Exercise is an important physical activity that the body needs to maintain fitness, proper metabolism of nutrients and utilization of fats for those who want to maintain or lose weight. It affects the energetics of muscle contraction and physiologic processes such as water balance, temperature regulation, and cardiorespiratory control.
The overall goal of exercise is to (1) mobilize fuel for ATP production that supports muscle contraction, (2) maintain blood glucose levels, (3) enhance cardiac output, (4) increase supply of blood to the active tissues, and (5) stabilize fluid and electrolyte balance for blood pressure maintenance (Brown, Miller, & Eason, 2006, p. 83). Metabolic responses to this kind of physical activities under the normal or usual dietary conditions are well explored but modification of the diet patterns and its effects on the metabolic processes in the body are still under few attempts of understanding (Gueye et al. 2003, p. 291).
Fasting has been proposed as a means of increasing utilization of fats in the body and spare glycogen in the muscle, and improving exercise performance. In humans, fasting results in increase in the concentration of circulating catecholamines, increased lipolysis (breaking down of fats), increased concentration of free fatty acids in the plasma, and decreased glucose turnover. Conversely, the muscle-glycogen ratio is not affected by fasting (Berning & Steen, 2005, p. 70). Total fasting involves only temporary starvation.
Health risks may be associated with it, which include excessive loss of lean tissue, metabolic changes such as ketosis and diuresis, and electrolyte imbalances, including kaliuresis and saliuresis. Exercise tolerance is greatly diminished during fasting, with physical activities not advisable anymore (Wadden & Stunkard, 2004, p. 255). Still, there are people who combine fasting and exercise to achieve desired body weight and figure. Both fed and fasted humans both have higher fuel requirements while in a muscular exercise.
The two major fuels consumed by the body are carbohydrates and fats, with the amounts depending on the intensity and duration of the exercise. Moderate exercise, or exercise in just a short duration run for only about a few minutes, consumes mostly the glycogen reserves to provide energy for the muscular activity. Exercises that take more than 1 to 2 hours are already considered heavy exercises, with additional 40% of the fuel consumption, are powered by the fuel supplied by the free fatty acids (FFA) (Felig & Frohman, 2001, p. 844).
FFAs tend to be the predominant energy source during mild to moderate exercise, especially when the exercise is prolonged. Intense exercise, on the other hand, utilizes carbohydrates as fuel (Becker, Bilezikian, Bremner, Hung, & Kahn, 2001, 1258). It is apparent that carbohydrates, particularly the glycogen found in muscles are important to maintain activity in an individual. Maximal and supramaximal intensity exercises are more specifically in need of this. Low and moderate exercise obtain energy from the free fatty acids stored in the adipose tissue.
FFAs also have small contributions to some other metabolic functions that carbohydrates have major involvement (Driskell, 2009, p. 32). Studies establish that consumption of carbohydrates during exercise causes delay in the onset of fatigue, which leads to a better performance in exercise as endurance is higher. Carbohydrate intake increases the availability of blood glucose needed for energy provision in muscle activity. In spite of this, evidences regarding the adaptation of the body from reduced carbohydrate in diet increase in number (cited in De Bock et al. , 2008, p. 045).
On a very low calorie diet, also known as therapeutic fast, those with body fats exceeding the range of 40% to 50% of the body mass as level of obesity may benefit (McArdle, Katch, & Katch, 2005, p. 608). Metabolic rates decrease as muscle proteins are mobilized for gluconeogenesis with the rate of muscle degradation lowering and lipolysis increasing after a few days during fasting (Leuholtz & Ripoll, 1999, p. 126). As previously mentioned, fats can also be used by the body for fuel during exercise. The storage of human fats is much larger than in carbohydrates.
Furthermore, since glucose is a requirement for the brain to function properly, the usage of fats during a moderate exercise in a fasted state is important to save glucose for the brain (Houston, 2006, p. 168). Fasting increases the amount of available lipid substrates, which result to increase oxidation of fatty acids at rest and during moderate exercise. Still, fatigue resistance and performance are weakened (Berning & Steen, 2005, p. 70). The carbohydrate utilization during exercise is under the influence of its availability.
In abundance, either endogenous or exogenous, carbohydrates are oxidized in higher rates during exercise. Individuals who have higher muscle glycogen reserves will most likely consume their glycogen reserves in the muscles first. This may sound contradictory, but faster utilization results from a more rapid completion of a fixed amount of exercise, even in a moderate exercise only (Driskell, 2009, p. 34). During the period of fasting and starvation, the body has sort of adaptation mechanism or metabolic responses.
Metabolic responses can be divided into three phases: the postabsorptive phase, short-term starvation, and long-term starvation. Specific metabolic changes occur in each phase graduall. The first phase of starvation occurs 2-3 hours after the oral intake of glucose. If the diet contains mixed components (protein, fiber, and fat), the gastric emptying takes 4-6 hours. The nutrients come from the body storage like adipose and liver because of the fall in the concentration of glucose in the blood. Glycogenolysis increases in rate and decreases in glycogen synthesis.
Alanine acts as the main source of glucose synthesis in the liver. Tissues that rely mainly on glucose as energy source continue to utilize blood glucose. The liver is responsible for almost all of the glucose output in the postabsorptive phase, either from gluconeogenesis (75%) or glycogenolysis (25%). On the other hand, falling insulin and glucose concentrations cause increase in lipolysis as previously mentioned by Berning and Steen (Sukkar, 2000, p. 148). Short-term starvation, in contrast, has 2-3 times greater rate of hepatic gluconeogenesis.
This is attributed to the increase in protein degradation that results to a negative N imbalance excreted in the urine in the form of urea. The liver takes up increasing quantities of free fatty acids with the ratio of insulin-glucagon concentration being low. This condition leads to increased ketone body production known as ketogenesis. This results to increase in ketone body concentration in the plasma, providing a significant contribution to the energy needs of muscle and brain tissue (Sukkar, 2000, p. 49).
In long-term starvation, almost 60-80% of the fuel needs of the body due to progressive rise in the blood ketone body concentrations. Unlike in diabetes, the levels of ketone bodies are in regulation, so it will not cause ketoacidosis. Another hormone relevant to adapt to starvation is the thyroid hormones (Sukkar, 2000, p. 152). For example, in the kidney, the concentration of lipid derived fuels – non-esterified fatty acids and ketone bodies become high in concentration in the plasma.
These chemical species are considered as biological acids that result production of hydrogen ions. As a consequence, the pH of the blood lowers. As a means of adaptation, the body excretes excess hydrogen ions in the urine in the form of ammonium ion, which carries a single proton. Glutaminase act on glutamine, whereas glutamate dehydrogenase act on glutamate in proteins to form the ammonia needed for hydrogen ion excretion (Frayn, 2003, p. 222).
Starvation can permanently disrupt regulation of the blood pressure, specifically heightening blood pressure because of the increase in the circulating concentration of growth hormone and cortisol, especially for those in the age of 9-15. This leads to excess in cases of mortality among ischaemic heart disease patients, and strokes (Bannon & Carter, 2007, p. 450). Since proteins in the muscles can also be substrate for fuel sources, as evidenced by weight loss, not just the proximal muscles may be affected, but organs such as heart are also likely to be affected (Waller, et al. , 2007, p. 19).