American Fitness Professionals & Associates

It has been well acknowledged for years that fat is an important fuel for muscle contraction. 1  Fat released from adipose tissue (fat cells) is the most studied of the triglycerides pools. Less studied but just as important to both the power output and endurance of a trained athlete is fat that is stored within the muscle tissue. Let me give you a brief overview of basic energy use in the body.

All foods that you eat are broken down in a series of physical and chemical actions to the final product of a molecule known as ATP (Adenosine triphosphate). When you first start to exercise the body primarily uses a form of energy stored within the muscles known as ATP-PC (CP= Creatine Phosphate).

This would be equivalent to sulfur on the tip of a match, it produces a lot of energy.4 After about 10 seconds the body starts to call upon it’s next available dominant fuel source which are carbohydrates stored within the muscle. This system is called the Lactic Acid system, and would be equivalent to the tip of the match burning long enough to light a candle or a stove. In the body theses intramuscular carbohydrate stores are dominate for about two minutes.

After about two minutes the body calls upon it longest burning fuel system, the Aerobic system. This would be comparable to the wood part of the match burning while the match lay in the ashtray. In the Aerobic system carbohydrates released into the blood from the liver (Glucose).

Fat that is being released from the fat cells and amino acids are used for fuel There are several factors that determine the absolute amount and type of fuel that the body uses.

The primary factors are duration and intensity. Factors such as physical conditioning (how fit you are for the activity you are doing). how many carbohydrates are stored in the muscle when you begin the exercise, and how many muscles are involved in the activity can have an influence also. It is well documented that after an overnight fast that fat provides the majority of the resting caloric needs in healthy people. 7,8   Less understood is the fact that the body does not use one fuel source exclusively but a mixture of fuels. Every calorie that we burn is a percentage of carbohydrates, fats, and amino acids.

Which fuel is dominate is determined by the same factors that determine absolute quantity. Just as there are carbohydrates stored within the muscle there is also fat stored there. We have already stated that when you first start to exercise the primary fuel after 10 seconds comes from carbohydrates that are stored within the muscle.At the same time that these carbohydrates are the primary source of fuel, fat maintains its role as the secondary source of fuel. 2,3,4   After you have been exercising beyond 2-3 minutes the body starts to burn a mixture of glycogen and glucose that is arriving to the muscle from the carbohydrates being broken down in the liver. Simultaneously the body starts to use a mixture of intramuscular fat and fat that is being carried in the blood.

The longer that you exercise in a continuous state the lower your intensity has to be to continue the exercise session, the more dominant fat becomes as a fuel. The longer you exercise and the more fat becomes available from outside of the muscle, mainly adipose stores. 5  A person who increases the size of their intramuscular stores would see an increase in muscle size and strength. What science has uncovered is a clear picture of how the body uses intramuscular fat stores and what muscle fibers should be used. 6   If you were to ask any professional or top level amateur bodybuilder how they peak for a contest you would probably hear one of two things:

Bodybuilders 1: Eats a fairly low carbohydrate diet, combined with a high repetition and rest between sets brief, while trying to work to muscular failure as much as possible.
Bodybuilder 2: Eats a diet high in carbohydrates/high calories, trains as heavy as possible keeping the repetitions between 6-10. While performing 1 to 2 hours of cardiovascular exercise per day. Without realizing it both bodybuilders are taking their bodies close to the same physiological state of utilizing more total fat. The difference in training that each bodybuilder choose will manifest themselves in the following ways:

Bodybuilder 1: Will tend to reach contest shape much harder (ripped)
Bodybuilder 2: Will tend to reach contest shape much fuller (more glycogen stored within the muscle).

Conclusion:

We know that the higher the higher the intensity the greater intramuscular stores are used. We can also determine what type of fuel source we want to work from for the type of training effects that are desired. Working to maximize the Lactic acid cycle between 45-120 seconds in conditioned athletes by using a repetition in the 5-6 second range for completion, then carrying out this timed repetition to between an 8-20 repetition set. This type of training will target primarily the type 2a muscle fibers (fast twitch red), that contain the highest concentration of intramuscular fat stores. 9  The greater physical condition an athlete has the longer work can be sustained within the lactic acid system (high intensity). There you have it, the skinny on intramuscular fat stores!

REFERENCES

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2. Drummond,G.I. and E.C. Black. Comparative physiology:fuel of muscle metabolism. Annu. Rev. Physiol. 22:169-190,1960
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4. Essen,B.L., Hagenfeldt,and L.Kaijoer. Utilization of blood borne and intramuscular substrateu Curing continuous and intermittent exercise in man. J.physiol, lond. 265:489506,1977
5. Martin, W.H. Effects of acute and chronic exercise on fat metabolismz Effect of training on Intramuscular Triglyceride use. Exerc. and Sports Science rev. 7:223-225 1996
6. Martin, W.H. Effect of acute and chronic exercise on fat metabolism. Exerc. and Spox,to Science rev. 7:203-227,1996
7. Kanalay,J.A., Jensen, M.D. Fatty acid kinetic responses to running above and below the lactic acid threshold. J.Appl. Physiol. 79:439-447, 1995
8. Klein, S., Wolf. R.R. Palmitate and Glycerol kinetics during brief starvation in norntal weight young adult and elderly Subjects. J. clin. invest. 78: 928-933, 1986
9. Romijn, J.A., Sidossis L.S. Regulation of endogenous fat and carbohydrate mete&bolism The relationship to exercise intensity and duration. Am. J. Physiol. 265(endocrinol.metab.28):E380E391,1993