Effects of coffee ingestion on oral glucose tolerance curves in normal human subjects

Effects

of Coffee Ingestion Curves in Normal

on Oral Human

Glucose Tolerance Subjects

By LEONARD J. FEINBERC, HERSCHEL SANDBERC,

OSCARDE CASTRO AND SAMUEL BELLET Oral, glucose tolerance tests were performed on 23 normal subjects and then repeated one week later. On one occasion, the test meal consisted of glucose dissolved in water and flavored with lemon juice; on the other occasion, 5 Gm. of instant coffee were also added to the meal. The order of administration of the respective meals was randomized. Serial blood samples were obtained and analyzed for blood glucose concentration, serum free fatty acid levels and the serum immunoreactive insulin values. Paired comparisons of the data were made and the following results were obtained: (1) The subjects ingesting coffee plus glucose had significantly lower blood glucose levels 30 and 60 minutes postprandium than those consuming the

glucose solution without coffee. (2) Three hours after ingestion of the test meal, the free fatty acid levels of the subjects receiving coffee with glucose were significantly higher than those receiving glucose without coffee. (3) No statistically significant differences between the two groups were found at any time period for the serum immunoreactive insulin levels. It is possible that coffee ingestion reduced the peak postprandial blood glucose levels by mobilizing a hormone from the gastrointestinal tract such as secretin, pancreozymin, or the newly discovered substance with glucagon-like immunoreactivity described by Unger et al.37 (Metabolism 17: No. 10, October, 916-922, 1968)

A

LTHOUGH THE EFFECTS of methyl xanthines on the kidney and central nervous system have been studied in great detail,lm3 relatively little is known about the ways in which these drugs influence carbohydrate metabolism.4 Recent studies from our laboratory5*8 have shown that subjects who have ingested caffeine show a significant and prolonged rise in serum free fatty acid levels. Since Butcher and Sutherland7 have shown that methyl xanthines block the inactivation of cyclic 3’,5’-AMP, a ribonucleotide of great From the Division of Cardiology, Philadelphia General Hospital, Philadelphia, Pa. Aided by a grant from the Sugar Research Foundation, New York, N. Y., and Grant HE 5165-11 from the National lnstitutes of Health, Bethesda, Md. Received for publication February 19, 1968. LEONARD J. FEINBERG, PH.D.: Chief, Biochemical Research Laboratory, Division of Cardiology, Philadelphia General Hospital. HERSCHEL SANDBERG, M.D.: Chief, Radioisotope and Coagulation Sections, Division of Cardiology, Philadelphia General Hospital. OSCAR DE CASTRO, M.D.: Research Fellow in Cardiology, Philadelphia General Hospital. SAMUEL BELLET, M.D., F.A.C.C.: Director, Division of Cardiology, Philadelphia General Hospital; Professor of Clinical Cardiology, Graduate School of Medicine of the University of Pennsylvania; Lecturer in Pharmacology, Univera’ty of Pennsylvania School of Medicine, Philadelphia, Pa. 916

COFFEE Table

917

INGESTION l.-Effects

Minutes

of Co&e Blood Gluep~

After Ingestmu

Nyfber

Teat Meal

Subjects

C~~olUUp

Subjects bx. 46)

80.3*

on Oral

Glucose

Tolerance

Curves

in Humans

Bload Glucose Concentration Of fhbjects Ingesting Coffee with Test Meal br. %)

0

23

30

18

139.1 zt25.4

60

23

121.7k24.3

90

15

90.1 f 25.7

83.0*

120

23

77.1 f21.7

180

23

68.2k

*Mean * SD. tDifferences not statistically

9.0*

hgeation

MWl Difference

Variance

St&%*d of the MWl

t

P

79.2 * 10.9*

1.1

117.7 f 18.6

21.2

694.30

6.21

3.41


98.6k24.0

23.2

615.06

5.17

4.48


12.8

N.S.t

7.1

N.S.

72.6~30.3

4.5

N.S.

14.1

72.8+ 12.3

4.6

N.S.

significant

at the 5 percent

level for a paired comparison.

importance in glycogenolysis, we decided to study the effects of coffee on the metabolism of glucose in human subjects. MATERIALS

AND

METHODS

All 23 subjects employed in this study were normal, healthy adults (15 males, and 8 females). The study group consisted predominately of students and technicians, 19-23 years of age; they were placed on a diet containing at least 300 Gm. of carbohydrate 3 days prior to the initial study and were advised to maintain a similar diet when the study was repeated one week later. No food was ingested after 10 p.m. of the night preceding the test. Smoking was prohibited. The following morning 1 Gm. of glucose per kilogram of body weight was administered orally to the test subject. The glucose was dissolved in 400 ml. of water flavored with lemon juice. On 1 of the 2 testing days, 5 Gm. of instant coffee containing 220 mg. of caffeine was administered along with the glucose. The number of subjects receiving coffee with glucose as the first test meal was equal to the number receiving glucose without coffee initially and the order of administration of the respective meals was randomized. Blood samples were obtained prior to the administration of the test meal and at serial intervals thereafter. In the earlier studies, these were obtained at hourly intervals. Later, test samples were also taken 30 and 90 minutes postprandially as well. Blood glucose concentrations were determined by the method of Somogyi-Nelson.8 Serum free fatty acids were determined in the initial 5 subjects by a modification of the method of Gordona and in the latter 18 subjects by the method of Antonis adapted for the autoanalyzer. The serum immunoreactive insulin @RI) was assayed by the method of Hales and Randle,rr a radioimmunoassay utilizing the principle of competitive inhibition and 2 antibodies, one to form a soluble complex with radioactive (and non-radioactive) insulin; the other to precipitate this complex and thus separate it from free radioactive insulin. RESULTS

Effects of Cofee Ingestion on Oral Glucose Tolerance Curve From Table 1, it can be seen that the glucose tolerance curve was significantly altered when 5 Gm. of coffee were added to the test meal. The zero time samples showed no important differences between the control and the coffee-treated group. Thirty minutes after ingestion of the test meal, the subjects receiving glucose without coffee averaged 139.1 -+ 25.4 mg. per

918

FEINBERG

Table

2 .-Efects

“%Inyftion

Nuzber

Test Meal

Subjecta

of Cofee Ingestion on Serum Free Fatty Course of Glucose Tolerance Test

FFA Concentrntion of control Subjects, ( % ofx~tl””

Acid

Levels

ET

AL.

During

FFA $m~;&tration i%c$ ( % of zero time sample)

Mean Difference

-1.7

%zd of tbe Mean

V~hl.X

t

P

30

13

84.5rt 18.0*

82.8f 18.5*

60

18

75.7f 24.3

81.1 + 24.8

5.4

N.S.

90

11

67.8f 20.3

67.9k 15.4

0.1

N.S.

120

18

73.3f 27.5

82.7 f 23.5

9.4

N.S.

180

15

86.5 k 33.3

117.6zt36.8

20.9

N.S.t

1281

9.565

2.196

<0.05

*Mean f S.D. tDiemnces not statist&ally significant at the 5 percantlevelfor a pairedcomparison.

cent while those also receiving 5 Gm. of instant coffee averaged 117.7 +- 18.6 mg. per cent. One hour after the test meal, the control group averaged 121.7 2 24.5 mg. per cent, while the coffee-treated group averaged 98.6 * 24.0 mg. per cent. T tests applied to a paired comparison gave values of 3.41 and 4.81, respectively, with P values of
on Serum Free Fatty

Acid Levels During

Oral

Table 2 shows the effect of adding coffee to the test meal on serum free fatty acid concentrations of human subjects during an oral glucose tolerance test. Both groups showed sharp drops in the free fatty acid levels after ingesting the test meal, but no significant differences in the free fatty acid concentrations of the two groups were found until 3 hours postprandium. At this time, the control group averaged 86.5 * 33.3 per cent of the zero time values, while the group receiving coffee averaged 117.3 + 36.8 per cent. T test applied to a paired comparison gave a value of 2.496 with a P value of < 0.05. Effect of Coffee Ingestion on Zmmunoreactive Oral Glucose Tolerance Test

Insulin Levels of Serum During

From Table 3, it can be seen that peak immunoreactive insulin values were found in both groups at 30 and 60 minutes postprandium and fell rapidly thereafter. No statistically significant differences in the immunoreactive insulin values between the two groups were found during the 3 hours of the glucose tolerance test. DISCUSSION

From the results shown above, the addition

of 5 Gm. of instant coffee to

glucose tolerance test meal produces a significant effect on both the free fatty acid concentration of the serum and the blood glucose concentration. The

COFFEE

919

INGESTION

Table

3.-Efiectr

of Coffee During

N”2br Subiecta

Ingestion on Serum Zmmunorscrctive Cwrse of Glucose Tolerance Teat

Immunoraactive Insulin Levela of Control Subjects w/mu

0

23

15.3f

30

17

60

Levels

Immunoreactive Indin Im$e of SubjecE

In&+~tC~~~

Gl/rd.,

mtb

MU&l Difference

P

13.2* 13.4*

2.1

N.&t

83.2 f 56.0

95.6 k 45.9

12.4

N.S.

23

93.9 * 60.1

85.5 f 45.1

8.4

N.S.

90

17

64.5k40.8

63.OIk42.8

1.5

N.S.

120

22

33.9 k24.3

28.1 k24.0

5.8

N.S.

180

22

13.0 27 14.5

13.8 f 18.4

0.8

N.S.

*Mean rt S.D. tDifferencea are not statistically

significant

11.2;

Znsulin

at the percent

level for a paired comparison.

effect on free fatty acids could possibly have been anticipated since carbohydrate ingestion has been shown to lower serum free fatty acid levels,” while caffeine has been found to elevate them.j,” When the glucose effect is dissipated and the effect of caffeine persists, the result would be a rise in the free fatty acid levels shown by the 3-hour sample of Table 2. The effect of coffee ingestion on the glucose tolerance curve, however, was unexpected since several earlier studies seem to suggest a hyperglycemic action of caffeine. Northrup and Parks]” reported that methylxanthines potentiated the hyperglycemia induced in rats with cyclic 3’,5’-AMP. Kuftinec and Mayer-’ observed that coffee produced a marked elevation of blood glucose in obese hyperglycemic mice. Biimer,‘:’ Hirayama,‘” and Karger’” have noted that caffeine produced hyperglycemia and glycosuria in dogs. In a recent study,16 a group of dental stud en t s, receiving caffeine in capsule form, showed significantly higher glucose levels than a control group matched by age and weight. Jankelson et al. l7 have reported that the ingestion of coffee by patients with maturity-onset diabetes results in higher blood glucose levels during an intravenous glucose tolerance test than those found when water was substituted for coffee. In vitro studies would also lead us to expect a hyperglycemic action of caffeine. Vaughan,18 as well as Anderson et al.,]” have shown that caffeine inhibits the uptake of glucose by rat adipose tissue. The latter group20 has observed that caffeine inhibits the production of Cl402 and the Cl4 lipid incorporation from glucose 1-C l4. Sutherland and his associates21-2” have noted that caffeine increases liver phosphorylase activity and glycogenolysis by inhibiting a phosphodiesterase which destroys cyclic 3’,5’-AMP. Other studies, however, lend support to our findings. Iancu et a1.24 observed that caffeine, administered subcutaneously, produced a decrease in the fasting blood glucose levels of dystrophic sucklings with total cortical inhibition. Although Kuftinec and Mayer-l observed that caffeine accentuates the hyperglycemia of obese mice, their lean litter mates were rendered hy-

920

FEINBERG

ET

AL.

poglycemic by the drug. Hankiewiczzs in a study of 226 patients (75 per cent of whom were diabetic) found that the administration of caffeine resulted in a drop in the fasting blood sugar levels. Deakins,26 who performed S glucose tolerance curves in a single subject, reported that large doses of caffeine depress the peak of the human sugar-tolerance curve. The mechanisms by which coffee lowered the peak values of the postprandial blood sugar are unknown, Several recent studies, however, may provide clues for this action. Lambert et a1.27 speculated that insulin release might be dependent on the intracellular accumulation of cyclic 3’,5’-AMP. When they added caffeine to an incubation medium containing foetal pancreatic tissue in the absence of glucose, they observed a sharp increase of radioimmunoassayable insulin released into the medium. When caffeine was added along with glucagon or tolbutamide, the insulin released from the pancreatic tissue was greatly increased. Perhaps caffeine works similarly in vivo, blocking the action of phosphodiesterase, prolonging the action of the cyclic nucleotide, thus stimulating an increase in insulin released from the pancreas resulting in a subsequent hypoglycemic action. Such a mechanism would offer an explanation for the significant decrease in blood glucose found in adrenalectomized rats treated with dihydroergotamine and subsequently with 3’,5’-AMP.12 The lack of a significant increase in the peripheral blood insulin levels in our subjects would not necessarily negate this hypothesis, since Anderson et al.28 have clarified mechanisms by which increasing insulin concentrations in the pancreatic veins may be associated with inhibition, by the liver, of insulin output into the periphery. Other mechanisms, however, may be operative. Since coffee contains noncaffeine compounds such as volatile aromatic, oils, furfuryl alcohol and tanninlike substances which have been stated to be local gastric irritants,2g it is possible that these agents produced a state of hypermotility, resulting in a rapid transit time of glucose through the gut and hence defective absorption of the test meal. Jankelson, lr however, failed to find evidence for such gastric irritation. Moreover, studies recently completed in our laboratory have shown that similar lowering of the postprandial blood sugar can be produced in dogs by adding a solution of caffeine sodium benzoate alone to the test meal. Several groups SO-33have shown that insulin-like activity for a given glucose load is considerably higher when glucose is administered orally rather than intravenously. Perley and Kipnis,33 in a well-planned study, also showed this difference to exist in patients with portacaval shunts. They therefore postulated that some mechanism, located in the upper gastrointestinal tract, mediated in part, the insulin response to an oral glucose load. Glucagon34*“” and pancreozymin 36 have been found to stimulate the secretion of insulin. Unger et al.3i recently extracted from the jejunum a substance with glucagonlike immunoreactivity but devoid of hyperglycemic, glycogenolytic or adenylcyclase activating activity, which is capable of stimulating insulin release. Gley and Hazard3s injected dilute hydrochloric acid into the duodenum of dogs and found that blood from these animals reduced the blood sugar of recipient dogs. Zunz and Le Barre3” performed pancreatico-jugular anast-

COFFEE

921

INGESTION

amoses on adrenal decapsulated dogs and found that secretin, injected intravenously into the donor animal, produced hypoglycemia in the recipient dog. Shay and Gershon-Cohen 4O found that stimulation of the duodenal mucosa with hydrochloric acid, fat, and hypertonic salt solutions could block, or significantly reduce, alimentary hyperglycemia. Since caffeine is known to profoundly augment gastric secretion in man, with peak levels of free acid appearing 4050 minutes after administration of caffeine41 it is possible that this mechanism might play an important role in the reduced post alimentary blood sugar peaks found in our study of normal subjects. The results of the present study are of interest. They show that marked decreases in blood glucose concentration may occur following the ingestion of coffee without concomitant changes in the serum immunoreactive insulin levels of the peripheral blood; that glucose ingestion inhibits, for several hours, caffeine’s ability to elevate serum free fatty acids. ACKNOWLEDGMENT We gratefully acknowledge Jacqueline Zavodnick, and Mrs.

the technical Barbara Trechak

assistance in carrying

of Miss Diane out this study.

Symons,

Miss

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922 Lancet 1:1299, 1967. 17. Jankelson, 0. M., Beaser, S. B., Howard, F. M., and Mayer, J.: Effect of coffee on glucose tolerance and circulating insulin in men with maturity-onset diabetes. Lancet 1:527, 1967. 18. Vaughan, M.: Effects of hormones on glucose metabolism in adipose tissue. J. Biol. Chem. 326:3125, 1961. 19. Anderson, J., Hollifield, G., and Owen, J. A., Inhibitory effect of caffeine on the in vitro uptake of glucose by rat epididymal adipose tissue. Diabetologia 3:50, 1967. 20. -, -, and -: The effects of caffeine, deoxyribose nucleic acid and insulin on the metabolism of glucose by adipose tissue in vitro. Metabolism 15:30, 1966. 21. Rall, T. W., and Sutherland, E. W.: Formation of a cyclic adenine ribonucleotide by tissue particles. J. Biol. Chem. 232:1065, 1958. 22. Berthet, J., Sutherland, E. W., and Rall, T. W.: The assay of glucagon and epinephrine with use of liver homogenates. J. Biol. Chem. 229:351, 1957. 23. Sutherland, E. W., @ye, I., and Butcher, R. W.: The action of epinephrine and the role of the adenyl cyclase system in hormone action. Rec. Progr. Hormone Res. 21: 623, 1965. 24. Iancu, A., Faucher, E., Fenesan, A., Nedelcu, L., and Fella, D.: Blood sugar variations induced in dystrophic sucklings under the action of caffeine. Roumanian Med. Rev. 1:42, 1957. 25. Hankiewicz, J.: Effect of the use of caffeine on the level of glucose in the blood. Pol. Tyg. Lek. 15:742, 1960. 26. Deakins, M.: Effects of caffeine on human sugar-tolerance curves. Proc. Sot. Exper. Biol. Med. 49:588, 1939. 27. Lambert, A. E., Jeanrenaud, B., and Renold, A. E.: Enhancement by caffeine of glucagon-induced and tolbutamide-induced insulin release from isolated foetal pancreatic tissue. Lancet 1:819, 1967. 28. Anderson, G. E., Kologlu, Y., and Papadopoulos, C.: Fluctuations in postabsorptive blood glucose in relation to insulin release. Metabolism 16:586, 1967. 29. Report of the Chemical Laboratory

FEINBERG

ET

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