As anyone working in the fitness industry will tell you, one of the most common questions I get asked is… “What’s better for burning fat, low intensity or high intensity training?” The short answer is…high intensity. The long answer is much more complicated, and a simplified version follows below.
There are two main sources of calories that we burn during activity: carbohydrates and fats. (We can also break down our own muscle tissue to supply energy, but this situation does concern the vast majority of the population in meaningful way, and so will be left for another time.) Some of the carbohydrates we burn are floating around in our blood, some are stored in our liver, and some are stored in our muscles. The fats we burn are stored in, you guessed it, fat cells.
Every physical activity (from simply being awake to sprinting) will use a combination of these two main sources. The intensity of the activity will determine what percentage of each is utilized. During light exercise, about 30-50% of our total energy is derived from carbohydrate, while the other 50-70% comes from fats. As energy intensity increases, so does the need for carbohydrates to supply our energy systems. This is because they are much faster and easier for our bodies to turn from raw materials into the final product (energy with which to continue the activity and maintain our basic bodily functions).
These facts have become more commonly known over the years, which has had both a positive and negative impact on the public’s knowledge. On one hand, it is encouraging that even light exercise can provide benefit to those unable to sustain a higher effort level. On the other hand, it has fooled some into thinking that low intensity exercise is the only answer to fat loss, luring them away from the benefits of moderate or high intensity activities. Let’s take for example 30 minutes of walking vs. 30 minutes of jogging. Even though the percentage of calories derived from fats is lower with the jogging, the total number of calories burned is so much higher that the total calories from fats is still greater than the walking. I like to explain it like this…would you rather make 100% of the annual income of a fast food worker, or 10% of what Bill Gates takes in per year? Or, with more traditional numbers…
30 min walking:
240 total calories @ 41% fat burn = 96 cal from fat
30 min jogging:
450 total calories @ 24% fat burn = 108 cal from fat
Now, 12 calories over the course of 30 minutes may not seem like a big deal. And I’m sure you’re thinking that the extra effort isn’t worth the small payoff, but another commonly held truth in the fitness industry is that every little bit adds up. Also, take a serious look at the comparison of total number of calories burned between the two activities. That’s nothing to sneeze at. And if you’re following a calories in-calories out philosophy, that difference is enough to cover a decent sized snack.
Higher intensity exercise provides other benefits as well. For example, your resting energy expenditure (REE) can stay elevated for hours after an intense exercise session. Cycling at 70% of your maximum can raise your REE by 5-14% for up to 12 hours after you’ve towelled off. That means you’re burning more calories while you shower, as you drive home, while you’re preparing dinner for the kids, everything! The cardiovascular changes associated with a long term program of higher intensity exercise also increases the activity of muscle enzymes that encourage the burning of fat over glycogen (stored carbohydrate).
Of course, there are a number of factors to consider other than the pure numbers of calorie expenditure. For example, are you able to safely participate in moderate or high intensity exercise? For someone who has never been active, a jogging or high intensity resistance training program is obviously not advisable for safety reasons. Also important is…do you enjoy the proposed activity? Adherence and motivation are huge issues that are far too often ignored. It doesn’t matter how often someone tells you that swimming is a great workout. If you hate the water, what’s the likelihood that you’re showing up every morning with your goggles in hand? So find something that challenges you, that you can participate in with minimal risk, and that you look forward to. Too often people forget that you don’t need to be in a gym to exercise. Find a weekly pick-up basketball game, organize a walking group with like-minded friends or co-workers, or take some salsa dancing lessons. But remember, continue to push yourself. The fat burn zone on your heart rate chart is not doing you any favours.
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