Cardio Exercise Intensity & How It Affects Weight Loss

cardio exercise

To Be Intense or Not To Be?

It's around that three-month mark since you made your New Year's resolution to lose some weight - “for real” this time. Your gym membership includes the big package (unlimited access, personal trainer follow up, etc.).

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Now, the big question is: what have you done to lose weight? High intensity or moderate intensity cardio?

Arguments on both sides are numerous but let's separate facts from fiction. What difference does intensity make in using carbohydrates or fat as fuel when you train? Experts on low-to-moderate intensity aerobics (cardio) for weight loss say that this kind of training will use fat over carbohydrates for energy/fuel. Results? Low to moderate intensity cardio will increase your weight loss. On the flip side, those who support high-intensity cardio argue that increasing intensity results in a higher level of energy expenditure over the same time, causing greater overall weight loss. Who's right and who's wrong?

High vs Low Intensity

Let's take a man between 30 and 39 years old with a body weight of 85 kg (187 pounds) and an aerobic capacity (cardio or V02 max) of 40 ml per kg per minute. We'll compare two training methods with the same number of calories burned (500 kcal), but at two different intensities of work. The lowest intensity will be set at 65% of the maximal heart rate (the zone generally prescribed for “optimal” fat loss) and the second the highest intensity will be set at 85% of the maximal heart rate (the intensity recommended to improve your aerobic capacity and that relies mostly on carbohydrates as fuel).

Moderate Intensity (65% Max)

To burn 500kcal, our participant needs to exercise for about 55 minutes at 65% of his maximal heart rate. By measuring the amount of oxygen consumed and the amount of carbon dioxide (CO2) produced with a gas analyzer, we can accurately determine the fat and carbohydrate contributions to the energy burned. For this task, the contribution of carbohydrates is about 63% and 37% for fats. We know that 1 g of fat will provide roughly 9kcal. Therefore, our participant will use - in 55 minutes - around 79g of carbohydrates and 20g of fat.

Now, what if we burned the same number of calories but at a higher level of intensity?

High Intensity (85% Max)

Of course the activity will take less time; it will require around 42 min to burn 500 kcal. As we increase the intensity, the body turns toward carbohydrates (83%) for fuel rather than fat (17%). This gives us a total of 104 g of carbohydrates used and about 9g of fat.

If we only look at what happens during the exercise, training at moderate intensity will allow a greater use of fats (about 11 g more than at high intensity) but will increase training time by approximately 13 minutes (for 500 kcal). Is this worth it? Before making your decision, you might consider this: If you lower your fat intake by 11g, you achieve the same effect of your body composition (reducing fat intake by 11g or increasing fat use by 11g is the same). What does 11g of fat represent? The next figure will give you some examples.

Foods Containing 11 g Of Fat

  • Home made muffin with 2% milk (5.1 cm x 7 cm in diameter)
  • 100 ml of ice cream
  • 85 g of protein bar
  • 30 g of cheese

Moderate Intensity Training (65% Max)

More fat used for the same calorie amount burned Absolute amount of fat used is insignificant
More fat used for the same amount of time It’s easy to cancel the effect of training with food intake
Training more accessible and less demanding Training benefits aren’t significant
  For the same amount of time, total energy

High Intensity Training (85% Max)

More energy burned for the same training duration Harder to perform
Greater benefits from training Lesser amount of fat used during training

Length of Workout

With the same example, we'll compare the two levels of intensity but this time for the same amount of time. Let's take a 30 - minute aerobic workout: with moderate intensity training, our participant burns about 275 kcal and uses 43g of carbohydrates and 11g of fat. On the other hand, the high intensity training costs about 365 kcal and requires 76g of carbohydrate and 7g of fat. We have a 4g advantage for the moderate intensity training...but a deficit of 90 kcal.

The Pros And Cons

Evidence shows that moderate-intensity training burns more fat than higher intensity training. But the absolute amount of fat is trivial (11g or 4g) and cannot account for significant weight loss, especially when the fat loss is easily replaced by a small fat intake. We also have to consider that the most important aspect of weight loss is still and always will be a negative energy balance (more calories out than in) and not the amount of fat used during exercise. As a results, higher intensity training will be more efficient at creating an energy deficit for the same duration, and could possibly produce a greater overall weight loss.

Unfortunately, training at higher intensity is not without drawbacks. It's more demanding and requires better preparation (medical check up, training periodization, etc.). Also, higher intensity training might fatigue you, causing you to rest more during the day and, therefore, lower your daily energy expenditure. Your bed might start to sound more appealing than a walk outside after your training session at the gym.

Now, you simply need to choose at which intensity you wish to train. A quick suggestion: whatever you choose, just try to be more physically active and try to eat/drink just a bit less - this might give you better results!

About The Author

This article was originally excerpted from the spring 2008 issue of Beyond Fitness magazine. Beyond Fitness magazine is published 5 times a year and is distributed nationwide at most fitness clubs, gyms and at GNC stores.

Author Maxime Saint-Onge, M.Sc. Nutrition and PhD candidate Exercise Physiology is the President of Synemorphose Inc. . He is a regular contributor to Beyond Fitness magazine.

The author has given permission to building muscle 101 to print this article.

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