merely an adjunctive training mode to further energy expenditure and cross-complement the adaptations specific to weight training. As far as cardio being absolutely necessary for cardiovascular health, well, that depends upon the overall volume and magnitude of your weight training – another topic for another time.
Chaos theory strikes again
On the surface, it seems logical to separate carbs from cardio if you want a maximal degree of fat oxidation to occur during training. But, there’s the underlying mistake – focusing on stored fuel usage during training instead of focusing on optimally partitioning exogenous fuel for maximal lipolytic effect around the clock. Put another way, it’s a better objective to coincide your carb intake with your day’s thermic peaks, where insulin sensitivity and lean tissue reception to carbs is highest. For some reason, this logic is not easily accepted, nor understood. As we know, human physiology doesn’t always cooperate with logic or popular opinion, so let’s scrutinize the science behind the claims.
Let The Research Speak
Carbohydrate ingestion during low-intensity exercise reduces fat oxidation
As far as 3 decades back, Ahlborg’s team observed that carb ingestion during low-intensity exercise (25-45% VO2 max) reduced fat oxidation compared to fasted levels . More recently, De Glisezinski’s team observed similar results in trained men at 50% VO2 max . Efforts to determine the mechanism behind this phenomenon have been made. Coyle’s team observed that at 50% VO2 max, carbohydrate availability can directly regulate fat oxidation by coordinating hyperinsulinemia to inhibit long-chain fatty acid transport into mitochondria .
Carbohydrate’s effect on fat oxidation during moderate-intensity exercise depends on conditioning level
Civitarese’s team found glucose ingestion during exercise to blunt lipolysis via decreasing the gene expression involved in fat oxidation in untrained men . Wallis’ team saw suppressed fat oxidation in moderately trained men and women when glucose was ingested during exercise .
In contrast to the above trials on beginning and intermediate trainees, Coyle’s team repeatedly showed that carb ingestion during moderate-intensity (65-75% VO2 max) does not reduce fat oxidation during the first 120 min of exercise in trained men [7,8]. Interestingly, the intensity margin proximal to where fat oxidation is highest was unaffected by carb ingestion, and remained so for the first 2 hours of exercise.
Horowitz’ team examined the effect of a during-training solution of high-glycemic carbs on moderately trained men undergoing either low intensity exercise (25% VO2 max) or high-moderate intensity (68% VO2 max) . Similar results to Coyle’s work were seen. Subjects completed a 2-hr cycling bout, and ingested the carb solution at 30, 60, and 90 minutes in. In the low-intensity treatment, fat oxidation was not reduced below fasted-state control group’s levels until 80-90 min of exercise. In the 68% group, no difference in fat oxidation was seen whether subjects were fasted or fed throughout the trial.
Further supporting the evidence in favor of fed cardio in trained men, Febbraio’s team investigated the effects of carb ingestion pre and during training in easily one of the best-designed trials on this topic . Subjects exercised for 2 hrs at an intensity level of 63% VO2 max, which is now known as the point of maximal fat oxidation during exercise. Result? Pre and during-training carbs increased performance – and there was no difference in total fat oxidation between the fasted and fed subjects. Despite the elevated insulin levels in the carb-fueled groups, there was no difference in fat availability or fat utilization.
Summing Up the Research Findings
• At low intensities (25-50% VO2 max), carbs during exercise reduce fat oxidation compared to fasted trainees.
• At moderate intensities (63-68% VO2 max) carbs during exercise may reduce fat oxidation in untrained subjects, but do not reduce fat oxidation in trained subjects for at least the first 80-120 minutes of exercise.
• Carbohydrate during exercise spares liver glycogen, which is among the most critical factors for anticatabolism during hypocaloric and other conditions of metabolic stress. This protective hepatic effect is absent in fasted cardio.
• At the established intensity level of peak fat oxidation (~63% VO2 max), carbohydrate increases performance without any suppression of fat oxidation in trained subjects.
Part 3: Fat Burning Zone and Fasted Cardio Discussion and Afterthoughts
Not an “Either-Or” Issue
The current facts have been presented in parts 1 and 2, and the bases for conclusion should be self-evident. Let me clarify that HIIT and linear high-intensity cardio are not the best and only ways to go. Many folks have perfectly legitimate orthopedic, cardiac, and even psychological reasons to avoid them. Not only that, I sincerely believe that both low and high-intensity cardio have unique benefits unto themselves. Optimally, both types should be done, since each has specifically different effects. Saying that one is bottom-line superior to the other for improvement in body composition is as false as blanketly saying 5 reps per set is superior to 15. On the contrary, there is well-established benefit in periodizing training variables, or as they say in the trenches, “mixing it up”.
Too Much of the Same?
I’ve heard it mentioned that high-intensity cardio shouldn’t be done concurrently with high-intensity weight training due to excessive stress on the central nervous system. Perfect excuse. My primary response is, there’s no solid proof of that danger. Certainly there’s no evidence of it in my observations as a professional in the field, working with bodybuilders, and all types of other competitive athletes such as gymnasts, sprinters, boxers, etc (you know, athletes whose incredible physiques have nothing to do with weights + high intensity cardio). It’s true that some folks regard a precociously low carb intake as a legit reason to keep intensity low. However, if your nutritional program doesn’t adequately support productive training, then you’ve designed it ass-backwards, painting yourself into a corner of compromised adaptation.
Options can be broken down in the following ways: If you’re pressed for time, and you can do HIIT without any delayed onset muscle soreness overlap (by virtue of doing a low frequency of HIIT), and you can tolerate it joint-wise and heart-wise, and you hate spending time doing cardio to begin with, then do HIIT. On the other hand, if you have the time to allot for low-intensity steady state (LISS), and you do a particularly high volume and magnitude of resistance training which raises potential recovery conflicts posed by a high frequency of HIIT, then do LISS. If you’re somewhere in between the aforementioned 2 camps and you don’t have a specific preference or tolerance limit, do both types on either a cyclical, rotational, or even combined basis. Also, it can’t be overstated that unless you undergo a very gradual progression towards the musculoskeletal tolerance for something like sprinting, you can get hurt pretty bad and there goes your productive training for several weeks.
Fasted = Suboptimal
Fasted cardio is not optimal for reasons spanning beyond its questionable track record in research. There’s unavoidable positive metabolic synergy in fed (read: properly fueled) training, regardless of sport. This effect increases with intensity of training; even in untrained subjects, whatever fat oxidation is suppressed during training is compensated for in the recovery period by multiple mechanisms, many of which are not yet identified.
Athletes are known for their gravitation towards self-sacrifice, but some rely on hearsay, while others rely on science. Did you know that way back in the 60’s, it wasn’t uncommon for coaches to tell athletes in various sports to avoid drinking water before and during training? No comment needed. Good thing researchers questioned it, and enough data surfaced to validate claims of the skeptics. Sometimes counterproductive dogma indeed dies, thank goodness. However, the myths addressed here are admittedly more subtle than the water example. Even on suboptimal protocols, athletes all over the world still inch along, although not at optimal rates, and not necessarily to optimal levels.
So…I see the bottom line like this.. Do the type you have a personal preference for, and also respect your physical limits. HIIT is quicker but riskier. LISS is safer but takes twice as long to accomplish the same thing. Again, do what you prefer and can tolerate, but do NOT make the mistake of assuming that LISS burns more fat. That’s misunderstanding the physiology of the matter.
I’ll end off by challenging you to diligently review the facts before blindly latching onto the myths.
About the Author:
Alan Aragon has over 15 years of success in the fitness field. He earned his Bachelor and Master of Science in Nutrition with top honors. Alan is a continuing education provider for the Commission on Dietetic Registration, National Academy of Sports Medicine, American Council on Exercise, and National Strength & Conditioning Association. Alan recently lectured to clinicians at the FDA and the annual conference of the Los Angeles Dietetic Association. He maintains a private practice designing programs for recreational, Olympic, and professional athletes, including the Los Angeles Lakers, Los Angeles Kings, and Anaheim Mighty Ducks. Alan is a contributing editor and Weight Loss Coach of Men’s Health magazine. His bookGirth Control is considered one of the most in-depth manuals for physique improvement and understanding nutrition for fitness & sports. Last but not least, Alan writes a monthly research review providing of the latest science on nutrition, training, and supplementation. Visit Alan’s blog to keep up with his latest shenanigans.
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