Fuel Selection and Metabolic Function

The body primarily relies on carbohydrate and fat oxidation as the main sources of ATP
for skeletal muscle contraction. The utilization of either carbohydrate or fat resources will
depend on the intensity and duration of the exercise However, contend that the ability of the
body to switch from glucose metabolism to fatty acid metabolism (metabolic flexibility) is
essential in determining exercise tolerance. The authors further note that the inability of the body
to switch from glucose to fatty acids can lead to metabolic disorders such as cardiometabolic
syndrome, diabetes, and insulin resistance, while metabolic flexibility is associated with
enhanced fatty acid oxidation, which preserves existing glucose reserves In this paper, the writer
will reflect on the best diet for increasing exercise tolerance and potential issues with the
selection of the appropriate fuels.
Metabolic Inflexibility
Metabolic inflexibility occurs in individuals whose beta-oxidation is impaired. These
disorders associated with beta-oxidation impairment also contribute to other problems such as
cardiomyopathy, hepatic steatosis. However, Li et al., (2015) contend that the mechanism
through which skeletal muscles switch from one fuel to another, and its impact on exercise is not
well known. In their study involving laboratory mice, found that Acyl-CoA Synthetase Long-
Chain Family Member 1 (ACSL1) is critical for fat oxidation in muscles. In the absence of the
factor, the body increasingly relies on glucose for APT production, which can lead to
hypoglycemia. The inability of skeletal muscles to switch from glucose to fatty acids leads to
suboptimal exercise performance, and low endurance (Stander et al., 2021).As such, any
mutation that affects the availability of ACSL1 can limit metabolic flexibility of an individual,
and therefore reduce their ability to endure intense exercise.

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Metabolic inflexibility may also occur due to other factors, other than ACSL1. During
exercise, the body will first oxidate glucose that is available in the liver or muscles. As the
intensity and duration of exercise increase, the body will draw on fat reserves. Cox and others
(2016) contend that the oxidation levels of either carbohydrates or fats will depend on serum
levels of free fatty acids (FFAs). They argue that a high level of FFAs can lead to a reduction in
oxidation levels of carbohydrates, as the free fatty acids suppress pyruvate dehydrogenase
complex activation. In their study of eight cyclists, found that a reduction of fat oxidation in
cyclists was due to the downregulation of carnitine palmitoyltransferase I. The scholars
concluded that it was likely that a decline in intracellular pH and free carnitine availability could
explain the findings. As such, it is likely that a change in pH and carnitine available could lead to
metabolic inflexibility since the body is unable to oxidate fats (Cox, 2016). It is likely that
similar mechanisms could explain the impairment of fat oxidation in people who are obese or
those who are diabetic.
A mismatch between oxidation and availability of lipids might induce lipotoxicity due to
ectopic fat deposition. Lipotoxicity occurs when lipid metabolites interfere with insulin signaling
pathways, leading to insulin resistance in such a context, high levels of oxidation may prevent
ectopic fat deposition and thus reduce the risk of lipotoxicity, which would prevent insulin
resistance. However, in people with type two diabetes (TD2), high levels of circulating non-
esterified fatty acids (NEFA) impede fat oxidation, which can lead to lipotoxicity. According to
Loon and others (2015), the presence of high levels of serum NEFA impairs metabolic flexibility
and thus can lead to lipotoxicity. The mechanisms through which these fatty metabolites
contribute to the condition are not unknown. However, there is evidence that skeletal
mechanisms are implicated in the problem.

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Fuel Selection and Exercise Tolerance
Selection of the appropriate fuel can be difficult. In their study, suggest that nutritional
ketones could increase resilience in individuals. The body produces ketones in response to an
energy deficit or calorie deprivation. Such conditions occur when an individual either has a
clinical manifestation of ill-health or when an individual is undergoing prolonged starvation or
exercise (Cox et al., 2016). Starvation ketones differ from nutritional ketones in that nutritional
ketones are produced when the body oxides fats instead of glucose. All tissues in the body can
easily oxidize ketones, with the exception of the liver, since it lacks succinyl-CoA: 3-ketoacid
CoA transferase the latter enzyme is responsible for the oxidative disposal of ketones (Li et al.,
2015).Athletes or other individuals engaging in high-intensity exercise can benefit from a direct
intake of d-β- hydroxybutyrate. However, direct intake of the monoester is not recommended.
Rather a ketonic diet or ketone esters can raise serum ketone
The most appropriate diet for exercise remains nutritional ketones. There are several
reasons for the conclusion. First nutritional ketones are easily oxidated by all tissues, including
skeletal muscles, with the exception of the liver Secondly, ketones yield more ATP energy per
mol of oxygen relative to fatty acids and glucose. Thirdly, ketones produce less reactive oxygen
species compared to other energy sources. Furthermore, ketones regulate their own production
by directing fuel oxidation, inhibiting spare glycogen oxidation, and lipolysis (Loon et al.,
2015).The role of ketones in starvation makes it an important fuel for the high duration and
intensity of exercise. d-β- hydroxybutyrate, a ketone body, plays a key role in starvation by
suppressing oxidative stress, increasing histone acetylation, and diminishing inflammation
response. According to Cox and others (2016) the ketone body also plays a key role in

FUEL AND METABOLISM 5
diminishing sympathetic nervous system activity. It also diminishes total energy expenditure by
blocking short-chain fatty acid signaling
Stander et al., (2021, pp.72) contend that nutritional ketosis can play the same role as
glucose in ATP production. The reason is that ketosis plays a key role in survival mechanisms,
where maintenance of normal homeostasis is essential. When competition for energy sources is
high, ketones can fulfil the energy demands of the body. Unlike other energy sources, ketones do
not lead to the production of reactive metabolites such as those that are produced by fatty acid
oxidation. In their findings, found that nutritional ketones were associated with higher levels of
skeletal muscle oxidation levels (Stander et al., 2021). On the other hand, intake of CHO did not
affect the oxidation levels of skeletal muscles. It is likely that nutritional ketones inhibited the
oxidation of fatty acids, thus increasing skeletal muscles oxidation levels.
While nutritional ketones can improve performance, it is critical for athletes or other
individuals engaging in high duration and intensity exercise to take time to recover. In their
study, reported that metabolic recovery of marathon athletes occurs within 48 hours (Li et al.,
2015). The reason for the recovery is due to a reduction in substrate catabolism. According to the
authors, a reduction in energy requirement after exercise reduces the need for fuel substrate
catabolism, which triggers intracellular glycemic flux and glycogenesis. Glycogenesis, on the
other hand, triggers cellular repair and re-esterification. It should be noted that energy use during
high duration exercise begins with carbohydrate oxidation, followed by lipid oxidation, and then
ketone oxidation. In the absence of these fuel sources, the body will resort to amino acid
metabolism. Generally, these fuel sources will recover after 48 hours.

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Conclusion

Enhancing the endurance of athletes remains a challenge. From the reviewed literature, it
seems that ketones are the best fuel source for people engaging in high intensity and long
duration exercise. Unlike other fuel sources, such as fatty acids, and carbohydrates, ketones
produce more energy per mole of oxygen. Furthermore, ketones produce less reactive oxygen
species. The role of ketones in reducing energy expenditure during starvation makes them a
particularly useful resource for athletes. As such, a nutritional diet comprising of ketones or a
ketonic diet is recommended.

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References

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   C.B., Muoio, D.M. and Coleman, R.A., 2015. Compartmentalized acyl-CoA metabolism in
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