How you get energy; how you use it

The Three Metabolic Energy Systems

We usu­al­ly talk of ener­gy in gen­er­al terms, as in “I don’t have a lot of ener­gy today” or “You can feel the ener­gy in the room.” But what real­ly is ener­gy? Where do we get the ener­gy to move? How do we use it? How do we get more of it? Ulti­mate­ly, what con­trols our move­ments? The three meta­bol­ic ener­gy path­ways are the phos­pha­gen sys­tem, gly­col­y­sis and the aer­o­bic sys­tem. How do they work, and what is their effect?

Albert Ein­stein, in his infi­nite wis­dom, dis­cov­ered that the total ener­gy of an object is equal to the mass of the object mul­ti­plied by the square of the speed of light. His for­mu­la for atom­ic ener­gy, E = mc2, has become the most rec­og­nized math­e­mat­i­cal for­mu­la in the world. Accord­ing to his equa­tion, any change in the ener­gy of an object caus­es a change in the mass of that object. The change in ener­gy can come in many forms, includ­ing mechan­i­cal, ther­mal, elec­tro­mag­net­ic, chem­i­cal, elec­tri­cal or nuclear. Ener­gy is all around us. The lights in your home, a microwave, a tele­phone, the sun; all trans­mit ener­gy. Even though the solar ener­gy that heats the earth is quite dif­fer­ent from the ener­gy used to run up a hill, ener­gy, as the first law of ther­mo­dy­nam­ics tells us, can be nei­ther cre­at­ed nor destroyed. It is sim­ply changed from one form to anoth­er.

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ATP Resynthesis

The ener­gy for all phys­i­cal activ­i­ty comes from the con­ver­sion of high-ener­gy phos­phates (adeno­sine triphosphate—ATP) to low­er-ener­gy phos­phates (adeno­sinediphosphate—ADP; adeno­sine monophos- phate—AMP; and inor­gan­ic phos­phate, Pi). Dur­ing this break­down (hydrol­y­sis) of ATP, which is a water-requir­ing process, a pro­ton, ener­gy and heat are pro­duced: ATP + H2O —© ADP + Pi + H+ + ener­gy + heat. Since our mus­cles don’t store much ATP, we must con­stant­ly resyn­the­size it. The hydrol­y­sis and resyn­the­sis of ATP is thus a cir­cu­lar process—ATP is hydrolyzed into ADP and Pi, and then ADP and Pi com­bine to resyn­the­size ATP. Alter­na­tive­ly, two ADP mol­e­cules can com­bine to pro­duce ATP and AMP: ADP + ADP© ATP + AMP.

Like many oth­er ani­mals, humans pro­duce ATP through three meta­bol­ic path­ways that con­sist of many enzyme-cat­alyzed chem­i­cal reac­tions: the phos­pha­gen sys­tem, gly­col­y­sis and the aer­o­bic sys­tem. Which path­way your clients use for the pri­ma­ry pro­duc­tion of ATP depends on how quick­ly they need it and how much of it they need. Lift­ing heavy weights, for instance, requires ener­gy much more quick­ly than jog­ging on the tread­mill, neces­si­tat­ing the reliance on dif­fer­ent ener­gy sys­tems. How­ev­er, the pro­duc­tion of ATP is nev­er achieved by the exclu­sive use of one ener­gy sys­tem, but rather by the coor­di­nat­ed response of all ener­gy sys­tems con­tribut­ing to dif­fer­ent degrees.

1. Phosphagen System

Dur­ing short-term, intense activ­i­ties, a large amount of pow­er needs to be pro­duced by the mus­cles, cre­at­ing a high demand for ATP. The phos­pha­gen sys­tem (also called the ATP-CP sys­tem) is the quick­est way to resyn­the­size ATP (Robergs & Roberts 1997). Cre­a­tine phos­phate (CP), which is stored in skele­tal mus­cles, donates a phos­phate to ADP to pro­duce ATP: ADP + CP© ATP + C. No car­bo­hy­drate or fat is used in this process; the regen­er­a­tion of ATP comes sole­ly from stored CP. Since this process does not need oxy­gen to resyn­the­size ATP, it is anaer­o­bic, or oxy­gen-inde­pen­dent. As the fastest way to resyn­the­size ATP, the phos­pha­gen sys­tem is the pre­dom­i­nant ener­gy sys­tem used for all-out exer­cise last­ing up to about 10 sec­onds. How­ev­er, since there is a lim­it­ed amount of stored CP and ATP in skele­tal mus­cles, fatigue occurs rapid­ly.

2. Glycolysis

Gly­col­y­sis is the pre­dom­i­nant ener­gy sys­tem used for all-out exer­cise last­ing from 30 sec­onds to about 2 min­utes and is the sec­ond-fastest way to resyn­the­size ATP. Dur­ing gly­col­y­sis, carbohydrate—in the form of either blood glu­cose (sug­ar) or mus­cle glyco­gen (the stored form of glucose)—is bro­ken down through a series of chem­i­cal reac­tions to form pyru­vate (glyco­gen is first bro­ken down into glu­cose through a process calledglycogenol­y­sis). For every mol­e­cule of glu­cose bro­ken down to pyru­vate through gly­col­y­sis, two mol­e­cules of usable ATP are pro­duced (Brooks et al. 2000). Thus, very lit­tle ener­gy is pro­duced through this path­way, but the trade-off is that you get the ener­gy quick­ly. Once pyru­vate is formed, it has two fates: con­ver­sion to lac­tate or con­ver­sion to a meta­bol­ic inter­me­di­ary mol­e­cule called acetyl coen­zyme A (acetyl-CoA), which enters the mito­chon­dria for oxi­da­tion and the pro­duc­tion of more ATP (Robergs & Roberts 1997). Con­ver­sion to lac­tate occurs when the demand for oxy­gen is greater than the sup­ply (i.e., dur­ing anaer­o­bic exer­cise). Con­verse­ly, when there is enough oxy­gen avail­able to meet the mus­cles’ needs (i.e., dur­ing aer­o­bic exer­cise), pyru­vate (via acetyl-CoA) enters the mito­chon­dria and goes through aer­o­bic metab­o­lism.

When oxy­gen is not sup­plied fast enough to meet the mus­cles’ needs (anaer­o­bic gly­col­y­sis), there is an increase in hydro­gen ions (which caus­es the mus­cle pH to decrease; a con­di­tion called aci­do­sis) and oth­er metabo­lites (ADP, Pi and potas­si­um ions). Aci­do­sis and the accu­mu­la­tion of these oth­er metabo­lites cause a num­ber of prob­lems inside the mus­cles, includ­ing inhi­bi­tion of spe­cif­ic enzymes involved in metab­o­lism and mus­cle con­trac­tion, inhi­bi­tion of the release of cal­ci­um (the trig­ger for mus­cle con­trac­tion) from its stor­age site in mus­cles, and inter­fer­ence with the mus­cles’ elec­tri­cal charges (Eno­ka & Stu­art 1992; Glais­ter 2005; McLester 1997). As a result of these changes, mus­cles lose their abil­i­ty to con­tract effec­tive­ly, and mus­cle force pro­duc­tion and exer­cise inten­si­ty ulti­mate­ly decrease.

3. Aerobic System

Since humans evolved for aer­o­bic activ­i­ties (Hochachka, Gun­ga & Kirsch 1998; Hochachka & Mon­ge 2000), it’s not sur­pris­ing that the aer­o­bic sys­tem, which is depen­dent on oxy­gen, is the most com­plex of the three ener­gy sys­tems. The meta­bol­ic reac­tions that take place in the pres­ence of oxy­gen are respon­si­ble for most of the cel­lu­lar ener­gy pro­duced by the body. How­ev­er, aer­o­bic metab­o­lism is the slow­est way to resyn­the­size ATP. Oxy­gen, as the patri­arch of metab­o­lism, knows that it is worth the wait, as it con­trols the fate of endurance and is the sus­te­nance of life. “I’m oxy­gen,” it says to the mus­cle, with more than a hint of supe­ri­or­i­ty. “I can give you a lot of ATP, but you will have to wait for it.”

The aer­o­bic system—which includes the Krebs cycle (also called the cit­ric acid cycle or TCA cycle) and the elec­tron trans­port chain—uses blood glu­cose, glyco­gen and fat as fuels to resyn­the­size ATP in the mito­chon­dria of mus­cle cells (see the side­bar “Ener­gy Sys­tem Char­ac­ter­is­tics”). Giv­en its loca­tion, the aer­o­bic sys­tem is also called mito­chon­dr­i­al res­pi­ra­tion. When using car­bo­hy­drate, glu­cose and glyco­gen are first metab­o­lized through gly­col­y­sis, with the result­ing pyru­vate used to form acetyl-CoA, which enters the Krebs cycle. The elec­trons pro­duced in the Krebs cycle are then trans­port­ed through the elec­tron trans­port chain, where ATP and water are pro­duced (a process called oxida­tive phos­pho­ry­la­tion) (Robergs & Roberts 1997). Com­plete oxi­da­tion of glu­cose via gly­col­y­sis, the Krebs cycle and the elec­tron trans­port chain pro­duces 36 mol­e­cules of ATP for every mol­e­cule of glu­cose bro­ken down (Robergs & Roberts 1997). Thus, the aer­o­bic sys­tem pro­duces 18 times more ATP than does anaer­o­bic gly­col­y­sis from each glu­cose mol­e­cule.

Fat, which is stored as triglyc­eride in adi­pose tis­sue under­neath the skin and with­in skele­tal mus­cles (called intra­mus­cu­lar triglyc­eride), is the oth­er major fuel for the aer­o­bic sys­tem, and is the largest store of ener­gy in the body. When using fat, triglyc­erides are first bro­ken down into free fat­ty acids and glyc­erol (a process called lipol­y­sis). The free fat­ty acids, which are com­posed of a long chain of car­bon atoms, are trans­port­ed to the mus­cle mito­chon­dria, where the car­bon atoms are used to pro­duce acetyl-CoA (a process called beta-oxi­da­tion).

Fol­low­ing acetyl-CoA for­ma­tion, fat metab­o­lism is iden­ti­cal to car­bo­hy­drate metab­o­lism, with acetyl-CoA enter­ing the Krebs cycle and the elec­trons being trans­port­ed to the elec­tron trans­port chain to form ATP and water. The oxi­da­tion of free fat­ty acids yields many more ATP mol­e­cules than the oxi­da­tion of glu­cose or glyco­gen. For exam­ple, the oxi­da­tion of the fat­ty acid palmi­tate pro­duces 129 mol­e­cules of ATP (Brooks et al. 2000). No won­der clients can sus­tain an aer­o­bic activ­i­ty longer than an anaer­o­bic one!

Under­stand­ing how ener­gy is pro­duced for phys­i­cal activ­i­ty is impor­tant when it comes to pro­gram­ming exer­cise at the prop­er inten­si­ty and dura­tion for your clients. So the next time your clients get done with a work­out and think, “I have a lot of ener­gy,” you’ll know exact­ly where they got it.

by: Jason Karp, PhD

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