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Physiology of metabolism and energy. The absorbed materials. Digested molecules of food, water and minerals from the diet, are absorbed from the cavity of the upper small intestine.
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The absorbed materials • Digested molecules of food, water and minerals from the diet, are absorbed from the cavity of the upper small intestine. • The absorbed materials cross the mucosa into the blood, and are carried off in the bloodstream to other parts of the body for storage or further chemical change.
Protein • Foods such as meat, eggs, and beans consist of large molecules of protein that must be digested by enzymes before they can be used to build and repair body tissues.
Digestion of proteins • An enzyme in the juice of the stomach starts the digestion of swallowed protein. Further digestion of the protein is completed in the small intestine. Here, several enzymes from the pancreatic juice and the lining of the intestine carry out the breakdown of huge protein molecules into small molecules called amino acids. • These small molecules can be absorbed from the hollow of the small intestine into the blood and then be carried to all parts of the body to build the walls and other parts of cells.
Protein Metabolism • The first step in protein metabolism is to break it into its constituent amino acids. These are absorbed into the blood stream. • The second step is to break down the amino acids into their constituent parts - catabolism. This removes the nitrogen or amino group from the amino acids. The process is called deamination. • Deamination breaks the amino group down into ammonia and what is termed the carbon skeleton. Ammonia is converted to urea, filtered through the kidneys, and excreted in urine. The carbon skeleton--which is composed of carbon, hydrogen, and oxygen--can then by used either for protein synthesis, energy production (ATP), or converted to glucose by gluconeogenesis.
Amino acids stimulate the release of both glucagon and insulin • In a healthy person, a rise in blood amino acid concentration stimulates the secretion of both glucagon and insulin, so their blood sugar remains stable. • The insulin is secreted to stimulate protein synthesis - the uptake of amino acids into muscle cells - making them less available for gluconeogenesis. The glucagon is secreted to stimulate the uptake of amino acids into the cells of the liver for gluconeogenesis.
Why are these two hormones battling for opposing uses of the same amino acids? Isn't that non-productive? • Actually, the phenomenon serves an important purpose. The release of these two opposing hormones ensures that the amino acids are used for protein synthesis (because of the extra insulin) but the blood sugar doesn't drop to dangerously low levels, even if the meal was low in carbohydrate. • As a result, blood glucose concentration remains reasonably stable during protein metabolism. • The insulin and glucagon essentially cancel each other out in terms of their effect on blood glucose, while the insulin is still able to promote protein synthesis.
Protein digestibility • An important aspect of protein metabolism is how well or how poorly a given protein is digested by the human body. Claims are sometimes made that protein powders (especially predigested or hydrolyzed proteins) are digested more efficiently than whole food proteins. • Protein digestibility is measured by seeing how much nitrogen is excreted in the feces compared to the amount of nitrogen which is ingested. A correction is made for the amount of nitrogen which is normally lost in the feces. Therefore, digestibility research examines how much more nitrogen is lost over normal levels when a given protein is fed. • If an individual were fed 6,25 grams of protein, 1 gram of nitrogen was excreted in the feces.
Protein quality • Protein quality is a topic of major debate, both in the research world, as well as in the realm of protein supplements. Arguments have been made that one protein is of higher quality than another, or that protein powders are superior to whole food protein. • Protein quality refers, in a general sense, to how well or poorly a given protein will be used by the body. • More specifically, it refers to how well the indispensable amino acid (AA) profile of a protein matches the requirements of the body. However, this should not suggest that the content of dispensable AAs in a protein is irrelevant to protein quality as the body.
Methods of measuring protein quality • There most spread methods available to measure protein quality are: chemical score, biological value, protein efficiency ratio, and protein digestibility corrected amino acid score. • The quality of a protein is directly related to the physiological needs of the subject being studied. • Diet and activity can affect how AAs are used in the body. For example, long-duration endurance activity tends to oxidize high quantities of the branch-chain amino acids (BCAAs). In all likelihood, there is no single protein that can be rated as the highest quality for all situations.
Chemical score • Chemical score is method of rating proteins based on it's chemical composition (more specifically it's indispensable AA levels). To determine chemical score, a protein is picked as a reference and other proteins are rated relative to that reference protein. • Typically, egg protein has been used as the reference protein, but this assumes that the amino acid profile of egg is the ideal for humans. • Since chemical score is a relative, and not an absolute scale, it is possible to have values greater than 100. If 5 grams of the reference protein contains 800 mg of a certain amino acid, and 5 grams of the test protein contains 1000 mg of that same amino acid, the second protein would be rated as 125% for that amino acid. • The chemical score has little to do with how a food protein will be used in the body and is rarely the only measure of protein quality used to rate a protein.
Biological value (BV) • Biological value (BV) is probably one of the most commonly used measures of a protein's quality. The BV of a protein is given as the amount of nitrogen retained in the body divided by the amount of nitrogen absorbed from that protein. Therefore, digestibility of that protein is taken into account. Thus: • BV = (nitrogen retained / nitrogen absorbed) * 100 % • A BV of 100 would indicate complete utilization of a given dietary protein, in that 100% of the protein ingested was stored in the body with none lost. • To measure BV, subjects are typically fed a zero protein diet so that baseline losses of nitrogen can be measured (i.e. the amount of nitrogen that is lost normally). Then the test protein is fed at varying levels (generally 0.6, 0.5, 0.4 and 0.3 g/kg are fed) and a nitrogen balance study is done. Some studies use longer periods of starvation and this is an important consideration in evaluating the data.
BV of some common proteins • Table 2 presents the BV of some common proteins. • Considering the high protein intakes of most strength athletes (2.0 g/kg or higher) it is hard to see how BV will play a meaningful role in rating proteins in this population. In all likelihood, any decent quality protein will be as good as any other at these types of protein intakes.
Protein efficiency ratio (PER) • PER is sometimes used to rate proteins and represents the amount of weight gained (in grams) relative the amount of protein consumed (in grams). For example, a PER of 2.5 would mean that 2.5 grams of weight was gained for every gram of protein ingested. • A recent animal study found that combinations of animal (30% of total) and plant based proteins (70% of total) had a higher PER value than the animal or vegetable proteins eaten alone. This may have to do with the proteins 'combining' to decrease the impact of the limiting AA. • Individuals who wish to decrease their intake of animal-based proteins may be able to achieve higher PER values with a combination of animal and plant based proteins than someone eating only animal based proteins.
Protein digestibility corrected amino acid score (PDCAAS) • PDCAAS is the newest method of protein quality to be developed. It has also been suggested as the ideal scale to rate proteins for their ability to meet human requirements. Similar to chemical score, it rates protein foods relative to a given reference protein. In this case, the AA profile used is that one determined to be ideal for children two to five years old as its reference protein for adults. • Using the PDCAAS method, along with the proposed AA reference patter, proteins which were previously rated at poor quality, such as soy, have obtained higher quality ratings. This is more in line with studies showing that certain purified soy proteins, such as Supro (tm) which is found in Twinlab Vege-fuel, can maintain adults in nitrogen balance.
Summary of protein quality • Although a variety of methods of measuring protein quality have been proposed, none are perfect in rating proteins for human use. While some methods of rating protein are based on how well (or poorly) an animal grows (or the nitrogen balance which is attained), these methods provide no information on specific amino acid requirements or protein synthesis at a given tissue. Rather, only data regarding growth in the whole body are obtained. • Another strategy to rate proteins is to compare the AA profile in food protein to some reference protein. Previously, food proteins such as egg or milk were used as a reference but there has been a recent move toward the use of an idealized reference pattern of AAs to rate proteins. This assumes that the true requirements for a given AA are known.
Carbohydrates • An average American adult eats about half a pound of carbohydrate each day. Some of our most common foods contain mostly carbohydrates. Examples are bread, potatoes, pastries, candy, rice, spaghetti, fruits, and vegetables. Many of these foods contain both starch, which can be digested, and fiber, which the body cannot digest. • The digestible carbohydrates are broken into simpler molecules by enzymes in the saliva, in juice produced by the pancreas, and in the lining of the small intestine. Glucose and other monocaccharides is carried through the bloodstream to the liver, where it is stored or used to provide energy for the work of the body.
Fates of dietary glucose • The major source of dietary carbohydrate for humans is starch from consumed plant material. This is supplemented with a small amount of glycogen from animal tissue, disaccharides such as sucrose from products containing refined sugar and lactose in milk. • Digestion in the gut converts all carbohydrate to monosaccharides which are transported to the liver and converted to glucose. The liver has a central role in the storage and distribution within the body of all fuels, including glucose. • Glucose in the body undergoes one of three metabolic fates: it is catabolised to produce ATP; it is stored as glycogen in liver and muscle; it is converted to fatty acids. Once converted to fatty acids, these are stored in adipose tissue as triglycerides.
Extracting Energy from Glucose • Two different pathways are involved in the metabolism of glucose: one anaerobic and one aerobic. • The anaerobic process occurs in the cytoplasm and is only moderately efficient. • The aerobic cycle takes place in the mitochondria and is results in the greatest release of energy. As the name implies, though, it requires oxygen.
Anaerobic Metabolism • Glucose in the bloodstream diffuses into the cytoplasm and is locked there by phosphorylation. A glucose molecule is then rearranged slightly to fructose and phosphorylated again to fructose diphosphate. These steps actually require energy, in the form of two ATPs per glucose. The fructose is then cleaved to yield two glyceraldehyde phosphates (GPs). • Finally, two more ATPs are produced as the phosphoglycerates are oxidized to pyruvate.
Aerobic Metabolism • Pyruvate is the starting molecule for oxidative phosphorylation via the Krebb's or citric acid cycle. • In this process, all of the C-C and C-H bonds of the pyruvate will be transferred to oxygen.
Summary of metabolism of glucose • Basically, the pyruvate is oxidized to acetyl coenzyme A, which can then bind with the four carbon oxaloacetate to generate a six carbon citrate. • Carbons and hydrogens are gradually cleaved from this citrate until all that remains is the four carbon oxaloacetate we started with. In the process, four NADHs, one FADH and one GTP are generated for each starting pyruvate.
Gluconeogenesis • The process of conversion of lactate to glucose is called gluconeogenesis, uses some of the reactions of glycolysis (but in the reverse direction) and some reactions unique to this pathway to re-synthesise glucose. • This pathway requires an energy input (as ATP) but has, due to kidneys, the role of maintaining a circulating glucose concentration in the bloodstream (even in the absence of dietary supply) and also maintaining a glucose supply to fast twitch muscle fibres.
Fats • Fat molecules are a rich source of energy for the body. The first step in digestion of a fat is to dissolve it into the watery content of the intestinal cavity.
Fat digestion • The bile acids produced by the liver act as natural detergents to dissolve fat in water and allow the enzymes to break the large fat molecules into smaller molecules, some of which are fatty acids and cholesterol. The bile acids combine with the fatty acids and cholesterol and help these molecules to move into the cells of the mucosa. In these cells the small molecules are formed back into large molecules, most of which pass into vessels (called lymphatics) near the intestine. • These small vessels carry the reformed fat to the veins of the chest, and the blood carries the fat to liver and than to storage depots in different parts of the body.
Fat metabolism and gluconeogenesis • Fatty acids cannot be used directly to produce glucose. However, gycerol, a product of fat metabolism, can and does go through the gluconeogenic pathway to produce glucose. Glycerol is a minor component in fats, and accounts for only 9 to 15% of the total mass. • Fats are much less important than proteins in the gluconeogenic process
Vitamins, water and salt • Another important part of our food that is absorbed from the small intestine is the class of chemicals we call vitamins. There are two different types of vitamins, classified by the fluid in which they can be dissolved:water-soluble vitamins (all the B vitamins and vitamin C) and fat-soluble vitamins (vitamins A, D, and K). • Most of the material absorbed from the cavity of the small intestine is water in which salt is dissolved. The salt and water come from the food and liquid we swallow and the juices secreted by the many digestive glands. In a healthy adult, more than a gallon of water containing over an ounce of salt is absorbed from the intestine every 24 hours.
Oxydation and ATP • Food energy is released through a chemical reaction with oxygen in a process called oxidation. When this occurs outside the body - for example the burning of oil (a fat) in a lamp or the use of a flaming sugar cube (a carbohydrate) as a decoration in a dessert - this energy is released as heat and light. In the body however, food energy needs to be released more slowly and in a form that can be harnessed for basic cell functions and transformed into mechanical movement by the muscle cells. • This is accomplished by "refining" the three basic food materials (carbohydrate, fat, and protein), converting them into a single common chemical compound adenosine triphosphate (ATP). It is this ATP, synthesized as the cell metabolizes (or breaks down) these three basic foods that transfers the energy content of all foods to muscle action.
Isodinamia of substances • The energy contained in equal weights of carbohydrate, fat, and protein is not the same. Energy content is measured in Calories. • Carbohydrates and protein both contain 4.1 Calories per gram (120 Calories per ounce) while the energy "density" of fat is more than double at 9 Calories per gram. • The disadvantage of fat as a fuel for exercise is that it is metabolized through pathways that differ from carbohydrates and can only support an exercise level equivalent to 50% VO2 max. It is an ideal fuel for endurance events, but unacceptable for high level aerobic (or sprint) type activities.
Energy Requirements for Daily Activities • An average man of 70 kilograms who lies in bed all day uses about 1650 Calories of energy. The process of eating and digesting food increases the amount of energy used each day by an additional 200 or more Calories, so that the same man lying in bed and eating a reasonable diet requires a dietary intake of aboul 1850 Calories per day. If he sits in a chair all day without exercising, his total energy requirement reaches 2000 to 2250 Calories. Therefore, the approximate daily en- ergy requirement for a very sedentary man performing only essential functions is 2000 Calories. • The amount of energy used to perform daily physical activi- ties is normally about 25 per cent of the total energy expendi- ture, but it can vary markedly in different individuals, depend- ing on the types and amounts of physical activities. For example, walking up stairs requires about 17 times as much energy as lying in bed asleep. In general, over a 24-hour period, a person performing heavy labor can achieve a maxi- mal rate of energy utilization as great as 6000 to 7000 Calo- ries, or as much as 3.5 times the energy used under conditions of no physical activity.
Poikilothermic and homeothermic organisms • Invertebrates generally cannot adjust their body temperatures and so are at the mercy of the environment. In vertebrates, mechanisms for maintaining body temperature by adjusting heat production and heat loss have evolved. These species are called "cold-blooded" (poikilothermic) because their body temperature fluctuates over a considerable range. • In birds and mammals , the ' 'warm-blooded ' ' (homeothermic) animals, a group of reflex responses that are primarily integrated in the hypothalamus operate to maintain body temperature within a narrow range in spite of wide fluctuations in environmental temperature.
Temperature balance • The balance betweenheat production and heat loss is continuously beingdisturbed, either by changes in metabolic rate (exercisebeing the most powerful influence) or by changesin the external environment that alter heat loss or gain. • The resulting changes in body temperature are detectedby thermoreceptors, which initiate reflexes thatchange the output of various effectors so that heat productionand/or loss are changed and body temperatureis restored toward normal.
Normal Body Temperature • In homeothermic animals, the actual temperature at which the body is maintained varies from species to species and, to a lesser degree, from individual to individual. In humans, the traditional normal value for the oral temperature is 37 °C (98.6 °F), but in one large series of normal young adults, the morning oral tem- perature averaged 36.7 °C, with a standard deviation of 0.2 °C.
Temperature receptors • There are cold and warmth receptors. Nerve fibers respond differently at different levels of temperature. So a person determines the different gradation of thermal sensation by the relative degrees of stimulation. • It is believed that the cold and warmth receptors are stimulated by changes in their metabolic rates or from chemical stimulation of the endings as modified by the temperature.
Central and peripheral thermoreceptors • There are two categories of thermoreceptors,one in the skin (peripheral thermoreceptors) and the other (centralthermoreceptors) in deep body structures, includingthe hypothalamus, spinal cord, and abdominal organs. • Since it is the core body temperature, not the skin temperature,that is being maintained relatively constant,the central thermoreceptors provide the essentialnegative-feedback component of the reflexes.
Central control of touch and temperature sensation • Almost all sensory information from the somatic segments of the body enters the spinal cord through the dorsal roots from the spinal nerves. • Sensory signals are carried through one or two alternative sensory pathways: 1) the dorsal colomn-medial lemniscal system; 2) the anterolateral system. All these fibers belong to spinothalamic tract. • Sensory information that must be transmitted rapidly or with great spatial fidelity is transmitted mainly in the dorsal colomn-medial lemniscal system. • Sensory impulses, which do not need to keep these conditions, are transmitted mainly in the anterolateral system. The anterolateral system can transmit pain, warmth, cold and crude tactile sensation. Because of the crossing of the medial lemnisci in the medulla, the left side of the body is represented in the right side of the thalamus, and the right side of the body is represented in the left part of the thalamus.
Central processing of impulses • Cerebral cortex processes somatic sensory information in somatosensory area I, and somatosensory area II. • Somatosensory area I has much more extensive spatial orientation of the different parts of the body. • Somatosensory area II helps in association of somatic sensory information with visceral sensation and body activity. In general, thermal signals are transmitted in pathways parallel to those for pain signals. On entering the spinal cord the signals travel for a few segments upward or downward and than terminate in dorsal horns. Then nerve fibers cross to opposite anterolateral sensory tract and terminate both the reticular areas of the brain stem and the ventrobasal complex of thalamus. • A few thermal signals are also relayed to the somatic sensory cortex from the ventrobasal complex. Furthermore, it is known that removal of the postcentral gurus in the human brain being reduced but does not abolish the ability to distinguish gradations of temperature.
