Fuel use during exercise: myths and misconceptions

Blog Post

Stephen W. Farrell, PhD, FACSM
The Cooper Team
Fit Tips
June 2, 2016

In order for the human body to function, it needs to be continuously supplied with fuel for energy. Most of this energy comes from the breakdown of carbohydrates and fats within the cells. These two fuels are delivered to our cells via the bloodstream. It is important to note that our cells need fuel for energy not only during exercise, but during the other 23 hours of the day as well! Obviously, our muscles needs more fuel during exercise than they do at rest.

Most of the carbohydrate in our body is stored in muscle and liver cells as a substance called glycogen, while a very small amount of carbohydrate is present in the bloodstream as blood glucose. Most of the fat in our body is stored in adipose tissue, which is found under the skin (subcutaneous fat) and deep among the internal organs (visceral fat). A much smaller amount of fat is found between muscle fibers and in the bloodstream. These fats are known as muscle and blood triglycerides, respectively. Scientists have been able to measure carbohydrate and fat use by the body since the 1920’s.

This is done by collecting and analyzing expired air while using a machine called a metabolic cart. The cart can measure a number of things, including caloric expenditure and how much oxygen the body is using per minute. The amount, as well as the percentage of carbohydrate and fat used per minute, are some additional parameters that are measured with the metabolic cart. This is where it gets a little sticky and confusing for the average person. The amount and the percentage of carbohydrate/fat used by the body are two entirely different things. Because of this, there are several common myths and misconceptions regarding the use of these two fuels relative to exercise and energy.

Let’s address some of these myths:

  1. Vitamins give you energy. While some B-vitamins are very important during the process of breaking down glucose and fats for energy, the energy is coming from the glucose and fats, not from the vitamins. An easy way to understand this is that in order for something to provide energy to the body, it must contain calories. Vitamins don’t contain calories; therefore, by definition, they cannot provide energy.
  2. We burn most of our calories every day through exercise. First off, only about 20% of American adults meet the minimal public health guidelines for physical activity! Even among that minority, only about 10-30% of their total daily calorie burn is from physical activity. Most of the calories that we burn every day (60-75%) come from our resting metabolic rate. You may already know this, but resting metabolic rate is the same thing as resting caloric expenditure. The rest of the calories that we burn each day (~10%) come from the work of digestion. Now let’s save the biggest myth for last:
  3. Low intensity exercise is best for weight loss because you burn more fat than carbohydrate. There is absolutely nothing wrong with performing low intensity exercise. In fact, if you are a beginner exerciser and/or are obese, that’s a great place to start. However, before you buy into popular myth number 3, let’s talk about the difference between absolute (amount) versus relative (percentage). I’m going to use an analogy with apples, assuming that you like apples. Let’s say that I have two apples and I give you 50% of my apples. How many apples do you have? Just one, of course! Now let’s say that I have 10 apples and I give you only 30% of my apples. How many apples do you have?  Three! So….would you rather have 50% of 2 apples or 30% of 10 apples? If you like apples, you most certainly would rather have 30% of 10 apples than 50% of two apples. This same type of analogy relates very well to carbohydrate and fat use during exercise. When we do low intensity exercise, the body does tend to use a greater percentage of fat than carbohydrate. When the intensity is increased, we tend to use a greater percentage of carbohydrate than fat. However, this is very different than the actual amount of carbohydrate and fat that we are using. Take a look at the chart below. This is a comparison of calories burned per minute, as well as the percentage and amount of carbohydrate and fat used when jogging at a 10 minute per mile pace (lower intensity) versus a 7 minute per mile pace (higher intensity) for 30 minutes in a person weighing 150 lbs. It’s pretty easy to see that while jogging at a lower intensity pace will burn more fat from a percentage standpoint than jogging at a higher intensity pace, the total caloric expenditure and the amount of fat burned is greater at the higher intensity. At the end of the day (figuratively not literally), body weight hinges primarily on the total number of calories consumed versus the total number of calories expended.

A similar type of result was seen in a study over a quarter of a century ago. Ballor and colleagues (1992) used a sample of 27 obese and sedentary women. Half of the women performed low intensity exercise 3 days a week for 8 weeks, while the other half performed high intensity exercise 3 days a week for 8 weeks. Both groups exercised for a length of time until they burned a total of 270 calories per exercise session. While the low intensity group burned a higher percentage of fat than carbohydrate and the high intensity group burned a higher percentage of carbohydrate than fat, both groups lost exactly the same amount of body fat and body weight because their total caloric expenditure per exercise session was the same. So, if loss of body fat is your primary focus, you should be far less concerned with the percentage of fat and carbohydrate that your body is burning during exercise and should focus instead on total caloric expenditure. Needless to say, you should also take a close look at daily total calorie intake as well!  You need not be a health and fitness professional to take part in our courses, everyone is welcome!

Reference: Ballor, D.L., McCarthy, J.P., Wilterkink, E.J. (1990). Exercise intensity does not affect the composition of diet and exercise-induced body mass loss. American Journal of Clinical Nutrition, 51(2), 142-146.


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