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Chapter 12

Kin 143 Chapter 12.docx

22 Pages

Biomedical Physio & Kines
Course Code
BPK 143
Tony Leyland

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FACTORS GOVERNING THE SELECTION OF FUELS FOR MUSCULAR WORK - Carbohydrates and fats are the major fuels used during exercise - Percentage utilization of fats, carbohydrates, and proteins in energy metabolism during exercise depends on the following factors: 1) Intensity of exercise - At rest, about two-thirds of the ATP produced in muscle tissue is derived from the breakdown of fatty acids, while the other third is derived from the breakdown of glycogen and glucose - Protein is not used as an energy source unless you are severely deficient in carbohydrate and not adapted to such a diet - At 20% of , approximately 60% of the energy comes from fat - at 50% of , only about 40% of the energy comes from fat - At work levels above 95% , carbohydrate is used almost exclusively Typical muscle glycogen utilization at various exercise intensities 2) Duration of exercise - At exercise intensities between 65% and 85% of , it commonly takes 80–120 minutes to deplete muscle glycogen stores - For athletes performing long-duration events, there is a real danger that they will deplete too much glycogen before their event is completed - As exercise duration increases, the proportion of energy production from fats begins to increase - There will be a gradual increase in the body’s reliance on fat as a source of fuel - At lower power outputs, a greater PERCENTAGE of fat is burned, not ABSOLUTE values of fat Carbohydrate (CHO) and fat metabolism during prolonged exercise Glucose metabolism versus exercise duration - The longer the event ( lower power output), the lower the percentage of energy will be obtained from glycogen 3) State of physical training and fuel utilization - The higher the subject’s , the greater the contribution of fatty acids to energy production at any given workload or running speed - For any given workload, the fitter person will be at a lower percentage of his or her than the less fit person - Endurance training brings about enzymatic adaptations in trained muscle fibres - Allows aerobically fitter individuals to get a greater proportion of energy production from fats at any given percentage of compared with the same percentage of for the less fit person - It is the glycogen-sparing effect in endurance training - In long-distance race, athlete who is fitter will not deplete glycogen stores as quickly and thus will go for a longer time at a faster pace DIETARY CARBOHYDRATE - The amount of carbohydrate in the diet will affect the amount of glycogen stored in the muscle fibres - Muscle glycogen levels at the start of moderate-to-high-intensity endurance exercise have a major effect on the length of time for which the exercise can be continued - Muscle glycogen content is positively related to work performance Relationship of exercise to volitional exhaustion and muscle glycogen on three diets - Carbo-loading: eating a large amounts of carbohydrate in the days prior to an endurance race - Athletes engaging in intense daily training, which relies heavily on muscle glycogen reserves, ensure that they obtain adequate carbohydrates in their diet - While brief spikes in carbohydrate intake may be beneficial, chronically high carbohydrate intake may be as detrimental for athletes as it is for the general public - If going with high-carbohydrate diets, at least go with natural foods - The type of carbohydrate ingested during the 24-hour period between exercise training sessions does not differentially affect muscle glycogen synthesis - Complex carbohydrates may be superior to simple carbohydrates during a three-day period of diet manipulation to super-compensate or overload liver and muscle glycogen stores - It is better to eat complex carbohydrates such as rice, potatoes, bread, and so on than to consume simple sugars that are found in candies and soft drinks CARBOHYDRATE LOADING (GLYCOGEN SUPER-COMPENSATION) - High glycogen levels can delay when athletes must slow down because of muscle glycogen depletion, thereby improving race times - Shorter duration events typically do not benefit from these specific dietary regimes - Depleting glycogen stores prior to the pre-event tapering (resting) period will result in higher pre-event glycogen stores - In effect, the body senses the low glycogen levels and is “hungry” to store carbohydrate when it gets the chance Glycogen depletion exercise and glycogen replenishment with rest - The longer the body goes without adequate glycogen stores, the most glycogen it will store once carbohydrate is again available in the diet Possible disadvantages and side effects of carbohydrate loading  An increase in muscle glycogen storage also results in an increase in muscle water content. Each gram of stored glycogen binds 2.7 grams of water. Therefore, a feeling of stiffness and heaviness may accompany large increases in muscle glycogen storage. On the positive side, the water liberated during glycogen breakdown is available for temperature regulation during exercise in the heat.  Irritability and other signs of hypoglycemia (low blood sugar) often accompany the high-fat, high-protein dietary period used in Method C.  Changing diet can be disruptive to athletes. There may be an increased probability of injury or sickness during this period. - Athletes involved in tournaments that play several games over a short period, or may find high glycogen stores advantageous - Although the duration of one game or one event may not deplete a very large amount of glycogen, the reduced recovery time between events mean they cannot eat enough carbohydrate to replenish glycogen - This can lead to a considerable reduction in glycogen stores, which may affect performance Blood glucose - Maintenance of an adequate blood glucose level is essential at all times because glucose is the brain’s major source of fuel - Blood glucose fluctuate throughout the day, usually in relation to timing and composition of meals and snacks, but also exercise routines - Blood glucose concentration is regulated by the hormones insulin and glucagon, which are produced in the pancreas - Liver plays an important role in maintaining blood glucose levels between meals - Immediately after meals, when blood glucose level are high, the liver removes blood glucose and stores it as glycogen - Later, when blood glucose levels decrease to a certain level, liver glycogen is broken down and glucose is released into the blood - The major source of blood glucose during prolonged exercise comes from the breakdown of liver glycogen - Toward the end of an endurance event, liver glycogen stores may depleted and blood glucose levels will then start to decrease - Once blood glucose level decrease below 50-60 milligrams per 100 milliliters, you can become hypoglycemic ( low blood sugar) - Muscle tissue cannot supply the blood with glucose, once glucose goes into the muscle cell, it cannot go back out - Muscle glycogen is used as a source of energy only for the specific muscle in which it is stored - Lactate can be transported by the blood from muscles to the liver to resynthesize glucose, thus, it is an essential fuel in avoiding hypoglycemia MUSCLE GLYCOGEN - The use of glycogen during exercise by a specific muscle, and muscle fibres ( cells) within that muscle, depends on the following : 1) Higher exercise intensities will result in greater glycogen use. This in turn means that continuous moderate power exercise tends to deplete glycogen less than high-intensity interval training 2) For a given muscle, the amount of glycogen used will depend upon tthat muscle’s involvement in the exercise 3) The type of muscle fibre or motor unit recruited during exercise must also be considered. In high-intensity training, fast-twitch fibres will be preferentially recruited and they will show greater glycogen depletion than slow—twitch fibres - Glucose supplied to muscle cells from blood glucose or muscle glycogen also has a role in protein sparing - If you don’t have enough glycogen for muscular contraction, the muscle can convert amino acids into glucose through a process called gluconeogenesis ( conversion of protein to glucose), which effectively breaks down muscle tissue - If carbohydrate is unavailable, fat will be used as the primary fuel source, along with some protein - Carbohydrates are metabolic primers and are needed to completely burn fat - Insufficient carbohydrate levels cause incomplete combustion of fat, result in ketone bodies - The state of ketosis is not necessarily damaging because the brain can metabolize ketones - Losing weight on low-carbohydrate diets may work for sedentary individuals, but not active individuals as they need at least a moderate supply of carbohydrates - Lack of carbohydrates causes protein to go through gluconeogenesis and be metabolized for energy- instead for improving muscle mass - Protein content in low carbohydrate diets is usually adequate and dietary protein can be used to produce some glucose - Fat cannot be converted to carbohydrate GLYCEMIC INDEX IN RELATION TO EXERCISE - The GI is a ranking of carbohydrates based on their immediate effect on blood glucose level - Carbohydrate that break down quickly during digestion have the highest GI - The rapid rise in blood sugar is accompanies by a rapid rise in insulin levels - The spike in insulin level prior to exercise is something athletes want to avoid, as insulin will trigger the body;s cell to remove blood glucose in an attempt to maintain normal blood glucose levels - Low-GI foods can prolong physical endurance, as blood glucose levels rise more slowly and will be available later in the workout - High-GI foods can help re-fuel carbohydrate stores AFTER exhaustive exercise - The muscle are most receptive to replacing glycogen within one hour after a hard workout Ingestion of glucose before and during endurance competitions - Eating refined carbohydrate before the start of the exercise can lead to two factors that will contribute to a fall in the blood sugar 1) High levels of insulin 2) Increased rate of muscle and blood glycogen utilization due to exercise - Results in rapid lowering of blood glucose and a quicker depletion of muscle glycogen as the blood cannot supply the muscles with as much glycogen - Rebound hypoglycemia: the situation when low blood glucose develops 20-30 minutes into the event when a subject is fed glucose Ingestion of glucose prior to endurance race - The beneficial effect of carbohydrate feeding during low-intensity aerobic exercise is not as great - Low intensity aerobic exercise is fueled mainly by the oxidation of fat, with a relatively small demand on carbohydrate metabolism - Fatigue can be postponed by 15-20 minutes, with carbohydrate feeding during exercise at intensities of 60-80% of aerobic capacity - The ideal glucose concentration of an athletes’ drink depends on the environmental temperature and humidity - Water replacement is the most critical factor to maintain performance levels, then glucose concentration should be low - The use of a 5% glucose polymer solution results in good glucose uptake without markedly reducing water uptake FAT AS A SOURCE OF FUEL - The other main source of energy from exercise is fat - One of the effects of endurance training is to make the body more efficient at using fat as fuel - Advantage of fat is that it is energy dense, providing nine kilocalories per gram and eoes not require additional water to be stored - Fat in the body is stored as triglycerides in adipose tissue and muscle ( intramuscular triglycerides) - As athletes start pushing the limit of their endurance, deplete of fat from muscle stores may become an issue - In low-intensity exercise, almost all the fat utilized as fuels is from fat stored in adipose tissue. It is then slowly broken down and transported in the blood to working muscles - As the intensity of aerobic exercise increases, the amount of fat obtained from the blood decreases, and the contribution from intramuscular triglycerides increase - Depletion of intramuscular triglyceride stores may limit endurance performance in much the same way as glycogen depletion - Low exercise intensities, mechanical stress, heat stress, dehydration, injury, and boredom, and depletion of intramuscular fat can all contribute to exhaustion and perceived fatigue - Fat provides protection for internal organs and act as an insulator - It is essential as a vitamin carrier and help depress the feeling of hunger PROTEIN AS A SOURNCE OF FUEL - Amino acids and proteins are integrally involved in the metabolic adjustments to exercise - Amino acids and proteins appear to be involved in at least three important ways: 1) Kreb’s cycle: It supports a number of functions, some of which deplete metabolic intermediates from the cycle. Amino acid conversion to Kreb’s cycle intermediates enhances the rate of oxidation of acetyl-CoA generated from glucose and fatty acid oxidation. 2) Glucose-alanine cycle—A process whereby a compound called alanine, which is formed in skeletal muscle during the breakdown of protein, is released into the blood circulation. When alanine reaches the liver, it is converted to glucose to help maintain blood glucose levels during exercise and thus helps to prevent hypoglycemia. 3) Oxidation of some amino acids – this may provide energy for muscular contraction. The contribution is probably no more than 5-10% of the total energy supply., but possibly reach 15% in events like marathon running - For the vast majority of exercise intensities and durations, protein plays an insiginificant role in providing energy SUMMARY OF MACRONUTRIENT ROLES Carbohydrates  Energy source (especially in high-intensity exercise).  Protein sparing.  Metabolic primer.  Fuel for the central nervous system. Fat  Energy source (at most intensity levels).  Energy reserve.  Protection and insulation.  Vitamin carrier.  Hunger depressor. Protein  Structural function (builds muscle and other tissue).  Enzymes (all of the nearly 2,000 different enzymes in the body are globular proteins).  Energy source (usually minimal).  Protein sparing (adequate dietary protein reduces chances of excessive breakdown of existing muscle and allows for required protein turnover). BASIC DIET COMPOSITION FOR ATHLETES AND ACTIVE INDIVIDUALS General macronutrient composition for various groups of athletes DIETARY ERGOGENIC AIDS TO ATHLETIC PERFORMANCE - The recommended dietary allowances (RDA) for nutrients are defined as “ the levels of intake of essential nutrients considered adequate to meet the known nutritional needs of practically all healthy persons” - RDA is neither minimums nor requirements, and they do not necessarily account for the lifestyles of athletes who push their bodies very hard - The attempt is to set the recommended RDA level for each nutrient higher than most people need, but that attempt is not always correct General use supplements 1) Protein intake - Research suggest that athletes need approximately 1.0 gram per kilogram of body weight as a minimum, and may benefit from up to 2.0 gram per kilogram - Animal protein is better than plant protein for muscle gain one study showed that milk protein was more effective than soy protein at simulating the uptake of protein in skeletal muscle because soy protein has been shown to decrease testosterone levels 2) Vitamin and mineral supplements - Because vitamin can be used repeatedly in metabolic reactions, the vitamins needs of athletes are generally not much greater than the requirement of sedentary people - Athletes consume more vitamins, mineral, and other nutrients simply because their overall caloric intake is higher - At high doses, vitamins stop acting as vitamins and act instead as pharmacological agents - Excessive amount of fat-soluble vitamins and many minerals accumulate in the body and can cause serious health problems - Vegetarians have deficiencies of calcium and iron 3) Vitamin D - Vitamin-D is vital to multiple bodily functions, including allowing body cells to utilize calcium, muscle fibres to develop and grow normally, and the immune system to function properly - Almost every cell in the body has receptors, which are protein molecules to which other important molecules can attach - Vitamin-D regulations is involved with increasing the amount of receptors for insulin like growth factor, which affects the growth and health of every cell in the body - An increase in the number of IGF receptors can increase the body’s sensitivity to the effects of IGF on the body - You will have more potent anabolic growth, which may potentially improve muscle gains 4) Omega 3 - Omega-3 fats are anti-inflammatory, which is a ig factor during recovery from exercise PERFORMANCE SUPPLEMENTS 1) Creatine monohydrate - It is stored as free creatine or bound to a phosphate molecule - Creatine supplementation can increase PCr and Cr stores by 10-40%, which can improve the ability of the muscles to re-synthesize ATP from ADP following high intensity, short-duration exercise - Increase fat-free mass, thigh volume, muscle strength, and myofibrillar protein content - Creatine causes some water retention 2) Caffeine - Improves fat oxidation - Increased performance on endurance tests - Taking too much will likely upset the stomach 3) Additional supplements - Genetic potential, smart/hard training, and sensible nutrition are the overwhelmingly important factors - Correct supplementation can give an already elite athlete a little bit of help in his or her persuit of very slight improvement NUTRIENT TIMING 1) Carbohydrate - Eating 100-200 grams of carbohydrate within two hours after endurance exercise is essential to building adequate glycogen stores for continued training - Waiting longer than two hours results in 50% less glycogen stored in the muscle - Carbohydrate consumption stimulates insulin production, which aids the production of muscle glycogen - Insulin will increase the rate at which blood sugar is stored - It is important to consume carbohydrate within 15 minutes post-exercise to help restore glycogen 2) Carbohydrate plus protein - Combining protein with carbohydrate within two hours of intense endurance exercise nearly doubles the insulin response, which results in more stored glycogen - Optimal carbohydrate to protein ratio is 4:1 - Eating more protein than that slows rehydration and glycogen replenishment - Insulin spikes post-exercise, it is chronically elevated insulin levels that are abd 3) Protein timing - Consume a protein shake within 30 minute of a resistance training session - Breakfast and 30-min post-workout meals should be high in quickly absorbed protein, such as whey and egg white - Evening and 90-minute post workout meals should be high in slowly absorbed protein, such as lean red meats, white means and casein 4) Cyclic diets - Some athletes eat varies quantities of the three macronutrients in cycles - The high protein, high fat diet increases blood serum levels of anabolic hormones, testosterone, growth hormone (GH) , and insulin-like growth factor (IGF-1) - The high fat diet decreases insulin levels, increase fat usage for exercise, and decrease muscle breakdown - The goals of cyclic diet are to better regulate the natural production of testoerone, GH, and insulin - This can shift the body from sugar burning to fat burning and decrease catabolic activity ( reduce muscle protein breakdown) - Cyclic diets have been shown to increase strength and endurance, and decrease excessive body fat - Intermittent fasting - The primary purpose appears to be that during fasting, insulin drops due to lack of incoming calories from the gut - Along with that, blood glucose levels decrease to baseline - With no influx of glucose and fatty acids from the gut, there is no need for the storage hormone, insulin - During regular fasts of 15-24 hours, the body utilizes stored fat, but never gets too much time to break down muscle tissue - Strength training and adequate deep sleep promote higher GH levels - Exercise in a fasted state and protein supplementation post exercise can further increase GH production - The nutritional variations of cyclic diets and intermittent fasting are designed to keep changing the stimulus that forces the body into continued adaptation - These strategies are intended to shock the body into a new growth phase and/or to reduce body fat stores Anabolic hormones Muscle growth (which will drive your appetite to provide sufficient nutrients) - Proper exercise and diet optimizes the balace of the anabolic homones ENVIRONMENTAL FACTORS - Whether the body gains or losses heat depends upon four different mechanism of heat transfer: 1) Radiation - The principle of radiation is based on the fact that molecules within a body are constantly vibrating and, as a consequence, heat in the form of electromagnetic waves is continuously being given off - All objects radiate heat to other objects in their environment - A human is warmed by radiation if objects in the environment ( especially the sun) radiate more heat energy to the person that he or she radiates to the surroundings - If the surface of the body is warmer than one’s surroundings, the body loses heat through radiation to its environment 2) Conduction - The transfer of heat between two objects of different temperatures that are in direct contact with each other - The direction of heat flow is always from the warm to the cooler object - Most of the body’s heat is transported to the surface of the body by blood circulation, a small amount continually moves by conduction directly through the tissues to the cooler outer surface - Heat conduction in water is about 25 times greater than in air 3) Convection - The transfer of heat between the surface of the body and the air or water because of the circulation of air or water molecules next to the skin - Larger differences in temperature between the skin and neighboring air or water molecules, and faster fluid flow over the skin, will result in greater heat transfer - Wind chill index gives the equivalent still air temperature for a particular ambient temperature at different wind velocities 4) Evaporation - The major physiological defense against overheating in humans - The transfer of heat from body surface through the change of liquid water on the skin to a gaseous water vapor in the environment - Body water must absorb heat form the body surface if it is to evaporate as water vapor - The heat required to turn boiling liquid water to a gas is called latent heat - Evaporation is vitally important to human temperature regulation in hot environment as it is the only avenue for heat loss - Evaporation is the diffusion of water molecules from the skin to the air, so no evaporation can occur if the air is 100% saturated with water vapor - Relative humidity: the ratio, expressed as a percentage, of the amount of water vapor that air could hold at the same temperature if it was saturated - Most cases of heat illness occur conditions of high heat and humidity - Most heat lost during exercise is through evaporation of sweat from the surface of the skin - Even while resting, what is referred to as insensible perspiration aids in ridding the body of excessive heat - A small amount of extra-cellular fluid is continually diffusing through the skin and evaporating, because it evaporates rapidly, we do not notice the moisture - If the sweat cannot evaporate and merely falls to the ground, no cooling of the body can take place - In skin covered by clothing, evaporative heat loss occurs only when the clothing becomes wet throughout - Evaporative cooling is thwarted by continually drying the skin with a towel before sweat has a chance to evaporate - Putting on a clean, dry shirt between games can give you a psychological boost and make you feel fresh again
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