METABOLIC RATE, BLOOD SUGAR AND THE UTILIZATION OF CARBOHYDRATE

Transcription

1 METABOLIC RATE, BLOOD SUGAR AND THE UTILIZATION OF CARBOHYDRATE H. T. EDWARDS, R. MARGARIA AND D. B. DILL From the Fatigue Laboratory, Morgan Hall, Harvard University, Boston Received for publication January 10, 1934 The many observations on blood sugar in exercise have revealed widely different conditions. Thus one may have extreme hypoglycemia as found by Levine, Burgess and Derick (1) in some marathon runners, or at the other extreme, hyperglycemia as in football players (2). A moderate decrease in the concentration of sugar in the blood (B. S.) has been found by Christiansen (3) in exercise of one hour s duration; in recovery following exercise both Rakestraw (4) and Christiansen report moderate increases. We have found that when exercise is carried on for many hours without emotional stress the value of B. S. may not vary much from the resting level until a terminal hypoglycemia develops (5). It seems probable that such a state reflects the exhaustion of glycogen reserves; if so it should be associated with a decreasing respiratory quotient. With this possibility in mind we have undertaken a series of experiments designed to study the relation between metabolic rate (M. R.), respiratory quotient (R. Q.), carbohydrate reserve,l and blood sugar (B. S.). The experimental procedure has been described in part elsewhere (6). In these experiments two rates of work were employed with alternation of rate each half-hour. The work was carried on for several hours, tiz., long enough to reduce materially the reserve of carbohydrate. During each half-hour blood was obtained from the finger while work proceeded and sugar was determined by the Folin-Malmros method (7). At the same time expired air was collected and analyzed in the usual manner. Since the rate of work did not exceed the oxygen transport capacity the rate of oxygen intake is a measure of the M. R. Our ablest subject was Y, aged 19, a skilled runner who has finished near the front in marathons. In three experiments he ran for 6 hours alternating at rates of 9.3 and 11.3 km. per hour with an energy output on these occasions averaging 3400 Cals. The second of these experiments 1 While the carbohydrate reserve cannot be measured directly, changes in it can be calculated from the M. R. and R. Q. En our experiments on subject Y the amount of carbohydrate used exceeded the usual estimates of glycogen content of man thus indicating that carbohydrate reserve was reduced to a low level. 203

2 204 H. T. EDWARDS, R. MARGARIA AND D. B. DILL TABLE 1 Subject Y-January 7, IO%9 Room temperature 14 ; body temperature in work 38.8 =t 0.2; urine 340 cc.; urine N., 2.5 grams; energy from protein, 2 per cent; duration of each work period 25 minutes; of each intervening rest period, 5 minutes. OXYGEN U8ED 1. par min BLOOD BODY ENEROY BUQAR WEIGHT PBODUCED mgm. per loo cc kgm Cd SOURCES OF ENEROY Carbohydrate - Cal Fat per cent Cd. m l SUYYA- TION OF CABBOHY- DItATE USED TABLE 2 Summary of 3 experiments on subject Y Energy derived from protein assumed to be 5 cals. per period; duration of each period, twenty-five minutes POBIOD OXYGEN U8ED 1. per min R.Q. BLOOD SUGAB mgm. per 100 cc ENERGY FROM CABBOHYDBATE per cent E CARBO- HYDBATE m

3 BLOOD SUGAR CONCENTRATION IN EXERCISE 205 is presented for an example in table 1 and a summary of the three experiments on him in table 2. Subject H was in good physical condition, but not a skilled runner. Beginning after a light breakfast, he worked for 7 hours on 4 occasions with glucose ingestion in the latter part of 2 experiments. The detailed experimental results on him are omitted; they are presented in condensed fashion in figures 1 to 3 and table 3. Two other subjects, B and C, were used in single experiments only. Their response was similar; the results on B. S., R. Q. and M. R. are included in figure 2 and table 3. Metabolic rate and blood sugar. The relation of B. S. to M. R. has been considered recently by Christiansen (3). Accordingly it will be of interest to see how our results agree with his conclusion that there is no relation Fig. 1. Blood sugar during exercise. Four experiments on subject H are averaged, 3 on subject Y. H used 163 grams of carbohydrate in 8 periods (4 hours) and Y used 374 grams in 12 periods (6 hours). Virtually all carbohydrate was derived from reserve glycogen. between these variables. The results of 9 experiments on 4 subjects including 118 pairs of observations are summarized in table 3. These results include a number of observations on subject H, after glucose ingestion. In consequence the mean B. S. in his case is much higher than that of subject Y. Apart from this, however, the two subjects showed a significant individual difference throughout. This is evident from figure 1 which gives B. S. as a function of time in the two subjects, excluding observations made on subject H after glucose administration. From the trend of these curves and from the summary in table 3 we may conclude that there is not an intimate relation between M. R. and B. S. under the conditions of our experiments. Furthermore it appears that 4 hours of hard work may be carried on before marked hypoglycemia develops. Thus these experiments support Christiansen s conclusion that B. S.

4 206 H, T, EDJjU.BDS, R. AEARGARIA AND D. R. DILL does not depend on M. R. It seems that, so long as exercise is continuous and reserves of carbohydrate ample, B. S. is maintained near the resting A 0 0 A 8 Fig. 2. BIood sugar and the proportion of energy derived from carbohydrate. Results after glucose ingestion are included. 0 Fig. 3. The reserve of carbohydrate and the proportion of energy derived from carbohydrate. The experiments are averaged as in figure 1. level by increasing glycogenolysis to a rate commensurate with the rate of carbohydrate oxidation. The data of Christiansen and unpublished ob-

5 BLOOD SUGAR CONCENTRATION IN EXERCISE 207 servations of our own commonly show an increase in B. 5. when vigorous exercise is interrupted; the more intense the exercise the more abrupt is the increase. This hyperglycemia may mean that glycogenolysis returns to normal more slowly than the rate of glucose utilization. In consequence of this phenomenon and the fact that most games are severe and discontinuous in character it is probably true that during them the B. 8. reaches and persists at high levels. This certainly is true in football playing-a game which lasts for 2 hours with only about 8 minutes of active Play (2). Utilization of carbohydrate and blood sugar. We have seen that in. muscular activity at two levels the B. S. dgers but little from its value in rest. From this one might guess that the proportion of carbohydrate used also remains constant. To test this a study has been made of the relation between B. S. and proportion of energy derived from carbohydrate BUBJXT TABLE 3 Blood sugar in relation to metabolic rate IUUMBIIB OB OBABBVATIONB MBAN OXYOBN INTAKID MlAN BLOOD BUOAB 1. per min. mgm. per 100 cc as determined by the R. Q. Figure 2 contains 125 pairs of observations on the 4 subjects. Obviously there is no more than a slight interdependence of these two variables under the conditions of our experiments. The correlation coefficient, calculated by conventional methods, comes out f-o.150 =t * The fact that it is positive is probably associated with s In more recent and unpubliehed experiments we find a much higher correlation between these variables. The experimental conditions, however, are significantiy different. The rate of work is constant, not variable; rest periods of one hour or more are interposed between working periods of two hours; the subject begins in a post-absorptive state and is given no food during the entire experiment. We are not able to say which dserence is most significant but it is probable that the last point mentioned is important. It was observed, for example, that after admititering glucose to subject H the R. Q. did not reach a maximum for two hours, i.e., some time after the blood sugar had passed through its maximum. This phenomenon will be referred to below; it means that the administration of sugar during exercise disturbs the existing relation between blood sugar and proportion of carbohydrate used.

6 208 H. T. EDWARDS, R. MARGARIA AND D. B. DILL the facts that at the beginning of exercise both variables are high and at the end both are low. In the intervening hours the B. S. fluctuates irregularly while the R. Q. decreases with a fair degree of uniformity. Utilization of carbohydrate, carbohydrate reserve and metabolic rate. We show in figure 3 the relation between these variables in subjects H and Y. On the basis of this figure and of additional observations of the R. Q. in rest it is evident that the ratio of carbohydrate burned to fat burned increases with metabolic rate. We cannot yet describe the relation precisely. From studies now in progress it appears that the proportion of carbohydrate utilized increases slowly with the first increments of M. R. and more rapidly as the capacity for oxygen transport is approached. It is also evident from figure 3 that even early in exercise when reserve of carbohydrate is greatest the utilization of carbohydrate is sign&antly less than 100 per cent even when the M. R. is 12 times its resting value. Fat continues to be burned even with a plethora of carbohydrate. From this evidence we can not conclude that carbohydrate is essential for strenuous exercise. While our subjects were nearly exhausted it does not follow that the principal source of their fatigue was carbohydrate depletion. Thus we have shown elsewhere that the M. R. of man may be 700 per cent of the resting level for 2 hours with less than 10 per cent of the energy derived from carbohydrate without excessive fatigue. And in the dog a much higher M. R. is possible for as long as 27 hours without fuel. Possibly in man the accumulation of acetone bodies in exercise of long duration without food is a primary source of breakdown; this question is now under investigation. The intimate relation between the proportion of energy derived from carbohydrate and reserve of carbohydrate is particularly interesting in view of the apparent lack of close relation between B. S. and carbohydrate utilization. Although the choice of fuel in exercise is determined with precision, its governor is at present unknown. Utilization of carbohydrate after glzdcose ingestion. The experiments of Carpenter and Lee (8) have already shown that the increase in utilization of glucose begins within 30 minutes after glucose ingestion and reaches a maximum within 60 minutes after ingestion. In one of our experiments on subject 4 the peak in the blood sugar curve coincides with the lowest value of R. Q. observed in that experiment. In the other experiment the maximum R. Q. comes two hours after glucose ingestion when B. S. has returned to its pre-ingestion value. The apparent lag in response of the R. Q. is all the more remarkable because of the depletion of glycogen reserve by the previous activity. It suggests that the utilization of glucose by the muscles takes second place to its use for replenishing glycogen stores. It is possibly significant in this connection that Carpenter

7 BLOOD SUGAR CONCENTRATION IN EXERCISE 209 and Lee (8) have found a more rapid and greater increase in R. Q. after ingestion of fructose or galactose than after glucose. SUMMARY A study has been made of exercise of su5cient duration and intensity to reduce the carbohydrate reserve to low levels. Altering the rate of work each half-hour alters the proportion of energy derived from carbohydrate (measured by the R. Q.) but not the blood sugar. With a plethora of carbohydrate some fat is used in exercise and as carbohydrate reserve diminishes the proportion of energy derived from fat may increase from 8 per cent to 77 per cent (table 1). There is no evidence that carbohydrate is more essential in work than in rest. Under the conditions of our experiments there is a small positive correlation between blood sugar and R. Q. While the choice of fuel is determined with precision, blood sugar concentration is not the governor. Thus, when glucose is ingested during work and after depletion of carbohydrate reserves, the blood sugar may reach a maximum before there is a response of the R. Q. REFERENCES (1) LEVINE, S. A., G. BUIWESS AND C. L. DEIUCK. Jdum. Amer. Med. Assn., 1924,, (2) EDWARDS, H. T., T. K. RICHARDS AND D. B. DILL. This Journal, 1931, 98, 352. (3) CHRISTIANSEN, E. H. Arbeitsphysiol., 1931, 4, 128. (4) R,AKESTRAW, N. W. Journ. Biol. Chem., 1921, 47, 565. (5) DILL, D. B., H. T. EDWARDS AND J. H. TALBOTT. Journ. Physiol., 1932, 77, 89. (6) DILL, D. B., B. F. JONES AND H. T. EDWARDS. Le Travail Humain. In press. (7) FOLIN, 0. AND H. MALMROS. JOWL Biol. Chem., 1929,80,115. (8) CARPENTER, T. M. AND R. C. LEE. This Journal, 1932,102,659.