Using Concepts of Exercise and Weight Control to Illustrate Biochemical Principles S. Scott Zimmerman Brigham Young University, Provo, UT 84602 An estimated 59% of all adult Americans engage in some tvDe of nhvsical activitv. (I). . . Fiftv million of these aet their exercise through jogging, and some 70,000 have a run a marathon (42.2 km or 26.2 miles) (I). This means that in a university biochemistry class of 50 "average" American adults, about 30 students would engage in regular physical exercise. (The actual number of active students would probably be hiaher because . vounger - neonle . . exercise more than older, those ofbigher socioeconomic levels more than those of lower, and those with hieher education more than those with less.) Among those same 50 biochemistry students, approximately 29 (58%) are or will become obese (2). Almost a l l of these will try dieting to reduce their weight, with probably less than 2% achieving long-term success (3). With this level of interest in fitness and weight control, students are enthusiastic about seeing basic facts of exercise and nutrition presented as a means of illustrating biochemical principles (4). These illustrations are beneficial in motivating and in helnine students better understand and remember the relationskp:hetween key metabolic pathways. The purpose of this paper is to provide some of the basic information on exercise and diet which I have found to be of interest and help to students.
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Forms of Stored Energy Energy for muscle contraction, active transport across membranes. and biosvnthesis comes nrimarilvfrom the e n e m "currency" bf the hody, ATP (adenosine 5'-t;iphosphate). B% the ATP concentration in muscle remains amazingly constant, a t only 5 to 7 mM, whether the muscle fibers are actively contracting or not (5). The cycling rate of ATP-ADP greatly increases, of course, as muscles go from rest to heavy exercise. This means that the muscle must be able to store, or obtain from other stores, the fuel necessary to regenerate ATP from A. np . -A Four energy stores are available for producing ATP to drive muscle contraction: creatine phosphate (CP), glycogen, fat, and protein. These same stores are also available for fuel t o drive the other life processes between meals, during fast or
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Table 1.
Energy (kcal per gram of dry substrate) Energy stored: (moles of ATP equivalents per kg storage tissue)
Oxygen requirement: (liters per mole of ATP equivalents) Maximal Power: (moles of ATP per minute) Maximal Exercise Duration: (in minutes)
Calorlc Balance, Exercise, and Weight Control Table 1also can he used to estimate the number of calories required t o gain or lose a pound of fat. Nine kcallg is equivalent to about 4000 kcalilb, or the more commonly quoted figure 3500 Calilb, where the dietary Cal equals one kcal. Thus, in order t o lose a pound of fat, a person must eat 3500 kcal less than his energy expenditure. Or stated another way, he must expend 3500 kcal more than he eats. Table 2 gives some typical activities and foods which equal about 100 kcal
Stored Fuels In the Human Bodva
Fuel Substrate:
Total energy stared: (moles of ATP equivalents)
starvation, and of course during a weight-reduction diet. The concentrations and energy content of use of these fuels vary considerably (6, 7),as is shown in Table 1. The last two data lines of Table 1 indicate that as the maximal power of a particular energy source increases, the maximal duration of the exercise decreases. Thus, an all-out sprint (maximal rate of ATP utilization) obtains energy almost exclusively from CP, but can last only 0.2 min (or about 12 sec), the world record in the 100-m dash being 9.95 sec. At the other extreme, the marathon derives its energy from aerobic metabolism of glycogen (about 80%) and fat (about 20%). Obviously, because of the time involved in covering the 42.2 km,the energy source for the marathon must be different from that for the sprint, and therefore the maximal power (and hence the rate of running) will be less. With the current (October 1981) world record in the marathon at 2:08:13 (128.2 min), the running speed is 5.48 mlsec compared to 10.05 mlsec for the 100 m dash (8). Table 1can also be used to predict the major energy source for other athletic events. For example, the world record in the mile run is 347 (227 sec), which is a time somewhere between the maximal duration utilizing glycogen anaerobically and aerobically. Therefore, the mile run requires both of these energy sources, with CP and fat contributing very little to the total energy requirement. Therefore, a miler must train his or her hody for both endurance (aerobic) and speed (anaerobic) running; a typical training program for the mile involves a combination of long distance running and fast interval work.
Creatine Phosphate 0.05 0.02 (muscle) 0.5 (muscle) 0 3.6 0.2
Glycogen Anaerobicb Aerobic 0.3 0.04 (muscle) 1.O (muscle) 0 1.6
0.8
4 3.0 (muscle) 75d (muscle) 3.5 1.0 90
Fat 9 1000 (adipose) 10,000 (adipose) 4.0 0.4 indet.
Mabilizabie Protein
3500 (muscle) 5.0
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indef.
MOSt n~mows818 YBP) appro%mate h y arlLmc among omsr m ngr s 70 kg man m average Wyr cal comnion wm 25 hp 01 muse s tloru, and 10 r g of adlposs A dash m3calss Uw datJm 1s not kno-n or hab no pnfnca m a n ng mess -la u n e d a v e d han Fox sno MaUwwa (61and Asfrand and Rcdan ( n %1\19umesB m a i l m i lactic acd tolerance of 2 0 - 2 3 0 o x Lo m ~ r c l e ~ic not Uw actual e n w prcducedas ATP. About 40% of the energy llned Is actually convenedm ATP Derived fmmthsoverall tee energy change far w d ~ b u ~ a i m e t a b opathway, erwrgy. Liver glycogm o n yield an addnlonal 25 moles of ATP equivalents.
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Journal of Chemical Education
Table 2.
Foods and Activities with Energy Values of about 100 kcal*
Approx. 100 kcal of Activity (9) Baseball. 20 mi" Basketball, 13 min Bicycling. 9 min (2.5 miles) Dancing, 20 min Football, 12 min Ho~sewWk,25 min Racket sports, 12 min (Tennis, racquetball, squash, etc.) Running. 8 min (1 mile) Sining or standing. 50 min Skiing (downhill).10 min Sleeping, 90 mi" Swimming. 20 min (440 yards) Walkina. 20 min (1 miid
Approx. 100 kcal of Food Energy ( lO) Apple, 1 med. Banana. 1 med. Bmer. or margarine, ITbsp. Bread, 1%slice Candy (wlo nuts), 1 oz. Egg, 1 fried in bmer Egg, 1% boiled Ground beef. %paw (1% oz.1
Ice cream, cup Milk, w h ~ l eCUP; ~ / ~skim. 1 cup. Nuts. 2 Tbsp. Potato, 1 med. Rice (cooked),%cup Suoar. 2 Tbm.
Figures are only approximate avera&s. Exercise data are for a 70 kg man. In most uues, olorie expenditure increases wim a person's weight and rim me intensity of me exetcise. me exerclse limo are for continu- (nonstop)activity.
(9.10). A more comnlete listine can be found in references (9) .. G d (10). Weieht loss. then. becomes a matter of creatine a caloric deficit by eating less and/or exercising more. However, the simolicitv of this statement belies the comolexitv of treatine obesity. ?he statement says nothing a b o i t the social, em; tional, and physiological influences on appetite and physical activity. T h e almost alarming rate of recidivism (near 98%) in weight-loss programs (3) indicates the difficulty of the problem. While it is true that intake and expenditure of energy can lead to weight increases and losses, individuals differ considerably i n i h e i r metabolic makeup: some obese individuals can maintain a diet of 1500 kcal/day, which should lead to a weight loss of one pound per week, yet still gain weight (11). On the other hand, some slender individuals have been known toeat up to l 0 , M kcallday (about four times the normal caloric intake) for 2Ml days without gaining any weight (12). In fact, most obese individuals eat no more (andperhaps fewer) calories than nonohese people (13-15). The reasons for these anoarent anomalies are not clear. However, several possible &planations have been proposed, includine differences between lean and obese oeoole in their content or activity of "brown fat," (16) of "futiie c&es" (17), and of sodium-notassium ATPase (18). See the indicated references for details of these hypot1;esks. There are other nroblems associated with trvine to lose weight by restricting caloric intake. For exam& dieting without exercise is known to cause loss of muscle mass as well as adipose tissue (19). This is due to the decreased levels of liver glycogen and hence the tendency toward hypoglycemia. The body degrades protein to amino acids, all but one of which are glucogenic and therefore can be converted to glucose through gluconeogenesis. Fats cannot be utilized for net glucose svnthesis but, in cases of severe glucose deficiencv, are conveited LO ketone bodies which supply energy to thebrain and heart. Kven with a high-protein (but low carbohydrate) diet, muscle degradation remains a prohlem because the hody has no way of storing excess amino acids. While some dietary amino acihs are converted to glucose, most are converted to fat or excreted soon after ingestion. T h e body is also known to adjust to decreased caloric intakes by reducing the basal metabolic rate, thereby decreasing or completely stopping weight loss even on a restricted diet (9,20). The metabolic adjustment, moreover, takes place more rapidly with suhsequent episodes of dieting, making weight loss almost impossible for chronic dieters (15). T h e wellknown nutritionist Jean Mayer (21) calls this the "rhythm method of girth control," and suggests that i t is one of the reasons for the high failure rate in dieting.
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T h e approach most widely accepted by professionals for weight reduction involves a combination i f diet and exercise. T h e benefits of including exercise are much more than the simple arithmetic of caloric balance: (1) Exercise not only expends calories during activity (see Table 2) but also followine exercise. because the increased metabolic rate persists for hours after exercise (9).Although a noneaerciser might look at Table 2 and become diacouragcd at the amount of exerrme required to lose one pound (e.g., run 35 miles),the long term effects can he significant: walking for one hour a day far a year can lead to a weight loss of 30 pounds. (2) Exercise is known to decrease appetite (22). In animals and in humans, regulation of hunger appears to he impaired by a sedentary lifestyle (22,231. But with increased activity, the appetite, relative to the numher of calories burned during the exercise, is suppressed. (3) A chronic effectof exercise is an increased basal metabolic rate, which means more calories are burned while the person is inactive and therefore can lead to weight loss (9,. (4) Exercise amelioratrs the problem of protein degradation. Thus, when individuals combine exercise with dietine I
