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Energy for exercise (or any physical activity) is derived from two primary, dietary fuel sources: carbohydrates and fats. Protein can also be used, but generally, durations of less than 60 minutes involve little protein burning, although under extreme low carb conditions, this can increase. For each fuel, there are also two primary sources working muscle can call on: intramuscular (within the muscle itself) or peripheral (derived from the blood).

When peripheral sources are utilized, glucose (the major carb or sugar form in the body) comes either from the blood itself or the liver which releases glucose it has either stored or produced in order to maintain blood sugar levels and prevent hypoglycemia.

Peripheral fat stores are any fat depots in the body including subcutaneous (directly under the skin) and intra-abdominal (fat stored underneath the abdominal muscles) fat. The process of releasing fat from peripheral fat stores into the blood is known as lipolysis. Intramuscular stores of carbs are known as glycogen (a large complex of glucose attached to itself). Intramuscular stores of fat are in the form of triglyceride, similar to the storage form of fat in peripheral fat stores.

How A Muscle Determines It's Fuel
During exercise four fuel sources (two carb and two fat) are typically utilized. How a muscle determines its fuel mixture, both the form of the fuel and its source, is based on:

- The intensity and duration of the exercise.
- The training status of those specific exercising muscles
- The general diet (relating to specific percentages of fats and carbs).
The time interval since the last meal prior to exercise.
In addition to the exercise itself there is also a recovery period after exercise where energy expenditure is greater than an equivalent period of time following rest. Finally, the exercise itself induces hormonal changes that impact longer term fuel selection and utilization as well as fuel (that is food) intake.

There has been a lot of confusion about intensity and duration with regards to fuel selection in terms of optimal training protocols specifically related to bodyfat reduction. A better understanding of the overall process will clear the air so stick with me! While carbs and fats supply the major energy fuels for most activities, they have dramatically different properties in our bodies. One major difference is that fats require more oxygen to burn than carbs. This implies 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. When oxygen is not limiting (that is at low intensity), fats are the preferred fuel. Now, the situation is not as simple a low intensity burns fat / high intensity burns carbs.

Carbs are utilized to some extent at all intensity levels with a gradual and progressive absolute as well as percentage increase as intensity is increased. Fats on the other hand, provide the bulk at low intensity and gradually increase with intensity (in this case, absolute amount of fat burning increases, even while the percentage contribution to the total is going down), but then taper off at moderate to high intensity because of oxygen limitations (during this phase, the absolute as well as percentage declines). However, the details of fuel selection get more complicated because of differences in where these fuels come from.However, the details of fuel selection get more complicated because of differences in where these fuels come from. The source of these fuels also has implications especially in the post-exercise period which in turn plays a role in bodyfat management.

Understanding The VO2max

To understand fuel selection in more detail, the concept of VO2max will be introduced. VO2max simply refers to the maximum rate of oxygen you can utilize during exercise. As intensity increases so does your heart rate and so does oxygen utilization. Eventually, at some point if you increase intensity further, your heart rate will not increase further (you've reached your max heart rate) and oxygen utilization won't go up anymore; your oxygen usage has maxed out (overall intensity can still increase, but you are going anaerobic at that point). Intensity then can be indicated by some percentage of that maximum aerobic intensity. Therefore, we can quantify intensity with some percentage of VO2max.

At rest, you are about 5-8% of your VO2max. Resting VO2 is sometimes referred to as a MET or metabolic equivalent. Some exercise equipment can relate the workload as the number of METs. Low intensity is typically in the range of 25 - 40% VO2max. A 25% VO2max effort would be a comfortable walking pace. 40% would be in the range listed as the fat burning zone on some cardio equipment. 45 - 70% VO2max is squarely in the range of moderate intensity and is labeled as the aerobic training zone.

A trained individual should be able to maintain this level of intensity for 1.5 - 3 hours or more. Above 70-75% VO2max is in the range of high intensity. Duration is severely limited because at these levels, the reliance on carb burning to generate a high percentage of the total energy begins to produce lactic acid. At some point (that is, a specific percentage of your VO2max), the level of lactic acid hits a threshold and begins to skyrocket in the blood and brings about muscle failure.3,26 Once you are above your lactic acid threshold, duration can only be sustained for 3-10 minutes.3

Now that intensity levels are set, we can follow fuel selection across the intensity spectrum, since both fuel usage and source is determined by intensity.21,22 At low intensities, fat is primarily used (assuming no pre-exercise meal as been ingested). In addition, the source of the fat is circulating fat in the blood derived from release of peripheral fat stores. In other words, at low intensity, such as casual walking, calories are derived from fat coming from peripheral fat stores and supply the major fraction (85%) of total calories expended. The remainder of the calories is supplied by carb burning from uptake of sugar in the blood. However, at this level of intensity the overall level of caloric expenditure is low.

A resting value of caloric expenditure is on the order about 1 calorie per minute which scales roughly with lean body mass. The more muscle you have, the more calories you will burn at rest. Low intensity exercise like casual walking ups this value to the range of 3 - 6 calories per minute. This is the basis for recommendations for low, sustained intensity levels for fat burning, sometimes called the fat burning zone, since about 85% of the energy expended will be derived from fat burning. However, total calories burned and hence the quantity of fat actually burned will be quite low. In addition, these intensities are too low for a substantial aerobic training effect to occur which has long term impact on fuel utilization, during and after exercise.

As an example, even at the upper range of 6 calories per minute, one hour at this intensity yields 360 calories expended. Since only 85% of these calories are fat derived, we've burned 306 fat calories or 34 grams of fat (1 gram of fat is worth 9 calories). In terms of weight control, that's not too bad since 34 grams of fat can be a substantial portion of one's daily intake, but from the standpoint of fat reduction, over 11 hours of this exercise would be necessary to burn off one pound of fat (about 3500 calories per pound).

The general rule of 20 - 30 minutes for these types of activity to "turn on fat burning" comes from the idea that as fat is pulled out of the blood by the working muscles, the level in the blood will eventually begin to drop after, surprise, 20 - 30 minutes. As this happens, there is a hormonal response to restock fat levels in the blood. These hormones (both adrenaline and noradrenaline, coming from adrenal glands and nerves, respectively) stimulate fat cells to break down their fat stores and release them into the bloodstream, through the process of lipolysis.7 This is also where diet and food composition is important, since insulin released in response to dietary intake of carbohydrate opposes the action of these hormones. Insulin is antilipolytic, shuts down fat release, and promotes fat storage in peripheral fat cells.

Increasing Exercise Intensity: More Or Less Fat Being Burned?
As the intensity of the exercise is increased, the whole process ramps up for greater energy expenditure. Both fat and carb burning are increased. Although carbs assume an increasing percentage of the total, the absolute level of fat burning still continues to increase. The major difference with moderate versus low intensity is for the source of the fat derived energy. As intensity is increased, the working muscles require more oxygen and hence more blood flow, which explains the increase in heart rate (more blood volume and hence more oxygen is pumped per minute). In addition, since muscle is only about 25% efficient in terms of work, 75% of the calories expended are lost as heat. This heat must be dissipated which is why we sweat. In order to sweat, some blood flow must be directed to the surface skin, which is different from the peripheral fat stores just below the skin.

These two factors (more blood to the working muscle and increased blood flow to the skin for sweating) combine to limit the blood flow that can be allocated to peripheral fat stores for loading the blood with released fat through.23 Thus, at some point with increasing intensity, the release of fat into the blood stream from peripheral fat stores levels off.4,21,23 Peripheral fat stores have a maximum rate of fat release into the blood which has been determined to be the primary limitation for this fuel source.7,23 However, the rate of fat burning by the working muscle continues to increase.

To meet increased energy needs for moderate intensity (relative to low intensity), the muscle begins to breakdown its own fat stores, the intramuscular triglyceride. Since this store is more limited than the peripheral fat store, the muscle prefers to preserve this store until absolutely needed. In other words, at low intensities, working muscles prefer to utilize fats from peripheral fat stores which are the most flexible in terms of fat storage (because they can become huge). But peripheral fat stores are limited in the rate at which they can release fat and so fat stores within the working muscle itself are involved when the intensity increases enough to require more fuel than peripheral fat stores can provide. Intramuscular fat stores are limited by the absolute amount of fat available.

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. imilar 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. 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. 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.

t 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.

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. To achieve a faster response than changing the diet, specific glycogen depletion exercise routines can be employed. 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.

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. Carb intake during exercise has a similar effect.
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