소화
Stomach enzymes produce partial digestion of dietary proteins, significant digestion of dietary fats, and little digestion of carbohydrates. (p88)
탄수화물
Carbohydrates are plentiful and constant in food supplies throughout the world. Carbohydrates generally supply about 45% of our energy requirement.(116)
Human have no dietary requirement for any specific type of carbohyhdrate. All carbohydrate of the body can be synthesized from dietary glucose or fructose. The body stores sugar in the form of glycogen. A naturally occuring deficiency specifically in cargohydrate is unknown. However, omission of carbohydrates from diet lead to specific problems. Glucose is required as an energy source by the central nervous system. When there is a deficiency of glucose, the body adjusts its metabolism to provide ketone bodies. nutrients derived from fat. Excessive production of the ketone bodies result in acidosis. (115-116)
Most aspects of carbohydrate nutrition are simpler than those of other nutrients. On the other hand, the nutrition of the carbohydrate that take the form of dietary fibers is very complicated. This complexity is due to the fact that they are metabolized by enzymes of the gut microflora (p116).
The actvity of alpha-amylase in salive is not significant compared with that in the pancreatic juice. Uncooked starch resists diestion. Increase in the plasma glucose continues untill 30 minutes after after a meal of raw wheat. The plasma glucose level dramatically increases after a meal of boiled wheat.(p109-113).
Fructose
Fructose presents in fruits and vegetables. Fructose does not provoke a release of insulin bu the pancreas. However, ingestion of fructose may be followed by rise in plasma insulin, because conversion of fructose to glucose in the liver results in a rise in plasma glucose level.(211) Ingestion of fructose can lead to a rise in plasma glucose higher than the rise in plasma fructoe.(211)
A mild fructose malabsorption syndrome may occur in certain individuals. Glucose have a protective effect by enhancing the rate of absorption of fructose. Apple juice, which is high in fructose and low in glucose, mayu provoke abdominal cramp.(211)
Lactose 요구르트
Galactose may be liberated from lactose by the bacteria present in the yogurt itself, but in human gut after yogurt consumption.(104)
In making yogurt, milk is initially heated at 80 degree celsius for 15-30 min in order to denature the whey proteins. This heating makes casein micelles, resulting in a yogurt with improved viscosity and texture. (425)
Lactose intolerance
Milk has a high content of lactose. With age,many children lose the ability to digest large amount of lactose. The lactose intolerance is most common in regions of the world where adults do not drink milk.(137)
Sucrose intolerance
Sucrose accounts for about half the solid material of a beet or peach. Sucrose intolerance occurs among Eskimos. This is consistent with the lack of availability of sucrose-containing staples. People in Greenland traditionally have consumed meat, not fruit or sugar cane. (137)
Starch 익히면 소화 증가
The starch in plants occurs in small microscopic granules that have a core of starch surrounded by a network of protein. In the granule, amylopectin has an orderly crystalline structure, whereas amylose is somewhat amorphous. The occurence of starch in granule prevent the digestion or up to 50% of the starch in these foods when they are consumed in the raw state. Cooking results in hydration, swelling, and gelatinization of the starch granules, with a consequent increase in digestibility.(107)
섬유소
Dietary fiber includes all the polysaccharides and lignin of the diet that are not digested by human enzymes. Plant cell walls are the major source of dietary fibers. Dietary fibers are classified as resistant starches, nonstarch polysaccharides, and lignin. Resistant starch resists to hydrolysis. Resistant starch occurs in raw starch granules and can accumulate during freeze-drying of foods or during prolonged cooling of some cooked foods. This form of starch escapes digestion but may be degraded substantially by bacterial enzymes in the large intestine. Nonstarch polysaccharides include pectins, gums, hemicelluloses, celluloses, and beta-glucans. The value of nonstarch polysaccharides for wheat bran is about 40%.(p140-142)
Dietary fibers are also classed according to its solubility. Soluble fibers - pectins, gums, and certain hemicelluloses- form viscus solutions and decrease the rate of passage of material and delay the rate of absorption of nutrients in the small intestine. They reduce the rate of rise of plasma glucose and reduce plasma cholesterol levels. Fruits, oats, barley, and legumes are high in soluble fibers.
Insoluble fibers-cellulose, lignin, and many hemicellulose-result in great fecal bulk and increase the rate of material through the large intestin. Vegatables, wheat, and most grain products are high in insoluble fibers (p144). Gut bacteria convert bile acids to compounds that appear to be carcinogenic. (p151). Insoluble dietary fibers reduce the concentration of cancer-producing chemicals by increasing bulk. An increase in flow rate through the colon reduces the time of exposure of the colonocyte to the cancer-producing chemicals supplied by the diet and microbial metabolism in the colon, (p147)
The primary fate of dietary fiber is digestion and catabolism by the gut microflora to short-chain fatty acids and carbon dioxide. The major products of this microbial metabolism -acetic, propionic, and butyric acid- are important sources of energy for ruminants. The fatty acids produced may supply 35-75% of the energy requirement of the ruminant. Short-chain fatty acids supply 50-75% of the energy requirement of the colonocytes of animals such as rats and humans, but only about 5% of the overall energy requirement of the animal itself. The initial step of short-chain fatty acids, whether in the gut of in the liver, is converted to the coenzyme A derivative. Acetate is converted to acetyl coenzyme A. The acetyl coenzyme A formed in the cytoplasm can be used for the synthesis of fatty acid, whereas that formed in the mitochondria can be used for immediate oxidation.(p143)
In the absence of short-chain fatty acids, the mucosa of the colon may become inflamed or atrophied. With total parenteral nutrition (TPN), continued sloughing off of enterocytes into the lumen. This results in diarhea when oral feeding is resumed due to the inability of the atrophied colon to absorb water.(p147)
Gut microflora
The human stomach contains about 100 bacteria per gram of contents, whereas the large intestine contains about one billion per gram. Over 99.9% of the bacteria are anaerobic. The most highly publicized bacterium of the gut is Eschericia coli which is able to use oxygen. This bacteria consume oxygen and help produce an oxygen-free environment that is required by many of the anaerobes. The anaerobes include carbohydrate-fermenting bacteria, pectinolytic bacteria, and methanogens. The gases produced by bacterial metabolism include CO2, H2, and CH4 (methane). About one-third of humans are methane producers. Different species of gut bacteria can supply nutrients to each other. One might recall ,the three forms of life which exist on earth are Eucarya, Bacteria, and Archae. Methanogens are Archae. (p147-149).
대장까지 도달시간이 3시간. H2 production was used to follow the time of passage of dietary gum. The results indicate that fermentation of the gum started within 3 hours of comsumption of the test dose. (p150)
Gut bacteria convert bile acids to compounds that appear to be carcinogenic. (p151).
Biotin , Vit K
Gut bacteria produce biotin and supplies half of our requirement. (p539) .Vitamine K synthesized and released in the large intestines can be absorbed by the human host, and this source has been estimated to supply about half of our requirement.(p538)
In adults, approximately one-third of the dry weight of feces is bacteria. Normally the body's defenses prevent invasion of such bacteria. (Hickman 1998;p234)
Hickman CP, Jr, Roberts LS, Larson A . Region of water absorption and concentration of solids. In: Hickman CP, Jr, Roberts LS, Larson A, ed. Biology of Animals. 7th ed. WCB/McGraw-Hill 1998;234.
가축은 fiber가 많이 필요하다.
Reduced levels of fiber can lead to impaired health in cow, i.e., acidosis, liver damage, and lower fat content in milk. With under 20% fiber, the farmer risks harming the cows, because of overproduction of short-chain fatty acid caused by over-eaten cereal grain. With over 20%, the farmer risks less milk production.
흡수
The transport of sodium ions is coupled to both glucose and amino acids. The cells of crypt mucosa in the small intestine continue to migrate toward the top of villus and are sloughed off within 2-3 days. The reason for the rapid turnover of the epithelial surface of the gut is the necessity of maintaining its function in the harsh environment of pancreatic enzymes. (p117)
---------------------------------------
지방
쓰임새
Dietary lipids are used as an energy source, as a structural component in the membranes of cells, as structural components of a small fraction of the proteins in the cell, and in the case of cholesterol, for the synthesis of bile salts. (312)
Dietary fats and oils are important for several reasons. There are excellent energy source and generally supply at least 30-40% of the body's energy requirment. They contribute to the palatability of the diet. Dietary fats and oils are deposited in adopocytes for storage as triglyceride and in membranes as structural phopholipids.
The major components of cell membranes are proteins, cholesterol, lipids with a glycerol backbone, and lipids with a sphingosine 0%)backbone. (312) The glycerol-based lipies are phospholipids with fatty acids esterified at the 1 and 2 positions of the glycerol, and a molecule of phosphocholine, phosphoethanolamine, phosphoserine, or phosphoinositol at the 3 position.(312-313)
The cholesterol:phospholipid ratio controls the viscosity of the membrane. Higher ratio result in a more viscous, less fluid membrane. The cholesterol:phospholipid ratio in the plasma membrane is about 1.0:1.0 but that in ER is about 0.1:1.0 (Straka et al., 1990) (327)
Cholesterol stabilizes the structure of membrane.(327). Cholesterol increaese the rigidity of the cell membrane. (p118).
The lipids of the diet include triglycerides, phospholipids, cholesterol, cholesteryl esters, and fat-soulble vitamine.(p91).
The triglycerides of foods are located mainly in the energy storage site of the animal or plant: specialized cells called adipocytes in meat (muscle), microscopic particles called lipoproteins in milk, and seeds in plants. (93) The phosphopipids of foods are located in the membranes that surround the cell (the plasma membrane), mitochondria, nucleus, lysosome, and endoplasmic reticulum.(93)
Cholesterol
Cholesterol is a structural lipids that is not required in the diet.(99) Cholesterol occurs as free cholesterol or as a covalent complex with a fatty acid bound to its hydroxyl group. Such complexes are called cholesterol esters or cholesteryl esters. About 90% of the unesterified cholesterol in the cell is located in the plasma membraine (327). Cholesterol is used by many organisms as a structural element in membranes and as the starting material for synthesizing bile salts and steroid hormones. (326). Plants do not contain cholesterol, but do synthesize sitosterol, ergosterol, and digitalis.(326)
A small portion of the body's cholesterol is obtained from food, but the molecule is completerly dispensable in the diet. DIetary cholesterol is absorbed fairly well (60-80%), but plant sterols are absobed poorly (<5%).(326).
The average daily intake of total dietary cholesterol is 400-500 mg. (326). Plasma cholesterol levels are not changed very much by changes in the quantity of cholesterol in the diet.(326). Reduction of dietary intake results in a change in available cholesterol that is small compared with the rate of endogenous synthesis. A 70-kg man synthesize about 700 mg per day. Increasing dietary cholesterol may not result in an increase in absorption. The greates amount that can be absorbed by the gut is about 1.0 g per day.(327).
The major sites of cholesterol synthesis are the liver and the intestine. About 1/3 of our cholesterol arises from the diet, while 2/3 is made in the body (Jones, 1997). Most of the cholesterol in the body is manufactured by extrahepatic tissues, the intestines. In cholesterol synthesis, the acetyl CoA is the starting material and is aquired by breakdown of dietary sugars or fat.(327)---> 단당류와 고기를 피해야 하는 이유.
Nearly all cholesterol synhesized in the liver is used for bile salt synthesis. (327)
The human body turns over about 800 mg of cholesterol per day. Most of this turnover involves bile salts. About 400 mg cholesterol is used to manufacture new bile salts. About 80 mg cholesterol is lost through the skin. About 50 mg is used for synthesis of steroid hormones.(99)
Lipid 운반
The biochemical apparatus used for assimilation and distribution includes bile salts, apolipoproteins, serum albumin, and vitamin-binding proteins. The term apolipoprotein is used when referring only to the protein, whereas the term lipoprotein refers to the complex of apolipoprotein and lipid (332).
Lipoprotein
Lipoproteins are synthesized primarily in the intestine and liver and are secreted into plasma. (312). The lipoproteins assembled in the small intestine are chyhlomicrons.(332). Dietary triglycerides are packaged into chylomicrons in the enterocyte and delivered to the circulatory system via the thoracic duct.(320). VLDLs and HDLs are secreted from the liver into the bloodstream. HDLs interacts with the chylomicrons and VLDLs, and promote their maturation and function. The chylomicrons contain a small proportion of protein, whereas HDLs have a high protein content.(332). Chylomicrons receive apolipoprotein C-II on entry into the thoracic duct.(333). Apo C-II activates the lipoprotein lipase that removes TGs from the chylomicron. Apo C-II functions in the transfer of TGs from lipoproteins to various tissue (334). The organs that produce lipoprotein lipase are muscles, adipose tissue, and mammary glands.(환경호르몬이 만이 가는 곳?) (335) Persons with genetic mutations in the gene coding for lipoprotein lipase may be at greater risk for artherosclerosis (335). During the progressive loss of TGs from a chylomicron, it becomes smaller until it reaches a stage where it is called a chylomicron remnant. The chylomicron remnants are hydrolyzed in the liver. Uptake of a chylomicron remnant by the liver requires apoprotein E, resides on the remnant itself. The chylomicrones contain apolipoprotein B-48, apo C-II, and apo A-I. The VLDLs contain apo B-100, apo C-II, and apo-E. The HDLs contain apo A-I and apo A-II.(333-334).
Apo B-100 and apo E facilitate the uptake of its associated lipoprotein by the liver. The rate of uptake mediated by apo E is greater (335). Type III hyperlipoproteinemia, results from mutaions in the apo E gene, the disease results in high plasma cholesterol, high TGs, premature atherosclerosis, and xanthomas.(355).
Various types of lipoproteins exist in the bloodstream; HDL, LDL ,VLDL ,and chylomicron. The lipoproteins contain protein and cholesterol.(312). Lipoproteins consist of a large core of triglycerides coated with a thin shell of phospholipid and protein.(317)
When speaking abouth health, the term "LDL" and "LDL-cholesterol" mean the same thing, and the term "HDL" and "HDL-cholesterol" mean the same thing. But to the biochemist, the terms "LDL" and "HDL" mean a specific combination of proteins, cholesteryl esters, triglycerides, and phopholipid- and not just the cholesterol component. (312)
Fatty acids classification
Fatty acids are classed as short chain, medium chain, or long chain. Short-chain fatty acids are 2-6 carbon atoms long, i.e., acetic acid. Medium-chain fatty acids are 8 to 12 carbon long. Long-chain fatty acids are 14-24 carbon long, i.e., palmitic acid (16:0), oleic acid (18:1), and linoleic acid (18:2). (320)
Short- and medium-chain fatty acids tend to be oxidized immediately to carbon dioxide, rather than deposited as TGs of phopholipids.(320). Dietary fat can influence the composition of membrane. Diet with long-chain fatty acids changes in the contents of plasma membranes of the liver to more long-chain fatty acids.
Long-chain versus medium-chain fatty acids
The fatty acid constituents of the dietary triglyceride are mainly long-chain fatty acids. The medium-chain fatty acids are more water soluble, and are absorbed by mechanisms independent of those used by long-chain fatty acids that bypass steps involving bile salts.(226). The medium-chain fatty acids have the greater tendency to be metabolized immediately in the mitochondria, rather than stored as fat. The long-chain fatty acids are first stored as fat. The medium-chain fatty acids are transported from the gut via the portal vein to the liver along with other water-soluble nutrients. The long-chain fatty acids are packaged into chylomicrons and transported through the lymphahtic system, and hydrolized by lipoprotein lipase and taken up by skeletal muscle, adipose tissue, and other tissue for storage or immediate oxidation.(226). The long-chain fatty acids entering into the liver may be packaed into VLDLs, as TGs, and shunted back to the bloodstream to be used for oxidation or storage.
버진 오일
Free fatty acids are not a major source of energy in the diet but are considered undesirable contaminants in vegetable oils. The term "virgin olive oil" means that the oil is of high quality and does not contain free fatty acids. (93)
방귀 냄새
The volatile fatty acids(VFA) are produced in the largest amounts in herbivorous animal species and especially in the forestomach of ruminants. The VFA, however, also are produced in the lower digestive tract of humans and all animal species, and intestinal fermentation resembles that occurring in the rumen. The principal VFA in either the rumen or large intestine are acetate, propionate, and butyrate.
Most of the butyrate is converted to ketone bodies or CO2 by the epithelial cells, and nearly all of the remainder is removed by the liver. Propionate is similarly removed by the liver but is largely converted to glucose. Although species differences exist, acetate is used principally by peripheral tissues, especially fat and muscle.
Bergman EN. Energy contributions of volatile fatty acids from the gastrointestinal tract in various species. Physiol Rev. 1990 Apr;70(2):567-90.
소에서 총에너지
Volatile fatty acid (VFA) contribute approximately 70% to the caloric requirements of ruminants, such as sheep and cattle, approximately 10% for humans.
Bergman EN. Energy contributions of volatile fatty acids from the gastrointestinal tract in various species. Physiol Rev. 1990 Apr;70(2):567-90.
섬유소
The amount of fiber in the diet undoubtedly affects the amount of VFA produced, and thus the contribution of VFA to the energy needs of the body could become considerably greater as the dietary fiber increases.
Bergman EN. Energy contributions of volatile fatty acids from the gastrointestinal tract in various species. Physiol Rev. 1990 Apr;70(2):567-90.
대장암, 고지혈증
VFA may indirectly influence cholesterol synthesis and even help regulate insulin or glucagon secretion. In addition, VFA production and absorption have a very significant effect on epithelial cell growth, blood flow, and the normal secretory and absorptive functions of the large intestine, cecum, and rumen.
Bergman EN. Energy contributions of volatile fatty acids from the gastrointestinal tract in various species. Physiol Rev. 1990 Apr;70(2):567-90.
트리아실글리세롤은 매우 농축된 에너지 저장형으로, 탄수화물과 단백질의 열산출량은 4kcal/g인데 비해서 지방산의 열산출량은 9kcal/g 이다. 탄수화물의 저장형인 글리코겐 보다 같은 무게에서 여섯 배의 에너지를 저장한다. 이것이 글리코겐 대신에 트리아실글리세롤리 진화에서 주요한 에너지 저장형으로 선택된 이유이다. 체중 70kg의 남자에게는 100,000kcal 의 트리아실글리세롤, 25,000kcal의 단백질, 600kcal의 글리코겐, 40kcal의 글루코즈가 비축되어 있다. 트리아실글리세롤이 체중에서 11kg을 차지한다. 이 만큼의 에너지가 글리코겐으로 저장된다면, 이 사람의 체중은 55kg이 더 무거워져야 한다. (Lubert Stryer. 지방산 대사. Stryer 생화학. 3rd ed. 서울외국서적주식회사.1992;499)
Essential fatty acids
The essential fatty acids (EFAs) are linoleic acid and linolenic acid. Arachidonic acid is relatively minor component of dietary fat and oils and has sometimes been called an essential fatty acid, even though it is readily synthesized from linoleic acid. Linoleic acid is converted to arachidonic acid. Linolenic acid is converted to eicosapentanoic acid (EPA).
Linoleic acid (18:2w6) --> arachidonic acid --> DPA(docopentaenoic acid)
Linolenic acid (18:3w3)--> EPA --> DHA (docohexaenoic acid)
필수지방산에서 합성된 동물성 지방산
Palmitic acid (18:0) and stearic acid (18:0) are the major products of fatty acid synthase (p639). Palmitic acid is a mojor fatty acid in cell membrane (p639). 포유동물에 있는 불포화지방산들은 필마톨레산, 올레산, 리놀레산, 리놀렌산 중 어느 하나로부터 유도된다. 포유동물은 리놀레산과 리놀렌산을 합성할 수 없다 (Stryer 생화학:518). 짧은 사슬 길이의 불포화는 지방산과 그들의 유도체들의 유동성을 증진시킨다. (p499).
필수지방산 결핍은 거의 일어나지 않는다.
EFAs are plentiful in the diet (p650). Because of our ample intake of fat, a deficiency in EFAs is quite rare.(p638) Wide variations in the intake of EFAs may have little of no effect on the fuctions of the body because of the stores of linoleic acid in adipose tissue and because of metabolic adjustments by the body(p650). The question of which dietary practice lead to EFA deficiency is difficult to predict.(p652)
PUFA와 vit E 와의 관계
Persons at risk for EFA deficiency tend to be the same as those at risk for vitamine E deficiency (p650). Greater intakes of PUFAs tend to increase the requirement for vitamine E. Foods highest in PUFAs such as vegetable oils are also good sources of vitamine E (p653).
동물성 음식의 지방산이 상하는 이유
The PUFAs are unstable in the presence of oxygem. Oxygen induced damage to polyunsaturated fatty acids (PUFA) in poultry occur during the mechanical deboning process, leading to off-flavors. Damage to the PUFAs in fish may occur during storage, leading to toughness. Damage to the lipids in meat may occur with prolonged heating, leading to warmed-over flavors. The off-flavors are due to the production of volatile aldehydes and acids. The toughness is due to the production of malondialdehyde, which can crosslink the proteins in the food.
These foods do not contain active oxidative enzymes and thus would not be expected to produce superoxide or HOOH. All foods, including fats and oils, contain trace amount of iron and copper. These metals, in their oxidized forms (Fe3+, Cu2+), may react directrly with PUFAs (653-655).
Bile salt & 콜레스티라민
Bile salts are required for supporting the activity of pancreatic lipase as well as for maintaining the polar products of fat hydrolysis in solution.(96) Biles salts support the migration of polar lipid in mixed micells to the enterocyte. (97)
Bile salts circulate many times per day through the enterohepatic system. This process can be interrupted by a resin, cholestyramine, that binds to the bile salts.(96) Cholestyramine simulates the loss of much more than 400 mg of cholesterol, Cholestyramine alone does not lower serum cholesterol because the liver responds by increasing its rate of cholesterol biosynthesis. Combination with other drugs that inhibit this biosynthetic pathway is useful. (99)
Taurine occurs as a componet of bile salts and plays an important role in the transport and absorption of lipids.(101). Taurine is found in animals and milk, but not found in plants. Most animals, including human adults, can synthesize sufficient taurine. Human infants, cat, dogs, and cats have little ability to synthesize taurine. Whether human can benefit from taurine in the diet is not clear. (103)
고지혈증 치료
The treatments are listed in order of importance: (1) diet; (2) statin; (3) nicotinic acid; and (4) Cholestyramine. (100)
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Protein
글루탐산의 조미료 역할
Glutamate is an efficient source of amino groups for the synthesis of the dispensable amino acids. (460)
Glutamate is the amino acid present in greatest quantities in many proteins. Glutamate is the source of amino groups for the synthesis of the dispensable amino acids. Glutamate and glutamine are frequently reported as a sum. The wheat protein, gliadin, is nearly 50% glutamate plus glutamine. The corn protein, zein, is about 25% glutamate plus glutamine. Glutamate is used as a flavoring agent. Cheese contains 0.5 to 1.0% glutamate by weight, while tomtatos 0.14%. This may contribute to the irriversible quality of pizza. (Brody 1999;p461).
작은 동물의 사료 효율이 높다.
A species difference has been found regarding the percentage of dietary protein used to support growth versus maintanance. In growing rats, about 95% of the dietary protein is used for growth. In the growing pig, about about 90% of the dietary protein is used for growth, with 10% for maintenance. In the 6-month-old human infant, about 50% is used for growth, while in the growing 5 year old child, only 5% of dietary protein is used for growth (Young and El-Khoury, 1995).
Young VR, el-Khoury AE. Can amino acid requirements for nutritional maintenance in adult humans be approximated from the amino acid composition of body mixed proteins? Proc Natl Acad Sci U S A. 1995 Jan 3;92(1):300-4.
Protein Daily Requirement
The protein requirement are derived by the followings;obligatory losses, catabolism of amino acids from dietary protein prior to use by the body, the mixed quality of proteins in the diet, and biological variability. The obligatory losses of protein with the carbohydrate-only diet are close to 0.34g protein/Kg or about 24g of protein for the 70-kg man. (p459).
단백질이 필요한 이유. Protein turnover
The amide groups of asparagine and glutamate of all proteins slowly and spontaneously hydrolyze over the course of several days or weeks, resulting in abnormal protein, and then resynthesized. This phenomenon of hydrolysis and resynthesis is called protein turnover. Small amounts of intact protein and intact amino acids are also lost in urine and feces. The amount of nitrogen excreted in the urine is many times greater than that excreted in the feces. Fecal protein and amino acids arise from the proteins of the pancreatic juice and from cells sloughed off from the gut mucosa. Creatinine and uric acid are lost in the urine (Brody 1999;p451-453).
Brody T. Why do adults need to eat protein? In;Brody T, ed. Nutritional biochemistry. 2nd ed. San Diego. Academic Press1999;451-453
The turnover of the body's protein affects all proteins of the body. Some proteins turn over rapidly between 0.2 and 15 hour, whereas others such as collagen and other structural proteins turn over quite slowly. Ornithind decarboxylase turns over with a half-life of about 2 hr and thus has a relatively rapid turnover rate. This means that any given molecule of the protein has a 50% chance of being degraded during a 12-min period.
What can be accomplished by protein turnover? One benefit is the removal of proteins that have sustained spontaneous hydrolytic damage or oxidative dmage. Another benefit is that protein turnover makes possible the control of enzyme levels by means of genetic regulation. If protein turnover did not occur, a decrease in the rate of transcription woud not result in a prompt drop in the enzymes's activity in the cell. Enzymes whos activity can be rapidly decreased by regulation at the genetic level might be expected to undergo a relatively rapid rate of turnover (Brody 1999;p451-453).
Brody T. Why do adults need to eat protein? In;Brody T, ed. Nutritional biochemistry. 2nd ed. San Diego. Academic Press1999;451-453
Protein sparing
Carbohydrate can spare protein. Similarly, one amino acid can spare another amino acid.
The concept of protein aparing was discovered during studies to create a better life raft ration. With total fasting, a 70-Kg man loses an amount of nitrogen equivalent to 80 g of protein per day. About half of these losses could be prevented by feeding carbohydrate to the fasted man. The proportion of nitrogen lost as ammonia increases to 25% of the total nitrogen by day 7 of fasting. The synthesis of ammonia requires less energy than making urea. The ammonium ion is a very weak acid. A shift from urea excretion to ammonium ion excretion aids in counteracting ketoacidosis during prolonged fasting (Boon et a1., 1996). The excretion of waste nitrogen via urea cycle results in net production of acid, whereas excretion of ammonium ions does not results in production of acid (Brody.1999:p455)
Boon L, Blommaart PJ, Meijer AJ, Lamers WH, Schoolwerth AC. Response of hepatic amino acid consumption to chronic metabolic acidosis.
Am J Physiol. 1996 Jul;271(1 Pt 2):F198-202.
Brody T. Changing style of waste nitrogen excretion with fastin. In;Brody T, ed. Nutritional biochemistry. 2nd ed. San Diego. Academic Press1999;455
이상적인 단백질 공급원
Protein quality refers to the ability of dietary protein to supply the amino adic needs of the body. Gelatin, for example, is a protein and cannot supply the body's needs. Gelatin is a low-quality protein .(Brody 1999;p469).
The ideal proteins are egg protein (albumin), cow milk protein (alpha-casein and beta-lactoglobulin), and human milk protein.(Brody 1999;p461).
채식에서 단백질 공급 요령
Oats, rice, peanuts are the proteins of moderate quality (p470). The soy protein is somewhat deficient in methionine. Thus, soy protein is regarded as a low-quality protein (Brody 1999;p451). Wheat and corns are also low-quality protein (p470). Wheat lacks of lysine and soybean lacks methionine(p474). Legumes contain lysine, threonine, and tryptophan, but are limiting methionine. Grains contains methionine, but are limiting in lysine and sometimes threonine or tryptophan. Therefore, a diet containing both legumes and grains supplies a mixture of amino acids that is of higher quality than a diet containing legume alone or grain alone. The complementary proteins are supplied by as the beans and rice, the tofu and rice, and the peanut butter sandwiches. One animal study demondstated that the most valuble sources of protein were obtained by the ratio of rice:regume about 50:50. They must be consumed together, or at about the same time, to be a maximal benefit as a protein source. For example, eating beans at breakfast and corn at dinner would fail to supply complement proteins. There are differences in human and animal nutrition. Persians may consume diets where grain may be the sole source of protein. Human children require a lower density of protein than do rats. A human infant grow well when its only source of protein is bread or beans.(p475).
Regulation of protein catabolism
There are several pathways to hydrolyze proteins: macro- and microautophage, the ubiquitin-dependent pathway, and the calcium-dependent proteases. In addition, the rate of protein breakdown is influenced by insulin and glucagon, as well as by specific amino acids.(446).
Glucagon often stimulates of protein breakdown in the liver, though it reduce protein breakdown in muscle. Insulin decreases protein degradation in liver and muscle. Brief fasting results in a large protein loss in the liver, but not in the muscle. In muscle, the noncontractile proteins are readily degraded, but with prolonged fasting the contractile proteins are lost through proteolysis.(446-7). Macroautophage is stimulated by an amino acid deficiency. Deficiency in leucine may stimulate macroautophagy. Contractile proteins are not deraded by macroautophage. They are catabolized by the calciym-dependent protease.(447).
One goal of the regulation catabolism is to increase the rate of amino acid catabolism when dietary proteins are in excess of the body's needs. Another goal is to allow the selective catabolism of nonvital proteins during starvation to support gluconeogenesis. The less vital protein includes close to half of the body's skeletal muscle; the more vital protein includes that of the nervous system.(444)
Additionally, the regulation of protein catabolism involves the regulation of enzymes of the urea cycle. (446) The activation of all five enzymes of the urea cycle increase with consumption of a high-protein diet (Kato et al., 1978; Schimke, 1962). This increase occurs slowly. Maximal adjustment takes place over the course of several days of high-protein diet.(446) The stimulatory effect of the high-protein diet has been traced to specific amino acids. Methionine, glycine, and alanine appear to be more potent than the other amino acids in inducing increases in the activities of the urea cycle enzymes (Snodgrass and Lin, 1981).
단백질 대사의 부산물
The primary end products of protein breakdown is ammonia, a highly toxic material. Fishs excrete ammonia across gills. Insects, reptiles, and birds convert into uric acid, a nontoxic, almost insoluble compound. This enables them to lose little water. Uric acid can be precipitated into solid crystals and stored harmlessly within the eggs until hatching (Hickman .1988;p170)
How terrestrial animals maintain salt and water balance. In: Hickman CP, Jr, Roberts LS, Larson A, ed. Biology of Animals. 7th ed. WCB/McGraw-Hill 1998;170.
다이어트시의 문제
The loss of lean body mass is an undesirable consequence when attmpting to lose weight by total fasting of by consuming carbohydrate-only diet. Human subjects achieve zero nitrogen balance from the third to the eighth weeks after a semistarvation diet supplied 60 g of protein and 54 g of carbohydrate per day. (Brody, 1999;456-7)
체중감량
Modified fasting using low-energy diet is used to treat obesty. The modified fast supplies 1.2 to 1.4 g of protein per kilogram of ideal body weight. The protein can be supplied by lean meat, fish, or poultry. Potassium losses are a vital concern because low plasma potassium can induce arrhythmias. (p476).
Total fasting results in catabolism of lean body mass which accounts for about 50% of the weight loss in the first month of a total fast and for 25 to 33% thereafter. Total fasting can provoke cardiac arrythmias, destruction of cardiac muscles, and hepatic and renal damage. (p476). Total fest induces the loss of urinary nitrogen equivalent to about 80g of the body's protein per day.(p458)
Carbohydrate-only diet induces the loss of urinary nitrogen equivalent to about 40g of the bodily protein per day. The losses of skeletal muscle protein tend to be minimized, and loss of other proteins such as serum albumin and retinol binding protein continue with the carbohydrate-containing, protein-free diet. These proteins are used by the liver for the synthesis of more vital proteins.(458-459)
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Sodium
Adequate dietary intakes of sodium and chloride for the adults are estimated to be 1.1-1.3 and 1.7-5.1 g per day, respectively (p118).
In societies accustomed to widespread use of foods heavily salted for preservation, daily intakes may approach or even exceed 100 g. Body weight remains normal under such conditions. However, with an acute ingestion of high salt intake, much above 20 g a day, the kidney cannot excrete sodium as fast as in enters. The unexcreted sodium chloride holds additional water in the body fluids and caused swelling, increase in body weight, and some increase in blood pressure.(Hickman .1988;p177)
Tubular reabsorption. In: Hickman CP, Jr, Roberts LS, Larson A, ed. Biology of Animals. 7th ed. WCB/McGraw-Hill 1998;177.
The usual intake of sodium is 1.8 to 5.0 g/day. About one-third of the sodium is supplied by bread and cereals, 20% by meats, and 14% by dairy products.(p702)
소금과 같이 먹으면 영양분의 흡수가 잘 된다. 탈수 치료시 포도당과 염분 같이 준다.
The transport of glucose, amino acids, and various ions across membranes may require the cotransport of sodium ions. The transport proteins that mediate the passage of these nutrients across the membrane recognize and bind both sodium and the nutrient. (p703).
When oral rehydration therapy is commenced, the glucose aids in intestinal absorption of sodium ions, because glucose transport is mediated by the Na-glucose cotransporter.(723)
운동시
The most vital health concern during prolonged exercise is maintenance of the body's temperature. A typical rate of sweat loss, during a marathon race, for example, is 1 litter per hour. A marathon taking place in a warm humid climate can evoke fluid loss of 2.8 liters per hours. The American Dieteric Association recommends that fluid losses occurring with exercise of moderate duration ( 1 hour or less) be replaced with plain cool water (Smith, 1987). In hot and humid environments, the fluid should be replaced by drinking cool water prior to endurance exercise and at 15-minute intervals during exercise. (726)
Research indicates that sodium and potassium supplements are not required during running races of duration less than 5 hour. Low salt diet should not be consumed with repeated days of moderate-duration (5 hours) exercise. (726-7)
During ultramarathons, races of 50 miles lasting 8 hours, the consumtion of plain water at a rate greater than that lost by sweating may produce hyponatremia. Adaptation takes a few days for maximal sodium conservation and about a week for maximal potassium conservation (728).
Potasium
The minimal requirement for potassium is about 1.6-2.0 g/day. Fruits and vegetables contain high levels of potassium.(702)
물
Water molecules are able to diffuse through phospholipid membrane. The permaeability to water of the cell membrane is much higher than to the other molecules. This increase permeability is due to the specific membrane-bound proteins called water channels or aquaporins (p121).
It is recommended that an intake of 1000 ml of water per 1000 kcal of energy expended for adults, About 1400 ml is lost in the urine, 100-200 ml with the feces, and about 1250 ml by insensible losses. The obligatory loss of urine for the young adult is 500 to 600 ml per day (699).
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Regulation of energy metabolism
Energy regulation involves the brain, liver, muscle, pancreas, and energy stores such as fat in adipose tissue and glycogen in muscle and liver.(157) The relationship changes with the body's activities, including feeding, brief exercise, prolonged exercise, fasting, and starvation. The overall direction of energy flux is controlled mainly by two hormones: glucagon and insulin. Insulin secretion is mainly regulated by glucose. Glucagon secretion is inhibited by glucose and insulin and stimulated by amino acids. ---- 단백질 섭취시 glucagon 증가, 대사 증가. (Philippe, 1991) The effects of these hormones are controled by not their plasma concentrations but the ratio of their concentration, (glucagon)/(insulin).(158).
Exercise nutrition
The fuel used for exercise are arranged in the following hierarchy:(1) creatine phosphate, (2) muscle and liver glycogen, (3) gluconeogenesis, and (4) fatty acids. Fatty acids are the major fuel of muscle at all times. Creatinie phosphate is important only during the early stage of exercise (i.e., during sudden bursts of work). Glycogen tends to become depleted during exercise of moderate duration. With this depletion, the importance of gluconeogenesis increases. During prolonged exercise, FFAs supply most of the energy requirements of muscle. Ketone bodies, which are made by the liver, are also used as fuel by exercising muscle, but their contribution is quite small compared with fatty acids.(195).
At rest, the muscle's main fuel is its store of fat. At rest, the brain is the major consumer of glucose in the body. WIth mild exercise (at about 25% maximal O2 utilization), most of the muscle's energy (80%) is supplied by the adipose tissue, that is, in the form of free fatty acids liberated from this tissue and released into the bloodstream. With moderate exercise (about 65% maximal O2 utilization), the muscle's energy is supplied in roughly equal proportions from muscle glycogen, fat stores within the muscle, and from the free fatty acids liberated from adipose tissue. Where moderate exercise is continued for two hours, or longer, the energy supplied by adipose tissue becomes increasingly important. With short bursts of extreme exercise (85% maximal O2 utilization) muscle glycogen supplies most (60%) of the energy source for muscle (Holloszy and Kohrt, 1996). (197)
Creatine phosphate
Creatine-P serves as an energy buffer in muscle. A sudden burst of exercise or brief period of exhaustive movements may deplete cellular ATP before hormonal changes can activate glycogen phosphorylase or hormone-sensitive lipase. Glucose is readily available, but, a decrease in glucose is not desirable, because glucose is required by the CNS.(201).
Creatine is synthesized in the liver from glycine, and is transported to the mitochondria, and is phosphorylated to creatine-P. The creatine-P travels to myofibril, where contraction of myofibril is coupled to the hydrolysis of ATP to ADP.(201)
The reservoir of creatine-P substantially is depleted after a human subject exercises vigorously for 6 minutes.
Creatinine
About 1.6% of the body's content of creatine spontaneously breakes down daily to form creatinine. The amount of creatine excreted closely reflects the body's skeletal muscle mass; 1 g of urinary creatine per day represents about 18 kg of muscle.(203). The human body generates and excretes about 1.7 g of creatinine per day. Cooking meat results in the conversion of creatine to creatinine, which, after ingestion, is rapidly excreted into the urine. Dietary creatinine may reach 0.5 g per day.(203)
Glycogen
Glycogen is depleted by about an hour of sternous exercise. A 70-kg man contains about 350 g of muscle glycogen, 80 g of liver glycogen, and 20 g of glucose in extracellular fluids. If this man exercised exhaustively for 2 hours, the glycogen content decreases to less than 20% of its initial. Much of the glycogen was restored over the course of half a day, and full restoration required about 2 days. (206).
Exhaustion, during prolonged exercise, occurs coincidentally with the depletion of muscle glycogen. This impairs the oxidation of the available free fatty acids. Therefore, eating sugar before the exaustion point does not work. Temporarily shifting to a mild rate of exercise or complete rest allows the re-synthesis of muscle glycogen, and then vigorous exercise can be resummed. Furthermore, the practice of exercising, then resting and eating sugar, has the end-result of building up muscle glycogen store to twice the usual size. This practice is called carbohydrate loading or glycogen supercompensation(207). Glycogen supercompensation occurs only in muscles that have been recently exercised. A maximal rate of muscle glycogen restoration occurs with glucose of surcrose consumption. Frutose consumption can restores glycogen, but only at half the rate produced by glucose or sucrose. (207)
마라톤 전략
The American Dietetic Association (Smith, 1987) recommended the following carbohydrate loading procedure for maximizing muscle glycogen deposits during the week before athletic competetion. This procedure is beneficial only to atheletes participatin in endurance events lasting more than 1.5 hours. About 350 g of carbohydrate is consumed for each of 3 to 4 consecutive days; this amount is increased to about 550 g for the 3 days prior to the event. The athlete should begin a tapered rest period at the start of the week and have complete rest the dat before the event.(207)
Gluconeogenesis
During exercise, the lactate produced and released by fermentative muscle is taken up by the liver. The glycerol released from TGs enters the bloodstream and used for gluconeogenesis. The amino acids released from skeletal muscle and sent to the liver are converted to glucose during exercise. The muscle proteins broked down during exercise are not those of the myofbrils.
During rest, the estimated contribution of lactate to glucose production is one-sixth the total glucose output. WIth prolonged exercise, lactate taken up by the liver is also one-sixth of total glucose output. These facts suggest that, during rest, the glucogenic nutrients taken up by the liver account for only about one-fifth of glucose ouput; however, with 240 minutes of exercise, this production increases dramatically to about 50% of glucose output. During both rest and exercise, the remaining glucose output results from release of glucose units from hepatic glycogen stores.(208)
The glycerol released during lipolysis in adipose tissue can be used for the synthesis of TGs or oxidized within the adipose tissue. Both of these uses would be expected to be minimized during exercise, when gluconeogenesis becomes increasingly important.(213). With exercise, most of this glycerol travels to the liver for conversion to glucose.(213).
Brain
The brain stores very little of its own enegy, and relies almost exclusively on a constant supply of glucose from the bloodstream.(156).
Unlike most other organs, the brain does not use free fatty acids which are abundant in the circulation. The brain cannot regulate its energy requirments in response to deficiencies in the diet. The brain does not have reserves of energy. It contains only a small amount of glycogen and is dependent on a continuous supply of glucose. The brain is not sensitive to glucagon or insulin. However the brain is flexible. After a few days of fasting makes use of a new fuel that appears in the bloodstream: ketone body. (161).
Glucagon and insulin
The pancreas detect changes in the concentration of plasma glucose, and responds by altering its rates of secretion of glucagon and insulin. Glucagon stimutates its target organs to liberate fuel stores. Insulin stimulates its target organs to store energy fuels.(161)
Insulin antagnozes the effects of glucagon. However, insulin also has effects that are independent of glucagon. One such independent effect is activation of glucose transport systems. Insulin stimulates the transport of glucose from the plasma into adipose tissue and muscle, but not into liver, brain, or red blood cells.(163)
Hormonal picture with exercise and fasting
The hormonal pictures, as far as glucagon and insulin are concerned, is similar during exercise and fasting. The concentration of plasma glucagon increases during both states. (163) Glucagon increases with both exercise and fasting, but, epinephrine increases only during exercise.(213)
Hormonal picture with feeding
Feeding provokes an increase in the concentration of plasma insulin. The most well-known glucagon-independant effect is an increase in glucose transport into muscle for the purpose of glycogen synthesis and into adipose tissue for the purpose of fatty acid synthesis.(181).
Different types of energy metabolism
There are pathways of oxidation of glucose, fatty acids, and other nutrients.
The first type is complete oxidation of glucose to carbon dioxide. The brain relies almost exclusively on a constant supply of glucose.
The second type is complete oxidation of fatty acids to carbon dioxide. The major fuel for most organs is fatty acids. The liver uses fatty acids as its main fuel. The liver's main fuction is to synthesize and store glucoes and to release glucose for the benefit of other organs.
In the third type of energy metabolism, partial oxidation of glucose by the pathway of glycolysis, glucose is oxidized only as far as pyruvate. The pyruvate is reduced to lactic acid, which is released into the bloodstream for further processing by the liver. This type of metabolism is called fermentation. Under certain conditions, muscle utilizes fermentative metabolism.
The fourth type of metabolism is complete oxidation of glucose and fatty acid. Exercising muscle generally uses this type of metabolism, whereas resting muscle tend to derive energy through the complete oxidation of fatty acids. (180-181).
Synthesis and Degradation of Glycogen
Carbohydrate is stored in liver and muscle in the form of glycogen.(183). Epinephrine and norepinephrine stimutate effects in muscles and liver silimar to those produced by glucagon, resulting in the breakdown of glycogen and fatty acids.(185). In the liver, glycogen can be converted to glucose-6-phosphate for use in glycolysis or hydrolyzed to glucose. In muscle, glycogen can be converted to glucose-6-phosphate for use in glycolysis, but cannot be hydrolyzed to glucose. Thus, muscle cannot supply glucose to the bloodstream.(185). Muscle does not contain glucose-6-phosphatase and thus cannot export glucose stored as muscle glycogen (188). On the contrary, in muscle, glucose derived from the bloodstream is converted to G-6-P, then to pyruvate. This pyruvate can be oxidized in mitochondria or can enter the bloodstream in the form of lactate.
What happens when you rest.
In resting state, liver and muscle derive most of their energy from fatty acids released from adipose tissue. In the resting state, glycogen in the liver is broken down to glucose.
What happens when you exercise
Fatty acids continue to be a major energy fuel in exercise, but the overall consumption of glucose by muscle increases. Some of this glucose is oxidized into carbon dioxide, some only to pyruvate. Most of the glucose oxidized in exercising muscle arises from the muscle's own glycogen. This pyruvate may be converted to lactic acid. This lactic acid is carried to the liver, and is used for synthesis of glucose, which reenters the bloodstream.(194).
What happens when you eat
Much of the dietary glucose is taken up by muscle and is converted to glycogen, some is metabolized via glycolysis, and some exits the muscle in the form of lactate for transport to the liver. Apparently, relatively little of the glucose entering the bloodstream from the diet is used by the liver for glycogen synthesis.
With the breaking of a fast, dietary carbohydrates are used first, mainly for the synthesis of muscle glycogen. Following the replenishment of this energy store, liver glycogen begins to be restored. Following replenishment of liver glycogen, the remaining dietary carbohydrate may be converted to fat. The liver is the main site of fat biosynthesis in some animals (humans and birds); adipose tissue is the main site in others (ruminants and pigs).(Bergman, 1990. Physiol Rev 70;567-590) (195)
The proportion of dietary carbohydrate converted to fat is small compared with that used for glycogen synthesis. Dietary fat is the major source of the fat in our fat stores. (195) 동물성 지방을 먹을 때가 탄수화물을 먹을 때보다 비만이 잘 생긴다.
Peripheral tissues such as muscle, adipose tissue, and the lactating breast contain both hormon-sensitive lipase and lipoprotein lipase. Lipoprotein lipase may be activated by changes in plasma glucagon and insulin. A decrease in glucagon/insulin ratio, as with feeding, may provoke an increase in the proportion of enzyme located on the luminal wall of the capillary and decrease in the proportion that is intracellular. Feeding appears to provoke activation of the enzyme in adipose tissue, but not of the enzyme located in muscle or other tissue.(215)
Issues in evergy nutriton
One aspect of obesity is the question of the energy cost involved in in the formation of energy stores. Deposition of various nutrients into energy sotores requires the expenditure of energy. Energy is required for the sysnthesis of glycogen and triglycerides. The energy cost of depositing dietary fat into adipose tissue is relatively low; the energy used for this process is equivalent to about 3.0% of the total amount of energy contained in the TGs. The energy cost of depositing dietary carbohydrates as glycogen is equivalent to about 7.0% of the energy contained in the carbohydrate. The energy cost of converting dietary carbohydrate to TGs is high; it is equivalent to about 23% of the energy of the carbohydrate.(Sims and Danforth, 1987). These figure indicate that a person desiring to build up his or her stores of fat could more effeciently do so by consuming excessive amounts of fat rather than excessive amounts of glucose. (305-6).
버섯은 생태계의 분해자로서, 모든 유기물을 자연으로 돌려놓은 환원자로서의 기능을 한다는