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“Fat Loss Zone” Myth Debunked

If you have read fitness magazines or been to an exercise class you have heard of the “Fat Loss Zone”. Typically, the “Fat Loss Zone” is centered around your heart rate, holding it at ~60 – 75% of your maximum heart rate. This is a moderate pace, typically obtained through steady-state aerobics and/or long running, swimming, or cycling. This is termed low-intensity steady state (LISS) exercise.
Unfortunately, LISS is not the most efficient method for weight loss and fat loss. Now, if you are an endurance athlete, it doesn’t mean this form of exercise is irrelevant, as performing at your race speed or working on technique at a slow speed is beneficial. Also, if you are overweight, you don’t need the most efficient method for losing fat, instead you need to lose fat safely over an extended period. However,  clearing the air about exercise response to exercise intensity is necessary for ease of mind, debunking myths, and setting a plan.  

Recently, high-intensity interval training (HIIT) and high-intensity training (HIT) has gained popularity, polarizing exercise camps: HIIT (HIT) vs. LISS. Unfortunately, this polarization is unnecessary, as forms of exercise can benefit and aide fat loss, as well as be the best method for fat loss for specific populations. 

Energy Metabolism

Eat carbohydrates, work out intensely (HIIT or HIT), mainly glucose with a small amount of fat and protein utilization. This is the sequence for energy metabolism during intense exercise. If you consume carbohydrates and are working out at a high-intensity, your body will mainly use carbohydrates for energy which is turned into glucose and stored as glycogen. Once again, this alters with food consumption, but most people (even paleo followers) will consume enough carbohydrates for fuel. 

Work out with slow steady exercise (LISS), nearly half glucose and fat utilization, with some protein usage. 

Sounds like low-intensity training would be more effective when it comes to losing fat, right? Unfortunately, substrate ratio is not the key for understanding fat loss, as total calories expended is more important. Even though the ratio of fat-to-carb calories might be higher during low-intensity exercise, fewer calories are used up overall. High-intensity exercise burns the most calories.

Specifically, at lower intensities the body may burn 50 percent of the calories from fat, while at higher intensities it may only burn 35 percent. But at higher intensities you burn way more total calories—and more fat calories overall—than you do at lower intensities.

Excess Post-Exercise Oxygen Consumption (EPOC)

Some have heard low-intensity exercise turns the body into a “fat burning machine”, implying more calories are expended during the day. However, EPOC increases most after HIT and HIIT. 

HIT and HIIT burn a lot of calories. After HIT or HIIT, metabolism remains elevated, previously referred to as oxygen debut, but is now known as excess post-exercise oxygen consumption (EPOC). Recovery of the metabolic rate back to pre-exercise levels can require several minutes for light exercise (LISS), several hours for very heavy exercise (HIT), and even up to 12 hours or longer for prolonged, exhaustive exercise (HIIT and heavy resistance trainin).

EPOC is a crucial and minimally discussed part of energy expenditure when totaled over the entire period of recovery. If oxygen consumption following exercise remains elevated by an average of only 50 milliliters per minute, and the metabolism remains elevated for five hours, this would amount to an additional expenditure of 75 kilocalories over that time period.

Muscle Mass

Some fear muscle gain, often associating bodybuilders with muscle mass. Yet, muscle also influences metabolism and fat loss. Each pound of muscle requires ~50 calories per day to maintain. This doesn’t take into account the calories burned developing that muscle, or the calories burned while maintaining that muscle. This equates to 18,250 calories per year, or the equivalent of a little over five pounds of fat. 

Adding one pound of muscle reduces five pounds of fat per year!

If you gain five pounds of muscle over a year, you’ll lose ~26 pounds of fat over the year. 


If looking for the greatest general method for losing fat mass, then HIT and HIIT are best. However, people are individual creatures with individual responses. Everything isn’t about efficency, but what will work for the person over the course of their life. Too many exercise programs and people want results now, when long-term health and improvement takes work over time. Many times athletes seek immediate results during their peak athletic years, damaging their long-term health and life. 

With all this, we still don’t know everything about energy metabolism. If EPOC increased when you begin exercise, is the effect cumulative? Personally, I think it is and creates greater improvements, but the research has yet to research the idea. 

Cost and risk ratios apply to life. Luckily, everyone can make their own exercise choices and put their best effort forward. At COR, we understand individuality and freedom of choice. There is no best exercise program for everyone, only programs helping people reach their goals of life, health, family, fitness, and whatever else makes you happy and a productive member of society. 


  1. Hackney AC, Kallman A, Hosick KP, Rubin DA, Battaglini CL. Thyroid hormonal responses to intensive interval versus steady-state endurance exercise sessions. Hormones (Athens). 2012a Jan-Mar;11(1):54-60.
  2. Hackney AC, Hosick KP, Myer A, Rubin DA, Battaglini CL. Testosterone responses to intensive interval versus steady-state endurance exercise. J Endocrinol Invest. 2012b Dec;35(11):947-50.
  3. Astorino TA, Schubert MM (2014) Individual Responses to Completion of Short-Term and Chronic Interval Training: A Retrospective Study. PLoS ONE 9(5): e97638.
Written by G. John Mullen, DPT, CSCS