For years, exercisers have been fed a misleading message regarding the best exercise intensity for burning fat. In a nutshell, it goes like this: if you want to burn more fat, you need to work out at a lower, more aerobic intensity, as opposed to higher intensities where you breathe more heavily. Exercisers are reminded of these recommendations every time they step onto a treadmill, elliptical trainer, exercise bike, or stair stepper, as there is usually a chart on the console display indicating the ideal fat-burning zone, typically expressed as a percentage of maximum heart rate or VO2 max, a popular and widely-accepted measure of aerobic fitness. Let’s face it — most people, exercisers and non-exercisers alike, are inclined to exercise within a comfortable intensity, one that comes closer to light effort than to vigorous effort. Modern humans are creatures of comfort and will often default to “ease” more than “effort” or challenge. All judgments aside, I think that it is important to set the record straight about the myth that low-intensity exercise is better for burning fat, especially for people who:
- Still subscribe to this short-sighted and antiquated view.
- Have been frustrated by the sluggish, if not absent, results from low-intensity training.
- Are on the fence about this issue but are looking for more information.
First, it might be helpful to shed light on how this belief came to be. In all cases, exercise and physical activity burn calories.
At lower intensities — such as walking, jogging, yoga, and other forms of exercise where you are able to maintain a conversation easily — the intensity is considered to be aerobic. At this intensity there is no significant challenge to getting oxygen delivered to the working muscles and cells of the body. You breathe easily and fully and never need to resort to panting or gasping. At such aerobic intensities, the body burns the bulk of its calories from fat, since fat requires an adequate supply of oxygen to be metabolized and used for energy production. In fact, it is not uncommon for people to burn 50 – 70% of all their calories from fat during low-intensity exercise.
In contrast, during higher-intensity exercise, the body is working harder and is more desperate to supply adequate oxygen to the working cells in the body. The breathing rate increases to accomplish this. As intensity continues to increase, the intensity becomes more anaerobic — the body cannot keep pace with the cells’ oxygen needs and must resort to burning some carbohydrate for energy. At very high intensities, most, if not all, of the energy is supplied by carbohydrate, and very little from fat.
Knowing this, it is tempting to conclude that low-intensity exercise burns more fat than high-intensity exercise. This is true in terms of the percentage of calories burned from fat, but it is important to remember that comparatively fewer calories overall are burned during low-intensity vs. high-intensity work. It boils down to a large percentage of fat calories from a small overall number of calories, vs. a smaller percentage of fat calories from a much larger overall number of calories burned.
Let’s use an example. A person weighing 145 lbs. walks on the treadmill for 30 minutes at 3.0 mph and burns 150 calories, 50% of which come from fat. The next day, the same person runs on the treadmill at 6.5 mph and burns 330 calories, 30% of which come from fat. While the first workout burns a greater percentage of the overall calories from fat, it actually burned only 75 fat calories. The second exercise bout burned a smaller percentage of its overall calories from fat, but actually burned 100 fat calories overall, 25 more fat calories than the lower-intensity effort.
At this point, one might suggest that 25 calories is not a big difference, and that is a fair observation. But what occurs over the following hours is very different — and very important. After vigorous, higher-intensity exercise, the body’s metabolic rate remains elevated for a much longer period of time. More calories are burned during recovery and into rest after a higher-intensity exercise session due to something called excess post-exercise oxygen consumption (EPOC). During EPOC, extra oxygen is needed to re-synthesize energy stores in the body’s cells, clear metabolic by-products from the blood, reduce the elevated body temperature, reduce the elevated hormone levels, restore breathing to resting levels, and contribute to cellular repair. All of these additional tasks require energy (calories) and create an afterburn that may extend for hours.
And there is more. Higher-intensity exercise recruits, and develops, more fast-twitching muscle fibers than low-intensity exercise does. The more muscle you have, the more calories you continue to burn — at rest and throughout the rest of the day — as these muscle fibers require energy (calories) simply to sustain themselves.
Long-term, the benefits of higher-intensity exercise are irrefutable. In sum, physical readiness is the most important variable in determining
an individual’s starting point when beginning a new exercise regimen, and higher-intensity exercise is most prudently performed once a baseline level of fitness has been established. To be sure, there is nothing wrong with low-intensity exercise. However, fitness level permitting, fitness goals such as improved body composition, muscular fitness, stamina, and cardiovascular function can be more effectively realized by committing to exercise outside of a strictly “conversational” aerobic intensity. At a minimum, the pervasive low-intensity
fat-burning myth ought to be reconsidered.
