Exercise Intensity and Fat Burning
Larry Birnbaum, PhD, FASEP, EPC
Professor
John Dargan, MA, EPC
Assistant Professor
Department of Exercise Physiology
The College of St. Scholastica
Duluth, MN  55811

A common question asked by individuals seeking to lose weight is, “What is the ideal exercise intensity for burning fat and losing weight?”  Many fitness professionals would argue that high intensity exercise is best, and some research supports that position [1-3].  However, not all studies agree [4-7].  Exercise intensity is obviously very important, but there are considerations that are often not addressed (e.g., goals and capabilities of individual clients).  

The exercise prescription has to be based in part on individual goals, and not all clients may be capable of high intensity exercise, nor may it be safe.  It is certainly safer in the short and long run to work out at a lower intensity, which also burns proportionately more fat [4], but it does require more time to burn the same amount of calories required at a higher exercise intensity.  Given these concerns, the exercise physiologist should consider the following questions:

1. How much time do clients have to work out each day and each week?
2. What is their current physical condition? 
3. Can their musculoskeletal system handle high intensity exercise? 
4. How motivated are they to lose weight? 
5. What is their attitude toward high, moderate, and low intensity exercise? 

These are important questions that need to be considered before writing the exercise prescription.  For example, if a client has little time for exercise, then higher intensity may be preferable provided the client is healthy and motivated to stay with the program.  Obviously, the client cannot have any predispositions to musculoskeletal injuries. 

Of course, if the client is claiming that she has little time for exercise, her level of motivation may be an issue.  The exercise physiologist may need to emphasize the importance of making a total commitment, including a time commitment, to regular exercise as part of a weight loss/weight management program.  If a client is obese and exercises very little, if at all, low intensity exercise would be more appropriate for the initial phase.  Intensity can always be increased incrementally as improvements in physical conditioning and weight are realized.

It may be helpful to provide clients with a basic explanation of their energy systems and how these systems are affected by exercise intensity.  This may help their motivation and adherence to an exercise program.  There are two different energy producing (calorie burning) systems in the body - fat burning (lipolysis and beta oxidation) and carbohydrate burning (glycolysis).  Each of these systems works best at different exercise intensities so each of these systems should be trained at the appropriate intensity (specificity of training). 

Proper training requires that the fat burning systems be trained first.  This means that to develop a training program for clients who have been relatively inactive, it is very important to "build the base".  They need to develop their fat burning (oxidative) mechanisms and enzymes so fat will be the major fuel used.  After the base is built the fat burning systems operate more efficiently at higher exercise intensities.  Building the base is done by training at a low (50-60%) heart rate maximum (HRmax) while building the duration of each exercise session from 15-20 minutes up to as much as an hour.  This can probably be done over a period of 8-12 weeks if they are working out 3-4 times per week. 

Once the base is built, the intensity can be increased gradually up to 80% of HRmax with elite athletes being able to maintain exercise at 85 or 90% of the HRmax.   If a client has been endurance training for a significant period, she probably does not need to begin at such a low intensity.  However, she should still "build her base" by exercising at what seems to be a low intensity (maybe 70-75% of her HRmax) if she wants to increase her training level.

Exercise intensity is a critically important piece of an exercise program, but individual goals, abilities, and motivation must also be considered.  Exercise prescriptions are written for individuals.  Generalizations provide guidance, perhaps a starting point, but they have to be adjusted for each client.  Additionally, the individual plan has to be adjusted as the client improves.

References

1.  Puhl SM, Clark K.  (1992).  Exercise Physiology:  Exercise Intensity and Body Fat Loss.  Journal of Strength and Conditioning Research. 14:16-18.

2.  Phelain JF, Reinke E, Harris MA, Melby CL.  (1997).  Postexercise energy expenditure and substrate oxidation in young women resulting from exercise bouts of different intensity.  Journal of the American College of  Nutrition. 16:140-146.

3.  Slentz CA, Duscha BD, Johnson JL, Ketchum K, Aiken LB, Samsa GP, Houmard JA, Bales CW, Kraus WE.  (2004.)  Effects of the amount of exercise on body weight, body composition, and measures of central obesity:  STRRIDE – a randomized controlled study.  Archives of Internal Medicine. 164:31-39.

4.  Thompson DL, Townsend KM, Boughey R, Patterson K, Bassett DR.  (1998).  Substrate use during and following moderate- and low-intensity exercise:  implications for weight control.  European Journal of Applied Physiology and Occupational Physiology.  78:43-49.

5.  Romijn JA, Coyle EF, Sidossis LS, Gastaldelli A, Horowitz JF, Endert E, Wolfe RR.  (1993).  Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration.  American Journal of Physiology. Endocrinology and metabolism.  265:E380-391.

6.  Coyle EF.  (1995).  Substrate utilization during exercise in active people.  American Journal of Clinical Nutrition. 61(4 Suppl):  968S-979S.

7.  Achten J, Jeukendrup AE.  (2004).  Optimizing fat oxidation through exercise and diet.  Nutrition.  20:716-727.