Relationship Between Fasting Plasma Insulin Level and Resistance to Insulin-Mediated Glucose. Uptake in Normal and Diabetic Subjects. Jerrold Olefsky, M.D. ...
Relationship Between Fasting Plasma Insulin Level and Resistance to Insulin-Mediated Glucose Uptake in Normal and Diabetic Subjects Jerrold Olefsky, M.D., John W. Farquhar, M.D., and Gerald Reaven, M.D., Palo Alto SUMMARY We have previously shown that a significant inverse correlation exists between the height of the plasma insulin concentration and the efficiency of glucose uptake in the fasting state. Subsequently, we devised an infusion technic that provides a more specific measure of cellular resistance to insulin mediated glucose uptake. We have used this technic to study the relationship between fasting plasma insulin level and resistance to insulin mediated glucose uptake in twenty-two patients with normal oral glucose tolerance, fourteen patients with impaired glucose tolerance, and fourteen patients classified as having chemical diabetes mellitus. The results indicate a highly significant positive correlation between the degree of insulin resistance and fasting plasma insulin concentration (r = .69, p < .0001). A positive correlation (r = .43, p < .005) was also found between the degree of insulin resistance and the two hour plasma glucose level during an oral glucose tolerance test. However, no significant correlation was found between per cent adiposity and fasting insulin level, and the relationship between insulin resistance and fasting insulin level appeared to be independent of obesity or any other known cause of decreased insulin responsiveness. These studies indicate that: (1) increases in fasting insulin levels and increase in resistance to insulin mediated glucose uptake are closely related, and that these changes can occur in the absence of any of the "known" causes of decreased insulin responsiveness; (2) in the types of subjects we have studied, deterioration of glucose tolerance is closely related to increases in insulin resistance; and (3) the increases in fasting insulin levels which we have observed appear to be compensatory attempts to overcome the resistance to glucose uptake. DIABETES 22:507-13, July, 1973.
In a previous study we indicated that rises in fasting plasma glucose concentration from 75 to 150 mg./lOO ml. were associated with commensurate decreases in ef-
ficiency of glucose uptake and increases in fasting plasma insulin concentration.1 One explanation for this relationship is that the decreases in efficiency of glucose uptake (and consequent fasting hyperglycemia) were due to resistance to insulin and that the observed increases in fasting plasma insulin levels were compensatory efforts aimed at overcoming the insulin resistance. Subsequently, we described an experimental technic devised to provide a specific quantitative estimate of this kind of insulin resistance.2 This approach is based upon the simultaneous constant intravenous infusion of epinephrine, propranolol, glucose, and exogenous insulin. With this technic, endogenous glucose and insulin release are inhibited and similar circulating levels of exogenous insulin are achieved in all subjects. Under these conditions, measurement of the steady state plasma glucose response provides a direct estimate of resistance to insulin mediated glucose transport.2 This direct method of assessing relative degrees of resistance to insulin mediated glucose uptake seemed to provide an appropriate way to further evaluate the possibility that resistance to the action of insulin leads to increases in fasting plasma insulin concentrations. With this thought in mind, we have carried out the present study in which the relationship between relative degrees of insulin resistance and fasting plasma insulin concentration was defined in fifty subjects with varying degrees of glucose intolerance. The results indicate that increases in fasting plasma insulin levels are directly related to the severity of the insulin resistance and are compatible with the suggestion that an elevation in fasting insulin level represents an effort to overcome the insulin resistance and thereby maintain glucose homeostasis. MATERIALS AND METHODS
From the Departments of Medicine, Stanford University School of Medicine and Palo Alto Veterans Administration Hospital, Palo Alto, California 94305. Accepted for publication April 11, 1973. JULY,
1973
Study group. Fifty subjects were studied at the Stanford General Clinical Research Center. All subjects were free of pharmacological or pathological states known to 507
FASTING PLASMA INSULIN LEVEL AND INSULIN RESISTANCE
affect carbohydrate metabolism. Their oral glucose tolerance was measured in response to a glucose load of 40 gm. per square meter body surface area. On the basis of the test results and previously published criteria,3 the study group could be seen to consist of twenty-two patients with normal oral glucose tolerance, fourteen patients with impaired tolerance, and fourteen with chemical diabetes mellitus. No subject had a fasting plasma glucose concentration in excess of 110 mg./lOO ml. Some characteristics of the patients are listed in table 1. It can be seen that.the three groups of patients differ only in their glucose tolerance and are comparable in regard to such factors as sex, age, and per cent adiposity. Experimental procedure. Subjects were hospitalized but ambulatory and received a liquid formula diet designed to maintain weight (35 cal. per kilogram), consisting of 42 per cent carbohydrate, 43 per cent fat, and 15 per cent protein. Blood for plasma insulin measurements was drawn after a fourteen hour overnight fast on the fourth, fifth, sixth, and seventh days of hospitalization, and the mean of the four determinations was used as the measure of each patient's fasting plasma insulin concentration. On the morning of the sixth day of hospitalization, the patients received a constant intravenous infusion of crystalline pork insulin (50 mU. per minute), glucose (6 mg. per kilogram per minute), epinephrine (6 fig. per minute), and propranolol (.08 mg. per minute). The infusion was begun five minutes after a 5 mg. loading dose of propranolol and was administered constantly via a Harvard pump over a period of 150 minutes. Steady state plasma concentration of insulin and
glucose were achieved within ninety minutes of the start of the infusion, and the concentrations of plasma insulin and glucose were then measured every ten minutes for an additional sixty minutes. The degree of insulin resistance was expressed as the mean of the seven steady state plasma glucose levels. This approach is based upon the ability of epinephrine and propranolol to suppress endogenous insulin secretion, which has been confirmed by finding no rise in plasma endogenous insulin levels during extreme hyperglycemia after infusion of glucose, epinephrine, and propranolol.2 Since comparable steady state plasma insulin levels are achieved in all subjects (mean steady state plasma insulin level = 102 fdJ. per milliliter with a coefficient of variation of ± 10 per cent), we are able to directly estimate the ability of identical circulating levels of exogenous insulin to promote disposal of comparable glucose loads in a variety of subjects. Endogenous glucose production is also inhibited under these experimental conditions, and glucose uptake rate equals glucose infusion rate.2 Thus, since insulin concentration and glucose uptake (infusion rate) are the same for all subjects at the steady state, the height of the steady state plasma glucose response is a direct reflection of the efficiency of insulin mediated glucose uptake, i.e., each subject's relative degree of insulin resistance. In order to be certain that increases in steady state plasma glucose levels are a reflection of decreases in efficiency of insulin mediated glucose uptake, it is necessary that the glucose transport mechanism not be saturated. In an effort to demonstrate lack of saturation of this mechanism over the range of steady state plasma
TABLE 1 Clinical characteristics
Glucose tolerance
Number Men Women
Age (mean + range) Men Women
All
Per cent adiposity (mean + range) Women Men
Normal
22
14
8
44 (27-69)
42 (23-69)
29.92 (22.81-43.78)
28.10 (22.81-41.10)
33.10 (26.70-43.78)
Impaired glucose tolerance
14
10
4
41 (24-61)
43 (31-61)
32.90 (23.38-41.34)
31.54 (23.38-40.80)
36.32 (31.29-41.34)
Mild diabetes mellitus
14
9
5
48 (37-61)
49 (26-61)
30.94 (19.59-36.19)
29.22 (19.59-35.54)
34.04 (30.80-36.19)
Total
50
33
17
Patients were divided into three groups on the basis of the following criteria: 1. Normal = one hour plasma glucose concentration < 185 mg./lOO ml., two hour plasma glucose concentration < 130 mg./lOO ml. 2. Mild diabetes mellitus = one hour plasma glucose concentration > 210 mg./lOO ml. and two hour plasma glucose concentration >145 mg./lOO ml. 3. Impaired glucose tolerance = one or more points between normoglycemic and mild diabetes. 508
DIABETES, VOL. 2 2 , NO. 7
J. OLEFSKY, M.D., J. W. FARQUHAR, M.D., AND G. REAVEN, M.D.
glucose responses and glucose uptake rates encountered in this study, we performed multiple infusion studies in one normal subject and in one subject with an abnormal oral glucose tolerance test. Each of these subjects underwent four separate infusion studies identical to those outlined above except that different glucose infusion rates were used during each experiment. These data are shown in figure 1, and it can be seen that the relationship between glucose infusion rate (and, thus, glucose uptake rate) and the steady state plasma glucose level is essentially linear until glucose infusion rates and steady state plasma glucose levels much greater than those seen in our study are reached. Although it is probable that glucose uptake is a saturable process, figure 1 indicates that it is highly unlikely that this process is saturated at the plasma glucose levels and glucose infusion rates achieved in our studies. Thus, a high steady state plasma glucose level indicates insulin resistance rather than saturation of the uptake process. Analytical methods. Samples for plasma glucose were collected in tubes containing potassium oxalate and sodium fluoride and measured by the Technicon AutoAnalyzer using the ferricyanide method of Hoffman.4 Plasma insulin was measured by the double antibody immunoprecipitation technic of Hales and Randle. 5 Per-
centage of body weight which is adipose tissue was calculated according to the anthropometric method found by Steinkamp and co-workers 6 ' 7 to be superior to other estimates of adiposity such as single skinfold measurements, relative weight, and ponderal index. Multiple series of anthropometric measurements were made by one observer, whose repeat data agreed within a ± 5 per cent coefficient of variation. These data are entered into a computer program that uses a regression equation to calculate percentage adiposity. This method correlates closely with isotopic methods of body fat determination/ RESULTS Figure 2 presents the relationship between fasting insulin level and relative degrees of insulin resistance for all fifty subjects. Fasting insulin is expressed as the mean of four separate determinations, and degree of insulin resistance as the mean of the seven steady state plasma glucose values obtained during the last sixty minute of the infusion study. It can be seen that a highly significant positive correlation exists (r = .69, p < .0001) between fasting insulin levels and degree of insulin resistance; the finding is compatible with the thesis that rises in fasting insulin levels represent com-
FIGURE I Relationship between steady state plasma glucose level and rate of glucose infusion.
4
JULY,
1973
6 8 10 12 14 GLUCOSE INFUSION RATE (mg/kg/min)
509
FASTING PLASMA INSULIN LEVEL AND INSULIN RESISTANCE
FIGURE 2 Relationship between fasting plasma insulin levels and resistance to insulin mediated glucose uptake. Degree of insulin resistance (vertical axis) is expressed as the mean 'of the seven plasma glucose measurements during the steady state (see Methods), and fasting insulin level (horizontal axis) is expressed as the mean of four separate determinations.
r*.69 P
