There are a number of ways ATP (the currency used to transfer energy within the body) can be made available when demand exists. The vast majority that will be hydrolyzed to drive energy-requiring reactions is formed by virtue of oxidative phosphorylation, a process that involves harnessing energy generated by electron movement. The source of these electrons is the food we ingest. Oxidative phosphorylation relies predominantly on sugar and fat as the substrates from which electrons are stripped.
A number of factors influence which fuel feeds the oxidative metabolic machinery. Foremost is how rapidly energy must be transferred. If transfer rate is low, fat can be used almost exclusively. Once it is accelerated, easier to-access sugar must be called into play. Fuel availability is also critical. Even a lean person has plenty of potential energy stored in adipose tissue (the main storage form of fat), but glycogen (sugar) stores are more limited. In fact, increasing expenditure for prolonged periods (exercising, for example) or restricting carbohydrate intake can readily deplete these reserves. Taking these aspects of metabolism into account, its easy to see why exercisers often employ strategies designed to maximize the proportionate contribution of fat. This includes limiting the sugar available to fuel the activity (dramatically restricting carbohydrate intake and/or exercising on an empty stomach when reserves are presumed to be low). In addition, limiting the rate at which energy transfer is required is also suggested. In this case, a challenging pace is eschewed for a comfortable one because fat use simply cant keep pace when a rapid rate of turnover exists. Unfortunately, these efforts ultimately undermine fat loss.
To understand why maximal fat use is contrary to effective fat loss, some number crunching is in order. As we rebuild ATP aerobically, oxygen is needed to dispose of electrons once their potential energy has been utilized. Consequently, measurement of pulmonary oxygen uptake provides a barometer by which aerobic energy turnover can be quantified. In addition, carbon dioxide is formed when electrons and their accompanying hydrogens are stripped from fuels. Because the ratio of carbon dioxide produced to oxygen consumed (known as the respiratory exchange ratio or RER) varies depending on the fuel being oxidized, the relative contributions of sugar and fat can be determined by analyzing the composition of expired air.
At an RER of .7, all of the ATP being reformed is derived from fat. Because accessing fat is a slow process, this is only the case when energy demand rate is extremely low. As a result, from a relative standpoint, fat utilization (in relation to sugar use) is greatest when we are least active (during sleep, for example). When the RER is 1.0, on the other hand, carbohydrate is the sole fuel source. And when the RER surpasses 1.0, non metabolic carbon dioxide is present, a sign that aerobic metabolism is not keeping pace and anaerobic transfer (incomplete glycogen breakdown accompanied by lactic acid accumulation) is present.
The fat-burning misconception is based on the fact that relative fat use decreases as exercise intensity increases. No one would advise sleeping as the best way to lose fat, but many believe restricting aerobic intensity is the ideal approach. But this reasoning fails if you consider the numbers. Imagine a 160-lb person walking on a treadmill for 30 minutes at 3.5 miles per-hour, with the grade at five percent. If this represented a relatively easy effort, its safe to assume this individual would be operating at a moderate RER, lets say .85. That means 50 percent of the energy requirement would be satisfied by fat breakdown. But how much actual fat does that represent?
The American College of Sports Medicine prediction equation indicates the aforementioned challenge necessitates an energy outlay of 225 kilocalories. If 50 percent of that were satisfied by fat breakdown, 112 kilocalories would be provided by fat. That means 12 grams of fat would have to be used (fat oxidation yields nine kilocalories per gram).
Compare this endeavor with one done at a more challenging pace (and, consequently, a higher RER). Assume the same exerciser jogged at five miles-per-hour, a degree of effort associated with an RER of .9. In this case, relative fat use would be reduced, as only 33 percent of the energy would be supplied by the slow-to-access option. But the overall energy need would be much greater. In fact, maintaining that pace for 30 minutes would cost 330 kilocalories, considerably more than the comfortable walk. Bottom line: Even though relative fat use would be lessened, the same amount of actual fat would be burned.
You might be wondering why an exerciser would jog five miles-per-hour to use the same amount of fat they could during a comfortable walk. To justify this approach, consider the big picture. Fat stores decrease when we use more energy than we ingest. Using 330 calories is better than expending 225, even if the extra amount is not satisfied directly by fat. Somewhere along the line, if a negative energy balance is present, fat will be reduced. Using as much energy as possible as you exercise facilitates this condition.
Fred DiMenna, a Certified Strength and Conditioning Specialist and Lifestyle and Weight Management Consultant is a two-time Natural Mr. United States and a WNBF drug-free professional bodybuilder. Visit him at www.freddimenna.com or email: firstname.lastname@example.org.