Understanding energy systems underpins the study of exercise and the effect it has on the human body
Any of a group of organic compounds, including the fats, oils, waxes, sterols, andtriglycerides, that are insoluble in water but soluble in nonpolar organic solvents, are oily tothe touch, and together with carbohydrates and proteins constitute the principalstructural material of living cells.Any of a diverse group of organic compounds that are grouped together because theydo not interact appreciably with water. One of the three large classes of substances infoods and living cells, lipids contain more than twice as much energy (calories) per unit ofweight as the other two (proteins and carbohydrates). They include the fats and edibleoils (e.g., butter, olive oil, corn oil), which are primarily triglycerides; phospholipids (e.g.,lecithin), which are important in cell structure and metabolism; waxes of animal or plantorigin; and sphingolipids, complex substances found in various tissues of the brain andnervous system. Since insolubility is the defining characteristic, cholesterol and relatedsteroids, carotenoids ( carotene), prostaglandins, and various other compounds are alsoclassifiable as lipids.Read more: http://www.answers.com/topic/lipid#ixzz1pPic9Lv7Simple LipidsTriglycerides, neutral fats: Found in adipose tissue, butterfat, lard, suet, fish oils, olive oil,corn oil, etc. Esters of three molecules of fatty acids plus one molecule of glycerol; thefatty acid may all be different.Waxes: beeswax, head oil of sperm whale, cerumen, carnauba oil, and lanolin.Composed of esters of fatty acids with alcohol other than glycerol; of industrial andmedicinal importance.Compound LipidsPhospholipids (phosphatides): Found chiefly in animal tissues. Substituted fats, consisting ofphosphatidic acid; composed of glycerol, fatty acids, and phosphoric acid bound inester linkage to a nitrogenous base.Lecithin: Found in brain, egg yolk, and organ meats. Phosphatidyl choline or serine;phosphatide linked to choline; a lipotropic agent; important in fat metabolism andtransport; used as emulsigying agent in the food industry.Cephalin: Occurs predominantly in nervous tissue. Phosphatidyl ethanolamine;phosphatide linage to serine or ethanolamine; plays a role in blood clotting.Plasmalogen: Found in brain, heart, and muscle. Phosphatidal ethanolamine or choline;phosphatide containing an aliphatic aldehyde.Lipositol: Found in brain, heart, kidneys, and plant tissues together with phytic acid.Phosphatidyl inositol; phosphatide linked to inositol; rapid synthesis and degradation inbrain; evidence for role in cell transport processes.Sphingomyelin: Found in nervous tissue, brain, and red blood cells. Sphingosine-containingphosphatide; yields fatty acids, choline, sphingosine, phosphoric acid, and no glycerol;source of phosphoric acid in body tissue.Glycolipids:
Cerebroside: myline sheaths of nerves, brain, and other tissues. Yields on hydrolysis of fattyacids, sphingosine, galactose (or glucose), but not fatty acids; includes kerasin andphrenosin.Ganglioside: brain, nerve tissue, and other selected tissues, notably spleen; contains aceramide linked to hexose (glucose or galactose), neuraminic acid, sphingosine, andfatty acids.Sulfolipid: white matter of brain, liver, and testicle; also plant chloroplast. Sulfur-containingglycolipid; sulfate present in ester linkage to galactose.Proteolipids: brain and nerve tissue. Complexes of protein and lipids having solubilityproperties of lipids.Terpenoids and SteroidsTerpenes: Found in essential oils, resin acids, rubber, plant pigments such as caoteneseand lycopenes, Vitamin A, and camphor. Large group of compounds made up ofrepeating isoprene units; Vitamin A of nutritional interest; fat soluble Vitamin E and K,which are also related chemically to terpenes.Sterols:Cholesterol: found in egg yolk, dairy products, and animal tissues. A consituent of bileacids and a precursor of Vitamin D.Ergosterol: found in plant tissues, yeast, and fungi. Converted to Vitamin D2 on irradiation.7-dehydrocholesterol: found in animal tissues and underneath skin. Converted to D3 onirradiation.Androgens and estrogens: (Sex hormones) Found in ovaries and testes.Adrenal corticolsteroids: adrenal cortex, blood.Derived lipidsFatty acids: occur in plant and animal foods; also exhibit in complex forms with othersubstances. Obtained from hydrolysis of fats; usually contains an even number of carbonatoms and are straight chain derivatives.Classification of fatty acids is based on the length of the carbon chain (short, medium, orlong); the number of double bonds (unsaturated, mono-, or polyunsaturated); oressentiality in the diet (essential or non-essential). A current designation is based on theposition of the endmost double bond, counting from the methyl (CH3) carbon, called theomega end. The most important omega fatty acids are: Omega 6 - linolein andarachidonic acids and Omega 3 - linolenic, eicosapentaenoic, and docosahexaenoicacids.Sample nomenclature for fatty acids:Name - Carbon Length : Number of Double Bonds(position of double bond)Butyric acid - 4:0
Palmitic acid - 16:0Oleic acid - 18:1 (9)Linoleic acid - 18:2 (9,12)Linolenic acid - 18:3 (9,12,15)Arachidonic acid - 20:4 (5,8,11,14)Eicosapentaenoic acid - 20:5 (5,8,11,14,17)Docosahexaenoic acid - 22:6 (4,7,10,13,16,19)Understanding energy systems underpins the study of exercise and the effect it has on thehuman body.Bioenergetics... or the study of energy flow through living systems is usually one of the firstchapters in any good exercise physiology text. But the current model of human energysystems is being challenged...Recent research and practical experience expose its limitations, in particular with regardto fatigue.This article outlines the three basic energy pathways, their interactions with one anotherand their relevance to different sporting activities. It finishes with a brief look at some ofthe more recent research and subsequent new models of human energy dynamics thathave been proposed as a result. ATP The Bodys Energy CurrencyEnergy is required for all kinds of bodilyprocesses including growth anddevelopment, repair, the transport ofvarious substances between cells and ofcourse, muscle contraction. It is this lastarea that Exercise Scientists are mostinterested in when they talk about energysystems.Whether its during a 26-mile marathon run or one explosive movement like a tennis serve,skeletal muscle is powered by one and only one compound... adenosine triphosphate(ATP) (2). However, the body stores only a small quantity of this energy currency withinthe cells and its enough to power just a few seconds of all-out exercise (5). So the bodymust replace or resynthesize ATP on an ongoing basis. Understanding how it does this isthe key to understanding energy systems.An ATP molecule consists of adenosine and three (tri) inorganic phosphate groups. Whena molecule of ATP is combined with water (a process called hydrolysis), the lastphosphate group splits away and releases energy. The molecule of adenosinetriphosphate now becomes adenosine diphosphate or ADP (2).To replenish the limited stores of ATP,chemical reactions add a phosphategroup back to ADP to create ATP. Thisprocess is called phosphorylation. If thisoccurs in the presence of oxygen it is
labelled aerobic metabolism or oxidative phosphorylation. If it occurs without oxygen it islabelled anaerobic metabolism (2). Energy Sources to Replenish ATPSeveral energy sources or substrates are available which can be used to power theproduction of ATP. One of these substrates, like existing ATP, is stored inside the cell and iscalled creatine phosphate.Creatine PhosphateCreatine phosphate is readily availableto the cells and rapidly produces ATP. Italso exists in limited concentrations andit is estimated that there is only about100g of ATP and about 120g of creatinephosphate stored in the body, mostlywithin the muscles. Together ATP andcreatine phosphate are called the high-energy phosphogens (1).FatThe other substrates that can the body can use to produce ATP include fat, carbohydrateand protein. Fat is stored predominantly as adipose tissue throughout the body and is asubstantial energy reservoir. Fat is less accessible for cellular metabolism as it must first bereduced from its complex form, triglyceride, to the simpler components of glycerol andfree fatty acids. So although fat acts as a vast stockpile of fuel, energy release is too slowfor very intense activity (5).CarbohydrateUnlike fat, carbohydrate is not stored in peripheral deposits throughout the body. At rest,carbohydrate is taken up by the muscles and liver and converted into glycogen.Glycogen can be used to form ATP and in the liver it can be converted into glucose andtransported to the muscles via the blood. A heavy training session can depletecarbohydrate stores in the muscles and liver, as can a restriction in dietary intake.Carbohydrate can release energy much more quickly than fat (5).ProteinProtein is used as a source of energy, particularly during prolonged activity, however itmust first be broken down into amino acids before then being converted into glucose. Aswith, fat, protein cannot supply energy at the same rate as carbohydrate. The rate atwhich is energy is released from the substrates is determined by a number of factors. Forexample, if there are large amounts of one type of fuel available, the body may rely moreon this source than on others. The mass action effect is used to describe this phenomenon(5). The Three Energy SystemsThere are three separate energy systems through which ATP can be produced. A numberof factors determine which of these energy systems is chosen, such as exercise intensity forexample. The ATP-PCr System
ATP and creatine phosphate (also called phosphocreatine or PCr for short) make up theATP-PCr system. PCr is broken down releasing a phosphate and energy, which is thenused to rebuild ATP. Recall, that ATP is rebuilt by adding a phosphate to ADP in a processcalled phosphorylation. The enzyme that controls the break down of PCr is called creatinekinase (5).The ATP-PCr energy system canoperate with or without oxygen butbecause it doesnt rely on thepresence of oxygen it said to beanaerobic. During the first 5 secondsof exercise regardless of intensity,the ATP-PCr is relied on almostexclusively. ATP concentrations lastonly a few seconds with PCrbuffering the drop in ATP for another5-8 seconds or so. Combined, theATP-PCr system can sustain all-outexercise for 3-15 seconds and it isduring this time that the potentialrate for power output is at itsgreatest (1).If activity continues beyond this immediate period, the body must rely on another energysystem to produce ATP The Glycolytic SystemGlycolysis literally means the breakdown (lysis) of glucose and consists of a series ofenzymatic reactions. Remember that the carbohydrates we eat supply the body withglucose, which can be stored as glycogen in the muscles or liver for later use.The end product of glycolysis is pyruvic acid. Pyruvic acid can then be either funnelledthrough a process called the Krebs cycle (see the Oxidative System below) or convertedinto lactic acid. Traditionally, if the final product was lactic acid, the process was labelledanaerobic glycolysis and if the final product remained as pyruvate the process waslabelled aerobic glycolysis.However, oxygen availability only determines the fate of the end product and is notrequired for the actual process of glycolysis itself. In fact, oxygen availability has beenshown to have little to do with which of the two end products, lactate or pyruvate isproduced. Hence the terms aerobic meaning with oxygen and anaerobic meaningwithout oxygen become a bit misleading (5).Alternative terms that are often used are fast glycolysis if the final product is lactic acidand slow glycolysis for the process that leads to pyruvate being funnelled through theKrebs cycle. As its name would suggest the fast glycolitic system can produce energy at agreater rate than slow glycolysis. However, because the end product of fast glycolysis islactic acid, it can quickly accumulate and is thought to lead to muscular fatigue (1).
The contribution of the fast glycolytic system increases rapidly after the initial 10 secondsof exercise. This also coincides with a drop in maximal power output as the immediatelyavailable phosphogens, ATP and PCr, begin to run out. By about 30 seconds of sustainedactivity the majority of energy comes from fast glycolysis (2).At 45 seconds of sustained activity there is a second decline in power output (the firstdecline being after about 10 seconds). Activity beyond this point corresponds with agrowing reliance on the The Oxidative System The oxidative system consists four processes to produce ATP: • Slow glycolysis (aerobic glycolysis) • Krebs cycle (citric acid cycle or tricarboxylic acid cycle) • Electron transport chain • Beta oxidationSlow glycolysis is exactly the same series of reactions as fast glycolysis that metaboliseglucose to form two ATPs. The difference, however, is that the end product pyruvic acid isconverted into a substance called acetyl coenzyme A rather than lactic acid (5).Following glycolysis, further ATP can be produced by funnelling acetyl coenzyme Athrough theKrebs CycleThe Krebs cycle is a complex series of chemical reactions that continues the oxidization ofglucose that was started during glycolysis. Acetyl coenzyme A enters the Krebs cycle andis broken down in to carbon dioxide and hydrogen allowing more two more ATPs to beformed. However, the hydrogen produced in the Krebs cycle plus the hydrogenproduced during glycolysis, left unchecked would cause cells to become too acidic (2).So hydrogen combines with two enzymes called NAD and FAD and is transported to theElectron Transport ChainHydrogen is carried to the electron transport chain, another series of chemical reactions,and here it combines with oxygen to form water thus preventing acidification. This chain,which requires the presence of oxygen, also results in 34 ATPs being formed (2).
Beta OxidationUnlike glycolysis, the Krebs cycle and electron transport chain can metabolise fat as wellas carbohydrate to produce ATP. Lipolysis is the term used to describe the breakdown offat (triglycerides) into the more basic units of glycerol and free fatty acids (2).Before these free fatty acids can enter the Krebs cycle they must undergo a process ofbeta oxidation... a series of reactions to further reduce free fatty acids to acetylcoenzyme A and hydrogen. Acetyl coenzyme A can now enter the Krebs cycle and fromthis point on, fat metabolism follows the same path as carbohydrate metabolism (5).Fat MetabolismSo to recap, the oxidative system can produce ATP through either fat (fatty acids) orcarbohydrate (glucose). The key difference is that complete combustion of a fatty acidmolecule produces significantly more acetyl coenzyme A and hydrogen (and hence ATP)compared to a glucose molecule. However, because fatty acids consist of more carbonatoms than glucose, they require more oxygen for their combustion (2).So if your body is to use fat for fuel it must have sufficient oxygen supply to meet thedemands of exercise. If exercise is intense and the cardiovascular system is unable tosupply cells with oxygen quickly enough, carbohydrate must be used to produce ATP. Putanother way, if you run out of carbohydrate stores (as in long duration events), exerciseintensity must reduce as the body switches to fat as its primary source of fuel.Protein MetabolismProtein is thought to make only a small contribution (usually no more 5%) to energyproduction and is often overlooked. However, amino acids, the building blocks of protein,can be either converted into glucose or into other intermediates used by the Krebs cyclesuch as acetyl coenzyme A. Protein may make a more significant contribution during veryprolonged activity, perhaps as much as 18% of total energy requirements (1).The oxidative system as a whole is used primarily during rest and low-intensity exercise. Atthe start of exercise it takes about 90 seconds for the oxidative system to produce itsmaximal power output and training can help to make this transition earlier (1).Beyond this point the Krebs cycle supplies the majority of energy requirements but slowglycolysis still makes a significant contribution. In fact, slow glycolysis is an importantmetabolic pathway even during events lasting several hours or more (2). Energy Systems & TrainingEach of the three energy systems can generate power to different capacities and varieswithin individuals. Best estimates suggest that the ATP-PCR systme can generate energy ata rate of roughly 36 kcal per minute. Glycolysis can generate energy only half as quicklyat about 16 kcal per minute. The oxidative system has the lowest rate of power output atabout 10 kcal per minute (4).The capacity to generate power of each the three energy systems can vary with training.The ATP-PCr and glycolytic pathways may change by only 10-20% with training. Theoxidative system seems to be far more trainable although genetics play a limiting rolehere too. VO2max, or aerobic power can be increased by as much as 50% but this isusually in untrained, sedentary individuals (4).
Energy Systems Used in SportsThe three energy systems do not work independently of one another. From very short, veryintense exercise, to very light, prolonged activity, all three energy systems make acontribution however, one or two will usually predominate (5).Two factors of any activity carried out affect energy systems more than any other variablethey are the intensity and duration of exercise. Here is a list of sports and approximatelyhow the each of the energy systems contributes to meet the physical demands: A New Model for Energy Systems?In the year 2000, Noakes and colleagues (3) questioned the classical model of energysystems. Their argument was based on the limitations this model has when it comes toexplaining fatigue. In particular, the general concept that fatigue develops only whenthe cardiovascular systems capacity to supply oxygen falls behind demand (thereforeinitiating anaerobic metabolism) is seen as overly simplistic. More specifically, theirargument centered around 5 key issues:i) The heart and not skeletal muscle would be affected first by anaerobic metabolism.ii) No study has definitively found a presence of anaerobic metabolism and hypoxia (lackof oxygen) in skeletal muscle during maximal exercise.iii) The traditional model is unable to explain why fatigue ensues during prolongedexercise, at altitude and in hot conditions.iv) Cardiorespiratory and metabolic measures such as VO2max and lactate threshold areonly modest predictors of performance.
Undoubtedly, fatigue is a complex subject that can result from a range of physical andpsychological factors. In an attempt to produce a more holistic explanation, Noakesdeveloped a model that consisted of five sub-models:i) The classical cardiovascular / anaerobic model as it stands now.ii) The energy supply / energy depletion model.iii) The muscle recruitment (central fatigue) / muscle power model.iv) The biomechanical model.v) The psychological / motivational model.Essentially this new model of energy systems recognizes what coaches have witnessed fordecades... that performance and fatigue is multifactoral and complex. It adds strength tothe synergistic and holistic approach to sport usually found in the most successful athletes.