Science Behind Interval Training III

[Sirtitan]

Your Maximum Fat Burning Zone

Maximum fat burning rate occurs during moderate intensity at about 60 - 65% VO2max which corresponds to about 75% max heart rate for most people (this assumes an aerobically trained individual).1 The absolute burn rate is size dependent, but will typically fall into the range of about 0.5 - 0.8 grams of fat per minute with about equal contributions from peripheral and intramuscular sources. Total caloric expenditure for moderate intensity exercise again is size dependent (larger people expend more calories because they are moving larger masses and working larger muscles) and falls in the range of about 8 - 15 calories per minutes with fat contributing on the order of about 50 - 70% of the total caloric expenditure. With longer durations, this amount tends to the higher value, mainly due to intramuscular glycogen depletion that occurs at this level of intensity. Most trained individuals can sustain this rate of expenditure for 1.5 - 3 hours, at least, but because of the level of carb burning at this intensity, the duration will be limited by total carb stores in the body.

As intensity is further increased (to 85% VO2max), oxygen supply begins to become limiting. This causes a further shift to greater carb burning with breakdown of the muscle's intramuscular store of carb in the form of glycogen1.21,22,23 Similar to fat burning, as intensity is increased, fuel source shifts to a greater reliance on intramuscular carb stores in the form of glycogen. If the level of intensity is held below the lactic acid threshold (which varies from the high 60's% of VO2max to the 80's%), the activity can be sustained for about 45 - 60 minutes until glycogen stores are exhausted necessitating a fall back to an intensity level where fat can supply the majority of energy needs.

If on the other hand, the intensity is above the lactic acid threshold, then in a matter of a few minutes, lactic acid levels will climb intolerably high in the blood and failure results.3 As will be seen in part II, interval training is designed to improve this situation.

An important aspect to this complex pattern of fuel usage is the effect of training on fuel selection. This is vital to understand because an understanding of training effects identifies the orientation of the overall system and permits exploitation of fuel selection for maximum desired results. Simply put, aerobic training serves to enhance greater energy generation from fat sources at all intensity levels.3,12,14 The rationale for this is simple, carbohydrate stores of energy are quite limiting and can be depleted during the course of a single exercise session of sufficient intensity and/or duration.

During a marathon race, 'hitting the wall' occurs when carb stores have been depleted and underscores the focus on carb loading regimes. Under conditions of glycogen depletion, the muscle must begin to breakdown protein since branched chain amino acids present in protein can substitute for carbs, in terms of supplying energy directly as well as substituting for carbs in the process of maintaining the system for aerobic energy generation (something fats cannot support). Alanine (another amino acid in protein) released from protein breakdown can also be converted by the liver into glucose further increasing carb supplies from the blood.

Training allows for a higher sustainable level of intensity to be performed by sparing carbs in working muscle (by reducing the rate of depletion) and generating a greater percentage of energy from fat derived sources. In other words, since at a given intensity level, a specific level of energy generation is needed, endurance training allows for higher energy outputs to come from fat burning and improves duration by sparing carbs. Alternatively, training will also result in a potentially higher intensity level for a specific time period.

Typically, an aerobic training effect increases VO2max by as much as 25% in as little as 3 - 4 months of consistent training 3 - 4 times per week in the range of 60 - 85% maximum heart rate for 30 - 45 minutes per session. Fuel usage at the same absolute workload pre and post training (the post training workload is a lower relative intensity because VO2max has increased as a result of training) differs so that a greater reliance on fat for energy occurs. In addition, the greater reliance on fat for energy is derived largely from the greater utilization of intramuscular fat stores rather than peripheral sources.11,14 While this may appear counterproductive for fat loss, part two will discuss the importance of this development with regards to post-exercise effects and interval training and the relationship to bodyfat management.

What Effect Does Your Diet And Food Intake Have On All This?

Finally, diet and food intake need to be addressed in the context of fuel selection. Diet refers to the macronutrient composition that occurs in the range of two weeks prior to the exercise period. Unfortunately, many dietary studies typically involve a short adaptation period of as little as three days, although prior work suggests as many as 10 - 14 days are needed for complete adaptation to changes in macronutrient composition to fully manifest.18,19

Specifically, the macronutrient composition that matters is the amount of fat and carbs in the diet. Simply put, the less carbs ingested over time, the greater the reliance on fat burning.6 To achieve a faster response than changing the diet, specific glycogen depletion exercise routines can be employed.24,25,27 A sustained, reduced carb intake leads to a reduction in carb utilization at comparable work intensities. After four weeks of complete carb elimination, moderate intensity exercise can be performed with no reduction in endurance capacity, but two-thirds reduction in carb burning with a corresponding increase in fat burning.18

In other words, the less carbs you eat, the less your body will try to burn carbs while you are exercising. This means that you will naturally be burning more fat!

Food intake in the immediate pre-exercise period also affects fuel usage. Carb intake prior to exercise will result in release of insulin which retards lipolysis and fat utilization during subsequent exercise. Insulin in general promotes carb utilization throughout the body including working muscle and limits peripheral fat stores from releasing fats. Exactly how long after eating the system takes to return to baseline (defined as an overnight fast) depends on the specific meal. Studies with mixed meals suggest that effects can persist for 4 - 6 hours.8,15 Carb intake during exercise has a similar effect.5,9,10,20

Conclusion And Summary Of Points


Part I has served to introduce the basics of fuel selection and utilization during exercise along with the effects of training. Here are the main points:


There is a lot of confusion about how to really burn fat.
During exercise, your body will either burn fat (from your ugly fat stores or from inside the muscle), carbs or in some cases, protein.
Your body will decide which type to burn based on the intensity and duration of exercising, your type of diet and when you last ate, and how advanced of a trainee you are.
By following an exercise routine, you will burn more fat and calories even while you are at rest.
Fats require more oxygen to burn than carbs. This means that as exercise intensity is increased, there is a natural shift to burning more carbs because less oxygen is needed to extract the needed level of energy output to support that intensity.
The more instense the exercise is, the more carbs you will burn, in most cases.
Maximum fat burning rate occurs during moderate intensity at about 60 - 65% VO2max which corresponds to about 75% max heart rate for most people (this assumes an aerobically trained individual).
The less carbs ingested over time, the greater your body's reliance on fat burning.
Carb intake prior to exercise will result in release of insulin which stops or slows down the fat burning process.
However, the exercise period itself is only part of the story, the post exercise period also has an effect that is substantial. Understanding its role and its relation to intensity levels will serve to introduce the role of interval training and its application to body fat management in part II.