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Fuel Substrate Utilization For Losing Weight While Pregnancy

The three energy systems illustrate the mechanisms by which energy is provided at different intensities and duration of exercise. Yet the picture is incomplete without seeing how fuel substrate not only supplies energy for activity, but also becomes the limiting factor for exercise duration and intensity. Understanding the importance of how food contributes to sustaining power output and delaying fatigue is essential for making appropriate dietary recommendations for athletes.

Think of a dimmer switch as an analogy for fuel substrate utilization. At low-intensity exercise, when there is sufficient oxygen, mitochondrial respiration predominates. Fat (fatty acids) is the primary energy source for ATP production, though some amount of carbohydrate is required to regenerate intermediates of the Krebs cycle. As exercise intensity increases, the rate of demand for ATP increases beyond what can be supplied by fat. This is primarily due to limited oxygen availability and blood flow, as well as other negative feedback mechanisms produced from metabolic intermediates that inhibit the enzymes involved in mitochondrial respiration. Additionally, fat is a dense molecule and must undergo a time-consuming process known as beta-oxidation before it can be converted into acetyl-CoA and enter into Krebs cycle. Because glucose (or glycogen) undergoes far fewer steps before entry into the Krebs cycle, it can be metabolized aerobically to yield ATP much more quickly than fat. If exercise intensity is to increase further, rate of ATP rephosphorylation must also increase. Absolute fat oxidation will continue to increase up to a point; however, its relative contribution will decrease. Therefore, other metabolic pathways (fast glycolysis and PCr) will have to contribute to meet ATP demand. Thus, it is the energy demand of the working cells that are required to maintain or increase a given power output or rate of muscle contraction that determines which energy system is utilized.

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Importantly, during high-intensity aerobic and anaerobic exercise, carbohydrate is the main source of fuel at higher intensities. This “fuel” will ultimately be used for ATP generation, which is directly involved in muscle contraction. The faster ATP can be generated the faster the muscle will contract. Fat will still supply some energy for exercise but the amount will be much less. Fat will also be used for much of the energy needed by inactive muscles and not the organs (heart, kidneys, stomach, etc.). So, while all three systems are active simultaneously, their relative contribution will vary according to exercise intensity and metabolic involvement in the exercise task.

Knowing that intensity determines which energy system and, consequently, which fuel source (fat, carbohydrate, or both) sustains activity, it is essential to be able to measure exercise intensity in order to provide appropriate nutrition recommendations. Heart rate can be used to measure exercise intensity, though the most precise measurement is volume of oxygen consumption (VO2). VO2 is the volume of oxygen consumed per minute per unit of body mass (mL/min/kg), and measures the metabolic demand of exercise. One’s maximal VO2 or VO2 max, is a measure of one’s cardiorespiratory fitness, or, generally speaking, is used as a measure of one’s aerobic capacity. VO2 max is the highest intensity of exercise that can be sustained aerobically. VO2 max can be measured using indirect calorimetry methods such as an open circuit spirometer or “metabolic cart,” as described in Chapter 2.

Aerobic exercise intensity can be expressed as a percentage relative to one’s VO2 max. For example, “low exercise intensity” relative to one’s maximal aerobic capacity corresponds to approximately 30 percent or less of VO2 max. At this intensity, individuals are utilizing mostly fat to meet energy demands. At moderate-intensity exercise, the relative contribution of fat and carbohydrate used to meet ATP demands are roughly equal (about 50 to 50). At vigorous or high exercise intensity such as 70 percent of VO2 max or greater, carbohydrate is the greatest contributor of energy used to meet ATP demands. See Figure 3.4 illustrating this concept.

At low exercise intensity as defined by percentage of VO2 max, fat predominates as an energy source. At moderate intensity exercise relative contributions of fat and carbohydrate are equal. At high intensity exercise carbohydrate contributes relatively more energy.

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