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Contribution of carbohydrate and fat relative to exercise intensity

If adequate carbohydrate is unavailable, fat must be used, albeit at a much slower rate, to meet ATP demands. In a scenario such as this, athletes risk “hitting the wall” or “bonking.” This is the experience of extreme reduction in aerobic power output and inability to maintain a given pace. It is often experienced by endurance athletes at some point in their career and is a result of insufficient carbohydrate availability. Even in the presence of adequate oxygen and fat stores, when carbohydrate, or glycogen, stores are depleted, exercise intensity suffers dramatically.

It is important to remember when discussing carbohydrate and fat contribution at different percentages of exercise intensity, relative contribution is what is being referenced. For example, relative contribution of fat and carbohydrate at 65 percent of one’s VO2 max is approximately equal, about 50 percent. However, this is distinct from absolute contribution. At low intensity, fat predominates as an energy source, yet the absolute rate of oxidation is only moderate, approximately 26 pmol/kg/min (a unit of measurement for fat oxidation) (Melzer 2011). At 65 percent of one’s VO2 max, when relative contribution of fat is only 50 percent, the absolute rate of fat oxidation is maximal, ~40 pmol/kg/min. So, even though the percentage of contribution of fat as an energy source is higher at lower intensities, the absolute fat oxidation is actually less than that at moderate exercise intensities.

The Myth of the “Fat Burning Zone”

Understanding the difference between relative and absolute rates of oxidation demonstrates why the “fat burning zone” is misleading in terms of weight loss. Many cardiovascular exercise machines, such as treadmills, elliptical machines, and recumbent bicycles show a “fat burning” zone promoting greater fat burning capacity at lower intensities. While fat provides relatively greater energy contribution at lower intensities, absolute fat burning is greatest at moderate intensities. Further, at high intensities, fat contributes relatively less energy, but total caloric expenditure is greatest (which is probably more important when trying to lose weight). While this will be discussed further in Chapter 8, which examines weight management, for now keep in mind that exercising in the fat burning zone may not prove to be an effective weight loss strategy.

Intensity is not the only variable that modulates fuel substrate utilization during exercise. Training status, exercise duration, and composition of the diet also affect the relative and absolute contribution of carbohydrate and fat to energy metabolism (Gonzalez and Stevenson 2012). Looking at training status, it has been shown that endurance training increases whole body rates of fat oxidation and decreases the rate of carbohydrate oxidation at both relative and absolute exercise intensities (Yeo et al. 2011). Specifically, endurance training increases mitochondrial volume allowing for greater fat oxidation and decreases activation of carbohydrate metabolism, with the result of sparing glycogen (Yeo et al. 2011). Glycogen sparing has significant performance benefits and is often a goal of endurance athletes. Any amount of glycogen that can be spared, or reserved, prevents insufficient glycogen availability from becoming the limiting factor in intensity or duration or both. Remember, the body has almost limitless fat stores but only limited stores of glycogen. The effect of these glycogen-sparing adaptations is that athletes rely upon great fat oxidation at given exercise intensities, potentially allowing athletes to train harder or longer with greater carbohydrate availability. Training status will also increase glycogen capacity, as well as the body’s ability to utilize carbohydrate and lactate. This will be discussed in Chapter 6 in “Carbohydrate Loading.”

Recent research has focused on dietary manipulations to increase fat oxidation with the hopes of sparing muscle glycogen. In fact, fat oxidation can be altered through specific nutritional strategies. Research has shown that increased fat intake (as a percentage of normal daily kcal or absolute increases beyond normal diet) increases fat oxidation and downregulates carbohydrate metabolism (Spriet 2014). Additionally, decreased carbohydrate availability increases the rate of fat oxidation at any given exercise intensity (Spriet 2014). Decreased CHO availability can be the result of depleted glycogen stores from exhaustive exercise, or due to insufficient dietary intake of CHO (purposefully or accidentally). Multiple adaptations occur on a cellular level that explain this shift in fuel substrate utilization. Adaptations that increase utilization of fat during exercise include improved fat transport into the mitochondria and enhanced regulation of intramuscular triglyceride synthesis and breakdown, and upregulation of enzymes involved in fat metabolism (Spriet 2014).

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These cellular adaptations have prompted questions regarding optimal fuel sources for endurance exercise. There have been investigations to determine if athletes should be encouraged to have a higher fat lower carbohydrate intake to promote greater fat oxidation, spare glycogen, and ultimately improve performance. Before these findings can be translated to revised nutrition and dietary strategies for performance, more research is needed. While research supports that dietary and training manipulations can increase fat oxidation at given exercise intensity, there is insufficient support that there are performance benefits to these strategies. Athletes may be able to increase fat utilization to a greater extent at higher exercise intensities, but the evidence is lacking that this results in performance benefits (Hawley et al. 2011; Ormsbee, Bach, and Baur 2014; Yeo et al. 2011). Rather, research has consistently shown an ergo-genic effect when carbohydrate is consumed before and during exercise at given intensities and durations (Burke et al. 2011; Cermak and van Loon 2013; Lima-Silva et al. 2013). The benefits of carbohydrate intake before, during, and after exercise will be discussed further in Chapter 4 on Nutrient Recommendations.

Understanding the concept “fuel substrate utilization” at various exercise intensities illustrates the importance of providing proper nutrition strategies. The body has almost endless fat stores, even in very lean athletes. This reliance upon fat at low intensity is what allows endurance athletes such as long-distance runners and cyclists to exercise for hours. Yet, even these athletes rely upon carbohydrate because, in order to metabolize fat, carbohydrates are required. As intensity increases, so does reliance upon carbohydrate, yet the body has limited carbohydrate storage capacity. Endogenous stores of carbohydrate (blood glucose, muscle, and liver glycogen) can become depleted during long-duration exercise, and thus the body becomes reliant upon exogenous sources (= consumption of carbohydrate from food and beverages).

While inadequate nutrition becomes a limiting factor in exercise, appropriate nutrition practices will sustain activity and enhance performance. This will be discussed in further detail in “Nutrient Recommendations.” The concept of the dimmer switch is essential for providing the best nutrient recommendations for athletes. Knowing which fuel substrates will be called upon, and which energy systems predominate when, will be necessary for ensuring dietary intake of optimal food sources.

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