By Len Kravitz, Ph.D.

Although there are countless strategies and approaches in weight management programs, the three overall objectives should include: 1) An aerobic and resistance exercise plan to increase caloric expenditure and maintain fat-free (muscle) mass, 2) a lifestyle/dietary approach emphasizing balanced nutrition with decreased caloric intake, and 3) a behavioral modification strategy that aids in the implementation of the exercise and lifestyle components. Perhaps the one topic (or obsession) with fitness industry professionals and personal trainers that will surely generate a discussion and debate in the exercise component of the weight control plan is ‘what is the best exercise fat burning zone?’ This article will attempt to bring clarity to where there is cloudiness, research where there is perception, and guidance where there is dissent on this contestable issue and its related matters.

It all Begins with Energy Balance Basics
A kilocalorie (expressed as calorie throughout the rest of the article) is a unit of energy, and since energy is neither created nor destroyed (First Law of Thermodynamics), the calories we eat will either be stored somewhere in the body or expended for fuel in metabolism (all reactions of the cell for life) for daily activities, occupational tasks and/or exercise. This basic caloric theory fundamentally specifies that if a person consumes more energy then what s/he is expending, it will lead to a positive energy balance and weight gain. When energy expenditure exceeds energy intake, a negative energy balance exists and weight loss will occur. However, due to individual differences in our body’s neurological, hormonal and metabolic regulatory systems, this caloric balance concept does not work precisely that same in all persons.

When a person is in a negative energy balance, the weight loss may come from three body sources: water, adipose tissue, and muscle tissue. Under most circumstances, body water will remain relatively normal as long as regular hydration is followed. Consequently, the goal of the weight loss plan is to lose fat while preserving muscle mass.

Is Low Intensity Exercise Better for Fat Burning?
We’ve all heard a number of ‘fitness claims’ that the best type of aerobic training to burn fat is lower intensity exercise, referred to as the ‘fat burning zone’. Thompson and colleagues (1998) have confirmed that at lower intensities (50% VO2max) there is a greater ‘percentage’ of energy from fat than at higher intensities (70% VO2max). However, at the higher training intensity the TOTAL energy expenditure will be greater, and a person will almost always burn the same amount (or more) fat calories as s/he would exercising at the lower intensities, providing the workouts are the same length in time. Another way of stating this is, the selective use of fat as fuel, such as in low intensity exercise, does not translate into greater fat loss. More importantly for weight loss plans, fitness professionals should focus on the exercise regime that yields the greater total volume of calories expended from the exercise bout. To further explain this association, I did a simple experiment where I had a 191 lb physically fit male student do treadmill exercise under two conditions for 30 minutes. Trial one was a leisurely pace at 55% of his heart rate maximum and condition two was at 85% of his heart rate maximum. Here are the results of this experiment.

%Heart Rate    Total Calories     Fat Calories    Carbohydrate Calories
55% 209.8 121.1 88.7
85% 457.1 191.3 265.8

At the higher intensity, the subject burned more total calories, more fat calories and more carbohydrate calories. Yet this student is a very fit individual. For those clients and students who are sedentary and/or perhaps at orthopedic, cardiac or health risk, high intensity exercise may be contraindicated. For their weight loss exercise plan, lower-intensity exercise, for a progressively longer duration would be recommended. In fact, since most people can’t do ‘high intensity’ exercise on a daily basis due to potential overtraining and over use concerns, perhaps the best strategy is to integrate and balance the long duration workouts with the high intensity workouts for optimal calorie (fat) burning.

Does Aerobic Exercise Actually Make You a Better ‘Fat Burner’?
Horowitz and Klein (2000) indicate that a number of physiological and metabolic adaptations occur with cardiovascular exercise that distinctively enhance fat metabolism, including the following:
1) An increased oxygen delivery (via blood flow) and extraction system (via capilarization) helps the cell oxidize (burn) fat more efficiently.
2) An enhanced sensitivity of muscle and fat cells to epinephrine leads to an improved release of fatty acids (disassembled triglycerides from their glycerol backbone structure) into the blood and within the muscle (where fat is in its triglyceride storage form).
3) An augmented circulatory blood flow system aids in the delivery of fatty acids to the muscle to be used as fuel.
4) An improvement in the specialized protein transporters that move the fatty acids into the muscle cell, thus making the fat more readily available for fuel.
5) An increase in the amount of fatty acids allowed to enter the muscle, which thus makes more fat available for fuel.
6) A meaningful increase in the number and size of the mitochondrion. The mitochondrion is the only place in a cell where fat is oxidized. It is the cell’s ‘fat burning furnace.’
7) Finally, an increase in the oxidative enzymes that ‘speed up’ the break down of fatty acids molecules to be used during aerobic exercise.
Thus, an important take home message to regularly tell all students and clients is that consistent, progressively challenging aerobic exercise will truly develop their body’s to be much better ‘fat burners’.

Where Does Resistance Training Factor Into this Fat Burning Debate?
Research by Bryner and colleagues (1999) has demonstrated that one of the most important benefits of resistance exercise in a weight loss program is the preservation of muscle mass, even on very low-calorie diets. In addition, Andrew Hill has shown that diet only programs can lower a person’s resting metabolic rate (RMR) by 20% (which may be approximately 300 less calories expended per day). Bryner’s research demonstrated that resistance training is one of the best protective interventions to maintain the RMR during a caloric restrictive weight loss program.

Fat Burning Summary Solution
It may be very gratifying for your clients to know that with consistent endurance exercise they truly will develop better ‘fat burning’ furnaces (mitochondria) in their bodies! As well, try to always focus workout designs on burning the MOST calories possible with the exercise plan––whether it be harder, longer or a combination of both types of aerobic workouts. Additionally, among the many benefits of resistance training is the fact that it helps to preserve the body’s muscle mass and resting metabolic rate in caloric restrictive states. Lastly, remember to exclaim to your students, “To burn more fat, burn more calories.

”How many Calories are Really in a Pound of Fat?
One pound of fat is 3, 500 calories. However, one gram of fat is equivalent to 9 calories. Therefore, multiplying 9 calories/gram x 454 grams = 4,086 calories. Why the difference? Fat stored in adipocytes (fat cells) contains minerals, water and small amounts of protein, reducing the caloric content of one pound of body fat to roughly 3,500 calories.

Why is Carbohydrate the Preferred Energy Fuel of Exercise?
From a caloric standpoint it seems that fat (at 9 calories/gram) should be a much better source of fuel for exercise than carbohydrate (at 4 calories/gram). However, carbohydrate is the most important fuel source for exercise. It is the only fuel source used proficiently in anaerobic and aerobic exercise. As well, there are two major reasons the body prefers carbohydrate to fat during endurance exercise. First and most prominently, the metabolic pathways of carbohydrate breakdown (glycolysis) are much more efficient than those for fat (mobilization, lipolysis, and beta oxidation). Second, more oxygen is required to oxidize (burn) fat. The energy yield of fat from one liter of oxygen is 4.69 calories as compared to the yield of 5.05 calories from carbohydrate. Thus carbohydrate is approximately a 7% more efficient fuel than fat.

Does the Exercise ‘After-burn’ Help Burn Fat?
The exercise after-burn, or EPOC (excess post-exercise oxygen consumption), is the number of calories expended above resting values after a workout. Although intensity dependent, both aerobic and resistance training programs may elicit an EPOC from 65-150 (primarily fat) calories post workout. Many fitness professionals suggest that since one pound of fat is equal to 3,500 calories, EPOC is an insignificant factor in the fat burning process. However, if someone exercises 5 days/week, over the course of the year EPOC would be calculated as follows:  5 workouts/week x 52 weeks x 100 EPOC calories/ workout totally 26,000 calories or 7 lbs of fat––that’s meaningful!

Bryner, R.W., Ullrich, I.H., Sauers, J., Donley, D., Hornsby, G., Kolar, M., and Yeater, R. (1990). Effects of resistance vs. aerobic traininig combined with an 800 calorie liquid diet on lean body mass and resting metabolic rate. Journal of the American College of Nutrition, 18: 115-121.Hill, A.J. (2004). Does dieting make you fat? British Journal of Nutrition. Suppl 1, S15-S18.Horowitz, J. and Klein, S. (2000). Lipid metabolism during endurance exercise. American Journal of Clinical Nutrition, 72: 558S-563S.Thompson D.L., Townsend K.M., Boughey R., Patterson K., and Bassett D.R. Jr. (1998). Substrate use during and following moderate- and low-intensity exercise: implications for weight control. European Journal of Applied Physiology and Occupational Physiology. Jun;78(1):43-49.