I have heard that if an exerciser does aerobic exercise in the morning after a fast, he or she will burn a greater percentage of calories from fat due to grenalin levels and low carb stores in blood. However, I have also heard it stated that an exerciser would burn more calories (a greater number of “fat” calories, if a lesser percentage) after eating and then performing aerobic exercise. Is there evidence to back this up?
Having spoken to several colleagues (nutritionists, doctors, exercise scientists, physical training instructors, etc) and after a thorough search of the research databases to which I have access, I have failed to find any reference to a grenalin and metabolism. So with this in mind, I will provide several points to cover the question/s.
- Exercising in the fasting state may lead to a metabolic acidosis caused by the accelerated requirement to use fat as an energy source. Simplistically, fat is broken down into an enzyme called Acetyl Co Enzyme A (Acetyl Co A). This enzyme enters the Kreb’s cycle, where it is further broken down and begins to “produce” energy. Oxaloacetic Acid is needed to allow Acetyl Co A to enter the Kreb’s Cycle. When there is insufficient glucose in the blood, the body begins to convert the Oxaloacetic Acid to glucose as glucose is the only fuel source the brain can use. Acetyl Co A builds up, and the liver starts to convert the Acetyl Co A to ketones. This in turn leads to a state of metabolic Acidosis.
- Adrenaline (epinephrine) is responsible for increasing fat metabolism AND glucose metabolism. Stimulation for release of epinephrine comes from the sympathetic nervous system’s (the switch on system) stimulation of the adrenal medulla during times of body stress including fright, exercise and hypoglycemia. The amount released depends on the type and intensity of the stressing stimuli.
- As previously mentioned, glucose is the only source of fuel the brain can use. A low blood glucose level can hence impact on higher order functions and impact on exercise technique/sports skills, decreasing performance and increasing the chance of injury.
- With a decreased glucose store, the body’s performance ability is decreased as fat oxidization is a slow and complex process; therefore, the total number of calories expended/performance will most likely be reduced.
- When considered in context: fat loss comes from the utilization of more calories (calories out) than calories consumed (calories in). [Note: It is important to remember that there is more to calories in and out than just numbers. Eating 1000 calories of steak is different to eating 1000 calories of apples. The thermogenic demand to break down the steak is vastly different, and there is also the overall nutritional content and hormonal responses to this content that must be considered.] The percentage of fat to glucose usage in this instance therefore becomes of little consequence (see "What is the Fat Burning Zone?" article under "related articles" at right), and having the energy to perform at a level that allows a high caloric requirement is more pertinent.
- In line with this, it is not just when food is eaten but what food is eaten as different fuel sources will have different effects on performance (e.g., high vs low GI/GL foods, proteins, etc).
Finally, setting a generic time to train (i.e., before or after meals) is very difficult due to the high number of influencing factors including:
- Individual preference (and the impact on training intensity and training value)
- Individuality (metabolic and hormonal profiles)
- Adaptability (the body’s circadian rhythms will adapt to the body’s requirements if the stimulus is standardized and repetitive)
The key (and often forgotten) factor must be how the client feels, as ultimately his or her motivation to train and continue to train will be the most influencing factor on any gains. If training before eating leads clients to feeling low on energy and lethargic, training may be better suited to take place after meals (usually 60 to 120 minutes after). Conversely, if clients feel ill training after a meal (even a very small one), then training may be better suited to take place prior to meals.
- Marieb, E.N., (2004). Human Anatomy and Fitness 6th Edition. Massachusetts, NW: Benjamin/Cummings.
- Mc Ardle, W.D., Katch, F.I. & Katch, V.I., (2006). Exercise Physiology. Energy, Nutrition and Human Performance 6th Edition. Malvern PA : Lea & Febiger.
- Sherwood, L., (2001) Human Physiology, from cells to systems, 4th Edition. Pacific Grove, Calif: Brooks/Cole
- Wilmore, J.H. & Costill, D.L., (2004) Physiology of Sport and Exercise 3rd Edition. Champaign,IL : Human Kinetics.