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Metabolism Unit (A.G., M.P., E.B., E.F.), C.N.R. Institute of Clinical Physiology and Department of Internal Medicine, University of Pisa School of Medicine, 56100 Pisa, Italy; and Diabetes Division (A.G., Y.M., S.M., E.F., R.A.D.), University of Texas Health Science Center, San Antonio, Texas 78229-3900
Address all correspondence and requests for reprints to: Ralph A. DeFronzo, M.D., Diabetes Division, University of Texas Health Science Center, 7703 Floyd Curl Drive MS 7886, San Antonio, Texas 78229-3900. E-mail: albarado{at}uthscsa.edu.
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Abstract |
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9 mM]. EGP was similar in nondiabetic controls and T2DM with FPG less than 9 mM but was increased in T2DM with FPG 9 mM (P < 0.001). Within the diabetic groups, obesity had an independent effect to further increase basal EGP (P < 0.01). In both nonobese diabetic groups, both the percent GNG and gluconeogenic flux were increased, compared with nonobese nondiabetic controls. In both diabetic groups, obesity further increased both percent GNG and gluconeogenic flux. In obese and nonobese T2DM, the increase in gluconeogenic flux was not accompanied by a reciprocal decrease in glycogenolysis, indicating a loss of hepatic autoregulation. By multivariate analysis, gluconeogenic flux was positively correlated with percent body fat, visceral fat, and the fasting plasma free fatty acid and glucose concentrations (all P 
0.02). We conclude that obesity per se, and visceral fat accumulation in particular, as well as poorly controlled diabetes are potent stimuli to augment gluconeogenic flux. |
Introduction |
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A role for elevated plasma free fatty acid (FFA) levels has been implicated in the accelerated rate of GNG and total EGP in T2DM patients (7, 8). Increased FFA concentrations promote GNG in rat liver (9); similar observations have been made in humans (10, 11). In the postabsorptive state, circulating FFAs are derived from lipolysis, and the fat cell has been shown to be resistant to the antilipolytic effect of insulin in both T2DM and nondiabetic obese individuals (7, 12, 13). Because of these associations, one might expect that excess total body fat or altered fat distribution would be related to the accelerated rate of GNG and EGP in diabetic and obese individuals. With regard to this, increased visceral fat (VF) has been associated with resistance to the antilipolytic effect of insulin in adipocytes (14, 15), reduced rate of FFA reesterification (16), and decreased insulin-stimulated glucose disposal as measured by the euglycemic insulin clamp technique (17). It has been postulated that accelerated release of FFAs (18, 19) and/or other adipocytokines (20) by visceral adipocytes into the portal circulation can induce or augment hepatic insulin resistance and enhance GNG.
The goal of this study was to examine the relationships among the rate of GNG, hepatic insulin resistance, and fat accumulation in normal glucose tolerant and T2DM individuals. Because visceral fat is related strongly to total adiposity (21), it is mandatory to account for influence of obesity when attempting to establish an independent role for visceral fat in glycemic control and the regulation of hepatic glucose metabolism. In the present study, we measured in the same individual total body fat content (by tracer), abdominal fat distribution [by magnetic response imaging (MRI)], EGP (by 3-3H-glucose infusion), and GNG (by the 2H2O method) in a large group of diabetic and nondiabetic subjects spanning a wide range of adiposity.
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Subjects and Methods |
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Forty-four T2DM patients (16 females, 28 males; 30 Mexican-Americans, 14 Caucasians) and 29 age-matched nondiabetic control subjects (12 females, 17 males; 16 Mexican-Americans, 13 Caucasians) participated in the study. Fifteen T2DM patients previously were studied (22). Based on World Health Organization criteria (23), subjects with a BMI greater than 30 kg/m2 were classified as obese. None of the diabetics had ever been treated with insulin or thiazolidinediones. For subjects who were taking metformin or sulfonylureas, the medication was stopped 2 wk before the study. Other than sulfonylureas or metformin, subjects were not taking any other drugs known to affect glucose tolerance. All control subjects had a normal 75-g oral glucose tolerance test (24). Body weight was stable in all subjects for at least 3 months before study. All studies were carried out at the Clinical Research Center of the University of Texas Health Science Center at San Antonio. The study protocol was approved by the Institutional Review Board of the University of Texas Health Science Center at San Antonio, and informed written consent was obtained from each subject before participation.
Study design
Within a 5- to 7-d interval, all subjects received: 1) measurement of lean body mass and fat mass (FM) using an iv bolus of 3H2O; 2) a euglycemic hyperinsulinemic clamp study in combination with 3-3H-glucose to measure basal EGP and hepatic and peripheral tissue sensitivity to insulin; 3) 2H2O given on the evening before the insulin clamp study to measure the contribution of GNG and glycogenolysis to EGP; and 4) in a subgroup of subjects (16 controls and 28 diabetics), quantitation of sc and intraabdominal visceral fat content at L4-L5 using nuclear MRI.
Lean and fat body mass
Subjects were admitted to the Clinical Research Center at 0800 h after a 10-h overnight fast. A catheter was placed into an antecubital vein and, after the withdrawal of a baseline blood sample, a 100-µCi iv bolus of 3H2O was administered. Blood samples were drawn for the determination of plasma 3H2O radioactivity after 90, 105, and 120 min. Lean and fat body mass were calculated as described previously (25).
Abdominal fat distribution
Intraabdominal visceral and sc fat depots were measured in a subgroup of subjects by MRI, using imaging procedures that have been published previously (22, 26). Briefly, images were acquired on a 1.9 T Elscint Prestige MRI system (GE Medical System, Milwaukee WI). A sagittal localizing image was used to center transverse sections on the line through the space between L4 and L5, and the field of view was adjusted for body size to ensure a 2-mm pixel spacing. Signal averaging (four signals averaged) was used to reduce the effect of motion-related artifacts. Additionally, respiratory gating was used to combat motion-induced artifacts and reduce the blurring of fat boundaries in the anterior region of the abdomen.
Euglycemic hyperinsulinemic clamp
Subjects were admitted to the Clinical Research Center at 0700 h, after an approximately 13-h overnight fast, and a spontaneously voided urine sample was obtained. The subjects were asked not to change their habitual dietary regimen, eat the last meal between 1800 and 1900 h on the night before study, and not eat or drink anything after the last meal. At 2200 h on the evening before study, all subjects drank 2H2O [5 g /kg of fat-free mass (FFM)] (Isotech, Williston, VT). A blood sample for the determination of baseline 2H2O enrichment was taken at 0800 h on the morning of the day before the study. Upon arrival, a polyethylene cannula was inserted into an antecubital vein for the infusion of all test substances. A second catheter was inserted retrogradely into an ipsilateral wrist vein on the dorsum of the hand for blood sampling, and the hand was kept in a heated box at 65 C. At 0700 h, blood was drawn for the determination of FPG. Then a primed (20 µCi x FPG per 5 mmol/liter) continuous (0.20 µCi/min) infusion of 3-3H-glucose (DuPont NEN Life Science Products, Boston, MA) was initiated and continued until the end of the study. During the last 30 min of the basal equilibration period (90–120 min after the start of 3-3H-glucose in nondiabetic subjects and 150–180 min for diabetic subjects), plasma samples were drawn at 5- to 10-min intervals for the determination of plasma glucose and insulin concentration and plasma tritiated glucose-specific activity. At the end of the basal equilibration period, a urine sample was obtained. After the basal equilibration period, insulin was administered as a prime-continuous (40 mU/m–2·min–1) infusion for 120 min as previously described (27). The plasma glucose concentration was measured every 5 min after the start of the insulin infusion, and a variable infusion of 20% glucose was adjusted based on the negative feedback principle to maintain the plasma glucose level at 5 mM with a coefficient of variation less than 5%. In diabetic subjects the plasma glucose concentration was allowed to drop to 5 mM, at which level it was clamped. Plasma samples were collected every 15 min from 0 to 90 min and every 5–10 min from 90 to 120 min for the determination of plasma glucose and insulin concentrations and plasma tritiated glucose-specific activity. Plasma samples for the determination of deuterated glucose and water enrichment were taken before starting the tritiated glucose infusion and at the end of the basal equilibration period.
Analytical methods
Plasma glucose concentration was determined by the glucose oxidase method (Beckman II glucose analyzer, Fullerton, CA). Plasma insulin concentration was measured by RIA (Diagnostic Products Corp., Los Angeles, CA). Glycosylated hemoglobin (HbA1c) concentration was measured by affinity chromatography (biochemical methodology, Drower 4350; Isolab, Akron, OH). Plasma FFA concentration was measured spectrophotometrically (Wako Chemicals GmbH, Neuss, Germany). Plasma tritiated glucose-specific activity was determined on barium hydroxide/zinc sulfate deproteinized samples (Somogyi’s procedure).
The pattern of 2H incorporation into plasma glucose after 2H2O ingestion was determined according to the method developed by Landau (28), as modified (22, 29). Briefly, the fraction of glucose produced via GNG from all precursors can be quantified from the ratio of 2H enrichment of carbon 5 (C5) to that of water. Because during glycogen breakdown there is no binding of hydrogen from body water to C5 of the glucose formed, enrichment at C5 in blood glucose vs. water reflects the fractional contribution of GNG to total EGP, i.e. from both phosphoenolpyruvate precursors and glycerol. Precision and accuracy of C5 have been reported previously (3). Water enrichment in the body water pool was monitored by reacting a sample of plasma or urine with calcium carbide (CaC2) to form acetylene (C2H2). The enrichment of acetylene was then determined by gas chromatography-mass spectrometry by monitoring peaks of mass 26 and 27 (30). All samples were run through the gas chromatography-mass spectrometry processing in duplicate or triplicate.
Data analysis
Total body water was calculated from the mean plasma 3H2O radioactivity measured at 90, 105, and 120 min after the iv bolus of 3H2O. Plasma 3H2O-specific activity was calculated assuming that plasma water represents 93% of total plasma volume and FFM was calculated by dividing total body water by 0.73 (31). FM was calculated as the difference between body weight and FFM.
Subcutaneous and VF areas were quantitated by magnetic resonance imaging at L4-L5 level as previously described (26). Briefly, images were processed using Alice software (Perceptive Systems Inc., Boulder, CO) to determine abdominal sc and intraabdominal visceral fat areas. The sc fat area was analyzed by selecting the outer and inner margins of sc adipose tissue as region of interest from the cross-sectional images and counting the number of pixels between the outer and inner margins of sc adipose tissue. The VF (intraabdominal) area was determined using histograms specific to the visceral regions. The histograms were summed over the range of pixel values designated as fat by fitting two normal analysis distribution curves to them.
All glucose fluxes were expressed per FFM (in micromoles per minute per kilogramffm) because this has been shown to best account for differences due to sex, obesity, and age (32). During the last 30 min of the basal equilibration period, both plasma glucose concentration and 3-3H-glucose-specific activity were in steady-state in all subjects, and total EGP was calculated as the ratio of the 3-3H-glucose infusion rate to the plasma 3-3H-glucose-specific activity (mean of five determinations). During the euglycemic insulin clamp, non-steady-state conditions prevail and total glucose rate of appearance was calculated using Steele’s equation. EGP during the last 30 min of the insulin clamp was obtained as the difference between rate of appearance and the exogenous glucose infusion rate. The insulin-stimulated rate of glucose disappearance (Rd) during the last 30 min of the insulin clamp was calculated from Steele’s equation. The hepatic insulin resistance index was calculated as the product of EGP x fasting plasma insulin. Over the range of insulin concentrations from 30 to 150 pmol/liter, i.e. those that exist under fasting conditions in T2DM and control subjects, the relationship between plasma insulin and EGP is linear (r = –0.92, P < 0.0001) (7).
Data are given as the mean ± SE. Proportions were compared by 
2 analysis; comparison of mean group values was performed by two-way ANOVA, with obesity and diabetes as the factors. Partial correlation analyses were used to estimate associations among continuous variables in the whole data set.
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Results |
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)In addition to subgrouping by obesity, individuals with T2DM were further classified according to severity of fasting hyperglycemia: FPG less than 9 mM (range 5.5–8.95 mM) or FPG 9 mM or more (range 9.1–14.4 mM). There were no significant differences between groups with respect to sex or ethnicity, but diabetics were older and heavier than nondiabetic subjects regardless of whether they were lean or obese. HbA1c and known duration of diabetes were not significantly different between obese and nonobese diabetics. Obesity, but not diabetes, was associated with an increase in plasma triglyceride concentration and decrease in plasma high-density lipoprotein-cholesterol concentration. The fasting plasma FFA concentration was increased modestly in both type 2 diabetic groups, and this increase was further enhanced by the presence of obesity (without reaching statistical significance). Fasting plasma insulin levels were significantly increased in diabetics and further increased by obesity in both diabetic and nondiabetic groups.
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FPG did not change significantly in nondiabetic subjects over the 2 h of tracer glucose infusion (5.4 ± 0.1 to 5.3 ± 0.1 mM, P = ns). In contrast, in both diabetic groups, the fasting plasma glucose concentration decreased over the 3-h period of tritiated glucose infusion: from 8.7 ± 0.3 to 7.4 ± 0.3 mM (by 8%) in diabetics with FPG less than 9 mM and from 12.1 ± 0.4 to 11.2 ± 0.5 mM (by 15%) in diabetics with FPG 9 mM or more (both P < 0.01).
Metabolic data (Table 2)
In the basal state, EGP was similar in nondiabetic controls and diabetics with FPG less than 9 mM, whereas it was significantly increased in diabetics with FPG 9 mM or more. Within the diabetic groups, obesity had an independent effect to further increase basal EGP. The hepatic insulin resistance index increased progressively, and the basal glucose clearance decreased progressively, from groups of nondiabetic control to diabetic with FPG less than 9 mM to diabetic with FPG 9 mM or more (P < 0.01 for both the effect of obesity and diabetes) (Fig. 1).
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); the bivariate model predicted an increase in flux of 2.6 ± 0.6 µmol/min–1·kgffm–1 in obese subjects (P < 0.0001), one of 0.7 ± 0.7 µmol/min–1·kgffm–1 in diabetics with FPG 9 mM or less (P = ns), and one of 2.2 ± 0.7 in diabetics with FPG greater than 9 mM (P = 0.001 vs. nonobese, nondiabetic controls). Glycogenolytic flux was reduced slightly, although not significantly, in nonobese and obese type 2 diabetics with FPG less than 9 mM, compared with the other groups.
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). EGP was suppressed by 75 ± 4% in the control group, 61 ± 5% in diabetics with FPG less than 9 mM, and 55 ± 4% in diabetics with FPG 9 mM or more (P < 0.005) similarly in obese and nonobese individuals. Clamp plasma FFA levels were less suppressed in obese than in nonobese subjects, the effect of diabetes per se not reaching statistical significance (Table 2). Correlations
In the whole data set, FPG was strongly and positively correlated with both the rates of EGP (r = 0.42 and P < 0.0001) and GNG (Fig. 3), whereas there was no significant relationship between FPG and glycogenolysis. None of the measured glucose parameters (EGP, percent GNG, GNG flux, glycogenolytic flux, hepatic insulin resistance index) was significantly associated with the plasma insulin concentration. Both percent GNG and gluconeogenic flux were strongly and positively related to the plasma FFA concentration (partial r = 0.36, P < 0.003 and partial r = 0.31, P = 0.01, respectively, after adjusting for sex, age, ethnicity, and BMI), whereas glycogenolysis was related to the plasma FFA concentration inversely (partial r = –0.31, P < 0.02). This result explained why no correlation was observed between plasma FFAs and EGP.
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Discussion |
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Whereas the preponderance of evidence supports enhanced GNG as the cause of increased basal EGP in human T2DM (3, 4), there is some evidence to suggest that glycogenolysis also may be increased (36, 37). Most importantly, the separate impact of factors such as degree of obesity and fat distribution on GNG has not been examined. In the present study, we found that the fraction of EGP that is derived from GNG was increased in obese patients at each degree of fasting hyperglycemia, whereas glycogenolytic flux tended to be decreased (although not significantly) in T2DM patients with mild fasting hyperglycemia and was unchanged in severely hyperglycemic diabetic subjects (Fig. 2). It is especially noteworthy that in our data a difference in BMI of only approximately 4 kg/m2 was associated with an increment in GNG flux similar to that associated with an increase in FPG of 6 mmol/liter. These results emphasize the quantitative effect of obesity to augment GNG flux, namely the impact of hyperglycemia, and the need to closely match diabetic and control subjects for adiposity when evaluating GNG. In addition to fasting plasma glucose and FFA levels, increased GNG flux was simultaneously related to percent body fat and VF area (independently of age, gender, BMI, and ethnicity).
In nondiabetic subjects an increase in GNG flux is counterbalanced by a reduction in glycogenolysis and total EGP remains unchanged (6). In mildly diabetic subjects, there was a tendency (although not significant) for glycogenolysis to decrease as GNG increased (Fig. 2). However, in severely diabetic individuals, hepatic autoregulation clearly was lost and the marked increase in GNG flux was not associated with any compensatory decrease in glycogenolysis. Thus, these findings support the results of Boden et al. (10), who failed to observe the normal compensatory glycogenolytic response to stimulation (with lipid infusion) or inhibition (with nicotinic acid) of GNG in diabetic subjects.
The hepatic insulin resistance index was influenced by both obesity and diabetes (i.e. severity of fasting hyperglycemia). Thus, for any level of glycemia, obesity approximately doubled the hepatic insulin resistance index, whereas, for any given level of obesity, diabetes increased the hepatic insulin resistance index by approximately 50% (Fig. 1). It is noteworthy that in both nonobese and obese diabetic individuals with severe fasting hyperglycemia (FPG 11.2 ± 0.5 mmol/liter), the marked increase in both EGP and FPG, compared with the diabetic group with mild fasting hyperglycemia (FPG 7.4 ± 0.3 mmol/liter and normal basal EGP), was associated with a failure of the fasting plasma insulin concentration to increase further to offset the worsening hepatic insulin resistance (Table 1). Moreover, during the clamp EGP was progressively less suppressed moving from control to diabetics with FPG less than 9 mM to diabetics with FPG 9 mM or greater (P < 0.005) (Table 2). Consistent with previous results from our laboratory (33), these observations stress the role that ß-cell incompetence plays in determining when EGP begins to increase in absolute terms and how the liver is sensitive to the effect of insulin.
In both nonobese and obese type 2 diabetic subjects, progressive increases in FPG were associated with a progressive decline in the basal glucose clearance (Fig. 1). This relation was observed, even after changes in the fasting plasma insulin concentration were taken into account. Likewise, Rd was reciprocally related to both obesity and diabetes. These observations confirm the inhibitory effect of hyperglycemia and obesity on glucose use and are consistent with previous publications from several laboratories (1, 39, 40).
Fat distribution and GNG
A significant body of evidence has accumulated to indicate that visceral, as contrasted with sc, adiposity is associated with hepatic and peripheral insulin resistance (41). Several hypotheses have been proposed to explain this association. One hypothesis proposes that increased FFA flux into the portal vein renders the liver resistant to the restraining effect of insulin on EGP (42). This hypothesis is based on the fact that visceral adipocytes are more resistant to the antilipolytic effect of insulin (18) than are sc fat cells. Several lines of evidence (10, 18, 20, 43, 44) support the concept that FFAs are an important regulator of EGP. In vitro studies have demonstrated that plasma FFAs increase the activity of pyruvate carboxylase and phosphoenolpyruvate carboxylase, the rate-limiting enzymes for GNG (9, 45, 46), and augment the activity of glucose-6-phosphatase, the enzyme that ultimately controls the release of glucose by the liver (47). In contrast, a decrease in plasma FFAs has been shown to inhibit glucose-6-phosphatase, leading to a reduction in GNG, an effect also seen in man (48). In normal subjects, an increase in plasma FFA concentration stimulates GNG (10, 49, 50), whereas a decrease reduces it (10, 49, 50). With regard to the latter, it has been shown that a significant portion of the suppressive effect of insulin on hepatic glucose production is mediated via inhibition of lipolysis and a reduction in circulating FFA concentrations (44, 51, 52). Moreover, FFA infusion in normal humans under conditions that simulate the diabetic state (43) and in obese insulin-resistant subjects (53) enhances EGP, most likely secondarily to stimulation of GNG.
In the present study, we found a strong linear relationship between GNG and the plasma FFA concentration. A strong correlation also was found between VF and both percent GNG and absolute gluconeogenic flux. These results are consistent with the possibility that increased release of FFAs by VF into the portal circulation may contribute to the stimulation of hepatic GNG. However, whether this stimulation results in overproduction of glucose by the liver (VF was not correlated with EGP) depends on the concomitant adjustment (or lack thereof) of the glycogenolytic rate. Because GNG is less sensitive to insulin inhibition than is glycogenolysis (34, 54), the increased fasting plasma insulin concentration in mildly hyperglycemic diabetic patients (Table 2) tends to down-regulate glycogenolysis, thereby maintaining EGP within normal limits. In the more hyperglycemic subjects, the ambient plasma insulin concentration is insufficient to restrain EGP, which consequently rises to levels that are elevated in absolute terms (Table 2). Thus, the combination of visceral adipose mass/visceral lipolytic activity, ß-cell incompetence, and insulin resistance could be viewed as a regulatory axis that controls EGP by independently regulating the individual components of EGP, GNG and glycogenolysis. The contribution of VF cell-derived adipocytokines to this visceral-portal-hepatic axis remains to be defined. This is in agreement with a recent paper (55), which observed that an increase in FM, primarily in the abdominal area, was associated with hepatic insulin resistance and inhibition of suppression of EGP during the clamp.
In conclusion, in diabetic subjects obesity per se and VF accumulation in particular induce hepatic insulin resistance and are potent stimuli to augment gluconeogenic flux. Poorly controlled diabetes and abdominal obesity are independent determinants of severe fasting hyperglycemia.
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Acknowledgments |
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Footnotes |
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Abbreviations: BMI, Body mass index; C5, carbon 5; 2DM, type 2 diabetes; EGP, endogenous glucose production; FFA, free fatty acid; FFM, fat-free mass; FM, fat mass; FPG, fasting plasma glucose; GNG, gluconeogenesis; HbA1c, glycosylated hemoglobin; MRI, magnetic response imaging; Rd, glucose disappearance; T2DM, type 2 diabetes; VF, visceral fat.
Received December 16, 2003.
Accepted May 6, 2004.
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