Metabolic Misconceptions: Part 9: Intensity vs. Duration
by Fred Dimenna, CSCS

The most prevalent metabolic misconception is that you must use a lot of fat during exercise in order to reduce your adipose stores. At face value, this appears to be a logical assumption. But if you consider how energy balance ultimately dictates whether the fat you've stored is accessed, you'll understand why emphasizing fat use can be counterproductive to the fat-loss objective.

The fat-burning misconception is based on the fact that relative fat use decreases as exercise intensity increases. It is simply not feasible to rely on fat as fuel when energy must be transferred rapidly. Consequently, the other predominant fuel our bodies store and use (glycogen) must be the main player when the going gets tough. But if you think that means fat can't be reduced as a result, guess again!

When you use more energy throughout the day than you take in, stored fuel must be "cashed in" to satisfy the deficit. If you maintain sufficient energy intake and proper nutritional balance, and include resistance training in your exercise regimen, chances are the use of protein (a structural constituent of the body, but also a potential fuel in the pinch) to fill this void will be minimal. That leaves fat and sugar (both glucose and glycogen) as the elements that will have to be used. Sufficient carbohydrate intake to support the sugar you burn ensures that, over the long haul, fat will be the "savings account" from which you make your "withdrawals." This will be the case, even though the exercise sessions that establish the deficit might involve relatively little actual fatty acid metabolism.

The true objective of exercisers looking to reduce fat should be to maximize energy use. The body expends energy simply to stay alive and, above and beyond that basal amount, all activities we perform cost an additional price. How much energy a physical endeavor requires depends on three factors how long and often we perform it, as well as the degree of effort associated with the task. The harder, longer and more often you are physically challenged, the more energy it will cost.

Endurance exercise is the best option for optimizing energy use. The best endurance exercises require using many of the body's muscles ideally, the larger ones and can be continued for an extended period of time. Typical examples of effective modalities include walking, jogging, stair climbing, cycling and exercising on an elliptical cross trainer. In theory, spending 3,500 calories more than you take in will cost you a pound of stored fat, assuming glycogen stores and body protein are unchanged. To put this into perspective, consider a 175-lb. Person walking 3.5 mph. In order to sustain this pace, this exerciser will have to expend approximately five calories per minute. That means if the energy they use throughout the rest of their day exactly matches the energy contained in the food they ingest, a pound of fat loss will require no less than 700 minutes of exercise! It is obvious that if they only perform this routine three times per week for 30 minutes, the actual fat cashed in will be negligible.

As this example shows, simply increasing your activity level without considering an appropriate intensity/duration/frequency mix won't ensure fat loss. When you begin to manipulate these variables, it's easy to see why many exercisers trying to lose fat fail in their attempt. It is simply a function of time: If you want to drop two pounds of fat per week (a realistic objective), you'd have to spend over three hours a day walking at the aforementioned pace. For most people, that ain't happenin!

Setting up a significant negative energy balance without spending all of your day exercising (or dramatically decreasing the amount of energy you ingest, which will circumvent your efforts in the long run) requires working at the highest intensity you can sustain. But identifying this pace can be challenging.

Many trainers espouse finding the right level by working within a specified zone, as indicated by heart rate. Unfortunately, this is another metabolic misconception. While it is true that heart rate can be a barometer of oxygen consumption (the actual determinant of energy expenditure during endurance exercise), simply establishing an elevated rate doesn't mean you're at any particular energy-using level. During resistance training, for example, heart rate elevation indicates a lack of blood returning to the heart and has little to do with oxygen uptake. And it's a sure bet if you get cut off on your way to the gym and narrowly avoid a serious car accident, your heart will be racing afterwards. Is this indicative of increased aerobic energy turnover? I think not!

Instead of worrying about how many times your heart is beating as you exercise (as long as it keeps pumping, you're good to go!), the key is to understand how you will feel as you transcend the spectrum from minimum (fast asleep) to maximum (working as hard as you can) energy outlay. Armed with this knowledge, identifying specific points along this continuum will allow you to find your ideal exercise pace.

Fred DiMenna, a Certified Strength and Conditioning Specialist and Lifestyle and Weight Management Consultant is a two-time Natural Mr. United States and a WNBF drug-free professional bodybuilder. Visit him at or email: