This is one of the most frequently asked questions I've been asked in my life as a fitness expert. So, here is the answer - hopefully it makes sense to you!
Your body’s first choice of fuel during exercise is carbohydrates. Carbohydrates are stored in the body as glycogen, both in the liver and muscle. The liver’s job is to break down glycogen into glucose to maintain suitable levels of glucose in the blood. The muscles on the other hand, tap directly into their glycogen stores for energy production.
Depending on the duration and intensity of the exercise, your body will rely on more complex carbohydrates (stored as glycogen in the muscles and liver) and fatty acids for its fuel.
What happens is that as the intensity of exercise increases, there is more reliance on carbohydrates for fuel. Typically, intensities greater than 70% of VO2 max (or maximum heart rate for our purposes) will have carbohydrates as the dominant fuel source.
NOW LISTEN UP...VERY CAREFULLY!
However, this doesn’t mean that working at high intensities doesn’t burn fat. Working at higher intensities allows more total calories to be burned and that’s what really matters if you’re looking to lose weight.
There is a phenomenon known as EPOC (or excess post-exercise oxygen consumption). What this refers to is a process whereby the body expends calories (and many more that in your workout in most cases) after the workout as your body is re-establishing homeostasis - it's point of balance.
That's why I'm a huge fan of intense workouts and interval cardio training sessions. They are by far the best way to burn calories and thus fat - not during the workout (as you'll see on many cardio machine screens) but after the workout. Some studies have shown that the body's metabolic rate can stayed elevated for up to 16 hours after just 20 minutes of interval running!
The point is that your body chooses different fuels during different situations. Because fats yield more calories and are slower burning as they are metabolized, they tend to be used over longer durations when compared to carbohydrates. However, the body wants to conserve its blood glucose and glycogen stores.
As you become fitter your body will become more efficient at using fats for fuel while sparing your vital carbohydrate reserves. Someone who is totally unfit will obviously fatigue sooner, one of the reasons being that their carbohydrate stores are more quickly dissolved and utilized. Fitter individuals can last much longer because they have developed the ability to use the slow burning fats as fuel, and therefore conserve their energy better.
Carbohydrates that are used during exercise come from both glycogen stores in muscle and from blood glucose. Within carbohydrate metabolism blood glucose and glycogen are used to varying degrees depending on the nature of the activity. However, the body’s first order of business is in maintaining healthy blood glucose levels.
Blood glucose is the main fuel source for the brain, so I’m sure you can appreciate why it’s pretty important. As intensity increases, muscle glycogen is increasingly used. Blood glucose use also increases slightly. However, to offset its usage and to maintain stable blood sugar levels (for the vital organs), the liver breaks down its glycogen stores into glucose for the blood.
So the point is this....intensity is king! Don't pay attention to those "cardio" and "fat burning" zone graphs on all the cardio machines - they do nothing but confuse you.
Yuri Elkaim is the owner and founder of Total Wellness Consulting, a leading health, fitness, and wellness company offering health-conscious individuals the most innovative information, programs, and technologies to help reach their goals of physical and mental well being. He is a Certified Kinesiologist, Registered Holistic Nutritionist, former Professional Soccer Player, Head Strength & Conditioning Coach for the University of Toronto men’s soccer program, and regarded by many as Canada’s leading fitness expert. You can see his information at www.yurielkaim.com