I Need To Stay In My Fat-Burning Heart Rate Range!

I have had this conversation twice in the past week with two different clients of mine:

Client: “I need to stay in my ideal fat-burning heart rate range.”
Sarah: “Do you have a heart rate monitor?”
Client: “No..”
Sarah: “Do you ever measure your heart rate?”
Client: “No..”
Sarah: “Then giving you numbers would be completely pointless. You can gauge your eventual fat-burning by how you feel, and exercising at a moderate level in what is typically considered the “fat-burning zone” is not the way to do it.”
*note* if the individual has been instructed by a doctor to stay within a heart-rate rage (due to surgery, cardiac rehab, medications, etc. for health reasons, do not neglect the heart rate. If the individual does not have a heart-rate monitor, it may be a wise investment for them.)

This is a GREAT time to educate your client!!  You don’t have to get as detailed as I will–I’ll include a summary at the end that should help consolidate everything I will be sharing with you.  The clients typically could care less about specific numbers or hormones, but you will have a few that will. In the case that you have a few of these, as I now do, I’ve also found that dropping a few terms or references, such as “hormonal response to exercise” and even referencing the actions of a few of them, help thee client’s perception of your credibility so they likely will listen and turn to you for advice, given the fact that you are a professional and clearly know your facts and research.

What (Ultimately) Is Responsible For Helping Us Burn Fat?
Easy: caloric expenditure. You should expend calories within the heart rate range that considered to be the ideal fat-burning zone.  If you get too heavy, you will begin burning sugar.  You don’t want to burn sugar, you want to burn fat.  Keep the intensity moderate!  This is what traditional exercise prescribers would tell you, and this was the fundamental belief for YEARS, even while I was in college. This was just prior to the brink of a major push for HIIT (High-Intensity Intemittent Training).  This prescription is mainly due to the percentage of calories burned that are fat vs. those that are glucose.  What they will not tell you is that in all reality, you are burning the highest percentage of calories from fat while you are merely sitting and reading this.  I repeat: you burn the highest percentage of calories from fat while you are doing absolutely nothing.  However, we know how this affects body composition—you don’t become less fat by literally doing nothing.  Just because the percentage is higher does not mean that the absolute number burned is higher.  At rest you might burn 60 kilocalories per hour.  If 70% of those kcals are fat, you are burning 42 kcals of fat and 18 kcals of glucose per hour.  Breaking this down to a per-minute analysis will be helpful as I progress, so doing that now would mean that your body at rest is burning 4.2 kcals of fat and 1.8 kcals of glucose per minute.  Jump to a steady-state aerobic exercise.  At 50% of your VO2max, your body is burning approximately 50% fat and 50% glucose.  VO2max is a measurement that indicates how close your body is to its maximal oxygen consumption aerobically—once you hit 100%, your body has moved from an aerobic source of energy supplication to entirely anaerobic–or, a respiratory exchange ratio of 1.0.  When your body is providing energy aerobically, fat is its primary source of fuel.  Combined with oxygen, it’s rather efficient.  However, anaerobic means “without oxygen”—the body is entirely dependent upon glucose to provide the necessary fuel.  The traditional fat-loss mindset would question why you would EVER want to go the anaerobic route.  Who cares about burning sugar? We want to burn fat!! Even if you could go anaerobic, the movements are too explosive and unsafe for deconditioned and obese individuals.
Revisiting solely the per-minute caloric expenditure reasoning, we have to remember that at rest someone may burn 4.2 kcals of fat vs. 1.8. kcals of glucose. If this same person is running a 10-minute mile, a conservative caloric expenditure expectation for them would be 100 kcals per mile or 600 kcals per hour.  This breaks down to a whole 10 kcals per minute.  If this is only 50% of their VO2max, this individual would be burning 5 kcals of fat and 5 kcals of glucose per minute.  This looks a little more promising in terms of caloric expenditure.  So, let’s jump up to 75% of their VO2max.  This individual is now running an eight-minute mile.  If this individual were to maintain this pace for an hour, they would be covering seven-and-a-half miles, burning approximately 750 kcals. This breaks down to 12.5 kcals per minute.  Rather promising in terms of caloric expenditure.  This person might be burning only 33% fat, and 67% glucose.  This means 4.2 kcals of fat are being burned, while 8.3 kcals of glucose are being burned.  This person’s 100% VO2max may be the pace of a 5-minute mile, or 12 miles per hour.  This would be 1200 kcals per hour, or 20 kcals per minute.  That would be 20 kcals of glucose.

“Maintaining that level of exertion for long enough to burn the number of calories I would need to burn for maintaining the caloric expenditure I currently am would be impossible!! Wouldn’t the lower-intensity exercise win out?”
I will argue no.  Here is why: your body responds to any change in homeostasis hormonally.  For example, if internal temperatures increase, your body will use its chemical messengers known as hormones to begin to attempt to cool itself—blood flow will prioritize away from the core and you will begin to sweat.  Kilocalories, you may or may not know, are actually measurements of heat.  1 kilocalorie is the amount of heat it takes to raise the temperature of 1 gram of pure water 1° Celcius.
That being said, heat is a byproduct of your body’s physiological response to exercise and needing to produce/use ATP (remember—energy currency).  Hence, heat is a pretty good indicator (during exercise) of what is happening regarding the amount of energy your body is using, and thus, calories you are expending.  Noting the above charts, if you are working at 100% of your VO2max, your body will be producing a lot more heat as a result of the ATP usage than if you are working at only 50% of your VO2max. Because your body enjoys homeostasis, the greater the disturbance to this homeostatic state, the greater the hormonal response to adapt.
Taking a look at what happens in the muscles during this bout of exercise: the production of ATP will use one of two cycles—the Krebs Cycle if it is aerobic exercise (remember—using oxygen at a lower intensity) or fast glycolysis if energy needs are so high they must be replenished anaerobically.  A byproduct of anaerobic (fast) glycolysis is lactic acid.  The lactic acid is responsible for the burning and numbness you feel in your muscles during exercise.  The higher the need for energy, the greater and faster the lactic acid accumulation.  Fatigue happens at a much greater rate as a result of decreasing pH within the cells as well as inhibition of the cells enzymatic activity for energy production.  Lactic acid triggers the release of other hormones, given its role as a stimulator for anabolic hormones 1 such as catecholamines (adrenaline, noradrenaline, and dopamine).  They prepare your body for physical activity—responsible for your “fight-or-flight” response.  As far as anabolic hormones (hormones that build muscle) go, it has been noted that there are hormonal responses that occur strictly as a result of resistance training and not as a result of endurance training. “With the notable exception of growth hormone, the anabolic hormones reviewed here appear to decline with endurance training.”  The endurance training being referred to is that which has been regarded as ideal for fat-burning–low-to-moderate-intensity.  Working around 100% of the VO2max is ideal for generating the hormonal responses that will best benefit fat-burning long-term.  There is a much greater afterburn created due to the hormones that are circulating following a maximal bout of exercise.   Working below this will not generate an increased hormonal response.  If an individual performs 10 minutes of exercise at VO2max—20 30s sprints, this individual will burn 200kcals. While it is true that this might take 20-30 minutes, and one could burn at least 300 kcal at a lower intensity steady-state, there is much research to back the fact that the hormonal response for fat-loss, and cardiovascular conditioning benefit, while exercising at a low intensity is nearly non-existent.  Also, consider the physiques, primarily the muscle mass, of a marathoner as opposed to a sprinter.  While the body composition percentages may be similar, the elite marathoner typically has a low amount of muscle mass.  The sprinter, however, has a much greater muscle mass and a stronger looking physique.
There are many misconceptions associated with the longer duration, lower intensity exercise being superior to the shorter duration, higher intensity. In research done by Tremblay, et. al (1990)*, “Despite the fact that the energy expenditure of exercise was twice as high in the LMICT (with a mean estimated energy cost of 120.4 MJ) as in the HIIT program (with a mean estimated energy cost of 57.9 MJ), there was a more pronounced reduction in subcutaneous fat (measured as the sum of six skinfolds) with the HIIT program compared with the LMICT program.” **   If not for body composition, then for cardiovascular conditioning.  However, research also defends the high-intensity intervals in increasing the VO2max and lowering resting heart rate, which are indicative of a more efficient and stronger heartbeat. A study comparing steady state conditioning and anaerobic interval training had the following results***:  “an improved work economy as demonstrated by a 15% reduced oxygen cost, an 8‐bpm lower heart rate, and a 59% lower blood lactate at a given submaximal walking speed. An improved anaerobic threshold and work economy makes a huge difference in the day‐to‐day functional capacity of anyone, but particularly for individuals that are severely deconditioned.”  All of the above results reveal a greater level of conditioning, and these are relative to the group that trained at a steady-state, moderate level.
There has been a complete, and research-based, revamping of the approach to fat-loss and conditioning for human beings, as well as a lot of logical support for why running for miles is a bad idea unless you have good running form.  Over the course of a mile, you will take at least 1,300 steps.  If I told you to hop on one foot 750 times, you would look at me like I am crazy and possibly even inform me that I am.  Yet, we do more than this when we run–we essentially leap, often with poor gait mechanics, from foot-to-foot TWICE that number of times.  Talk about a recipe for injury that results from chronically poor movement!  I’m saving for another day my longer elaboration about why running is, in fact, an advanced activity that a random person off the street should certainly not do for long durations without proper preparation and training.

(This is for my trainer friends who need an idea of how essentially talk about why HIIT is superior to the traditional mentality of long-duration, low-to-moderate-intensity exercise for fat-burning.)
In summary, to blast fat and get lean, you don’t need to run a marathon, or even a 10k.  To exercise at that level is not intense enough to challenge your body to change its composition.  The human body is a master at adapting, but it needs to be challenged to do so.  The only time this happens and will affect body composition positively is while exerting energy at a maximal level.  You know you’re doing this in a few ways: you feel the heat (you’re sweating), you’re breathing hard (when you’re walking up a flight of stairs, you don’t typically breathe heavily until you’ve reached the top–your body has to refuel and is using energy to do so!), and you feel the deep burn and numbness in the muscle (the lactic acid).  When these are taking place, your body is responding in a way that will elicit the greatest fat-burning response that goes all the way down to the cellular level!  That is where change begins.  You’ll also benefit from a cardiovascular strengthening with results that are potentially better than running for endless miles.

The excuses of having no time or being too out-of-shape are being thrown under the bus, leaving us without justification for not taking care of ourselves.  Yes, it burns.  Yes, you will be out of breath.  Yes, you will be better conditioned and deveop a better physique.  Yes, you can call your trainer “drillmaster” and tell everyone they “torture you”, but when all is said and done, you will be better conditioned, stronger, and more confident because of it.

You’re welcome.

*Tremblay A, Simoneau JA, Bouchard C. Impact of exercise intensity on body fatness and skeletal muscle
metabolism. Metabolism. 1994; 43(7): 814‐8.

** Smith, Mark. Sprint Interval Training: “It’s a HIIT!”. 2008.

***Senni M, Tribouilloy CM, Rodeheffer RJ, Jacobsen SJ, Evans JM, Bailey KR, Redfield MM. Congestive heart failure in the community: a study of all incident cases in Olmsted County, Minnesota, in 1991. Circulation. 1998; 98:2282‐2289.


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