Diabetes Care 31:311-315, 2008 DOI: 10.2337/dc07-1593 © 2008 by the American Diabetes Association
Decreased Non–Insulin-Dependent Glucose Clearance Contributes to the Rise in Fasting Plasma Glucose in the Nondiabetic RangeFrom the Division of Diabetes, University of Texas Health Science Center at San Antonio, San Antonio, Texas Address correspondence and reprint requests to Muhammad Abdul-Ghani, MD, PhD, University of Texas Health Science Center at San Antonio, Diabetes Division, 7703 Floyd Curl Dr., San Antonio, TX 78229. E-mail: albarado{at}uthscsa.edu
OBJECTIVE—To assess the contribution of decreased glucose clearance to the rise in fasting plasma glucose (FPG) in the nondiabetic range. RESEARCH DESIGN AND METHODS—A total of 120 subjects with normal glucose tolerance received an oral glucose tolerance test and euglycemic insulin clamp with 3-[3H]glucose. The basal and insulin-stimulated rates of glucose appearance, glucose disappearance, and glucose clearance and the basal hepatic insulin resistance index were calculated. Simple Pearson's correlation was used to assess the relationship between variables. RESULTS—The increase in FPG (range 75–125 mg/dl) correlated (r = 0.32, P < 0.0001) with the increase in BMI (20–50 kg/m2). The fasting plasma insulin (FPI) concentration also increased progressively with the increase in BMI (r = 0.62, P < 0.0001). However, despite increasing FPI, the basal glucose clearance rate declined and correlated with the increase in BMI (r = –0.56, P < 0.0001). Basal hepatic glucose production (HGP) decreased with increasing BMI (r = –0.51, P < 0.0001) and correlated inversely with the increase in FPI (r = –0.32, P < 0.0001). The hepatic insulin resistance (basal HGP x FPI) increased with rising BMI (r = 0.52, P < 0.0001). During the insulin clamp, glucose disposal declined with increasing BMI (r = –0.64, P < 0.0001) and correlated with the basal glucose clearance (r = 0.39, P < 0.0001). CONCLUSIONS—These results demonstrate that in nondiabetic subjects, rising FPG is associated with a decrease (not an increase) in basal hepatic glucose production and is explained by a reduction in glucose clearance.
Abbreviations: FPG, fasting plasma glucose FPI, fasting plasma insulin HGP, hepatic glucose production OGTT, oral glucose tolerance test
Hyperglycemia is a sine qua non in type 2 diabetes. The hyperglycemia is manifested both as fasting and postprandial hyperglycemia. The mechanisms that regulate the plasma glucose concentration during the postabsorptive state are distinct from those that regulate postprandial plasma glucose levels (1–3). Following glucose ingestion, approximately two-thirds of the glucose load is taken up by the skeletal muscle and one-third by the liver (4,5), while glucose-stimulated insulin secretion causes the suppression of hepatic glucose production (HGP) (5). During the postabsorptive state, the liver is responsible for the majority of endogenous glucose production, while most of the glucose uptake takes place in insulin-insensitive (brain and splanchnic) tissues. Only 25% of glucose uptake occurs in insulin-sensitive tissues, primarily skeletal muscle, during fasting conditions (6). During the postabsorptive state, tissue glucose uptake is closely matched by HGP (7,8). The primary determinant of basal HGP is the fasting plasma insulin (FPI) concentration, and small increases in the portal plasma insulin concentration markedly suppress HGP (9,10). Previous studies have demonstrated that the increase in fasting plasma glucose (FPG) concentration in subjects with type 2 diabetes is primarily due to an increase in HGP, which occurs in the presence of fasting hyperinsulinemia, indicating the presence of hepatic insulin resistance (8). Studies examining the relationship between HGP and FPG concentration in type 2 diabetic individuals have demonstrated that basal HGP does not start to increase until the FPG exceeds 140–160 mg/dl (8,11). Since basal HGP remains unchanged with FPG concentrations up to 140–160 mg/dl (8,11), the etiology of the increase in FPG remains unclear. We postulated that a decrease in tissue glucose uptake was responsible for the increase in FPG over this range. The aim of this study was to assess the relationship between tissue glucose clearance during the postabsorptive state and FPG concentration in the nondiabetic range of plasma glucose levels.
The participants included 120 normal healthy subjects (67 female and 53 male, aged 38 ± 1 years, BMI 29.1 ± 0.6 kg/m2, FPG 93 ± 1 mg/dl, 2-h plasma glucose 115 ± 2, FPI 9 ± 1 µU/ml, and 2-h plasma insulin 65 ± 01). All subjects were of Mexican-American origin and had a normal (75-g) oral glucose tolerance test (OGTT) (FPG <126 mg/dl and 2-h plasma glucose <140 mg/dl). All subjects had normal liver, cardiopulmonary, and kidney function as determined by medical history, physical examination, screening blood tests, electrocardiogram, and urinalysis. No subject was taking any medication known to affect glucose tolerance. Body weight was stable (±2 kg) for at least 3 months before study in all subjects. The study protocol was approved by the institutional review board of the University of Texas Health Science Center, San Antonio, Texas, and written informed consent was obtained from all subjects before participation. All studies were performed at the general clinical research center of the University of Texas Health Science Center at 0800 h following a 10- to 12-h overnight fast.
OGTT
Euglycemic insulin clamp
Calculations
Statistical analysis
The increase in FPG over the nondiabetic range of FPG levels (75–125 mg/dl) in the present study correlated positively (r = 0.32, P < 0.0001) with the increase in BMI (range 20–50 kg/m2) (Fig. 1A). However, over this same range, HGP decreased with the increase in BMI (r = –0.51, P < 0.0001) (Fig. 1B). We also examined the relationship between HGP, expressed per lean body mass, and BMI. The negative relationship between HGP and BMI remained when HGP was expressed per lean body mass (r = –0.31, P = 0.004).
To further examine the relationships between obesity, FPG, and HGP, subjects were divided into four quartiles. The metabolic and anthropometric characteristics of subjects in the four quartiles are shown in Table 1. The four groups were comparable in age and sex, but FPG progressively increased from quartiles 1–4 (P < 0.001 with ANOVA). However, HGP, whether expressed per total body weight or lean body mass, progressively decreased (not increased) (P < 0.001 with ANOVA).
Since FPI is the primary regulator of HGP, we examined the relationship between FPI and BMI. As expected, FPI concentration progressively increased in subjects in quartiles 1–4 (P < 0.0001) (Table 1). FPI correlated strongly and positively with BMI (r = 0.62, P < 0.0001; Fig. 1C) and negatively with HGP (r = –0.34, P < 0.0001). The product of HGP and FPI, an index of hepatic insulin resistance, progressively increased from quartiles 1–4 (P = 0.0002) and correlated closely with BMI (r = 0.52, P < 0.0001; Fig. 2A).
However, despite the marked increase in FPI, the glucose clearance rate was decreased with increasing BMI in quartiles 1–4 (P < 0.0001), and the plasma glucose clearance rate correlated inversely with BMI (r = –0.56, P < 0.0001; Fig. 2B).
To assess the contribution of obesity to the relationship between FPG and glucose clearance, we compared the correlation coefficient between the two variables in lean (BMI <27 kg/m2, n = 48) and overweight/obese subjects (BMI >27 kg/m2, n = 72). The correlation coefficient was –0.43 in lean subjects and –0.27 in obese subjects. In a multivariate model, using FPG as the dependent variable and BMI, FPI, HGP, and glucose clearance as the independent variables, all four independent variables significantly correlated with FPG, which was explained by the following equation:
60% of the variability in FPG, where more than half of this was explained by glucose clearance and basal HGP. When related to FPG, both HGP (r = 0.17, P = 0.05) and glucose clearance rate (r = –0.42, P < 0.0001) displayed a negative correlation (Fig. 3A and B). A positive correlation between the glucose clearance rate and total body glucose disposal during the insulin clamp (r = 0.39, P < 0.0001) was observed.
The results of the present study demonstrate that in nondiabetic subjects, the increase in BMI is associated with an increase in FPG and, paradoxically, with a decrease in HGP. Because HGP is the main contributor to the elevated FPG concentration in type 2 diabetic subjects (8), one might have expected that the increase in FPG observed with increasing BMI would be associated with a rise in HGP. However, the results of this study demonstrate the opposite. The inverse relationship between FPG and HGP is most striking when one compares subjects in the highest BMI quartile with subjects in the lowest BMI quartile (Table 2), and it excludes the possibility that an increase in HGP is responsible for the increase in FPG in the nondiabetic range. Because under postabsorptive conditions steady-state conditions exist with respect to the FPG concentration, the elevated FPG concentration in obese subjects must be explained by a decrease in tissue glucose clearance. Indeed, when the glucose clearance in obese subjects is compared with that in lean subjects, there is a 34% decrease in glucose clearance rate. Furthermore, the glucose clearance rate correlates negatively with the increase in BMI. These results indicate that the increase in FPG, which accompanies the increase in BMI, primarily results from the decline in glucose clearance and not from excess production of glucose by the liver. Obese subjects, as expected, had a 2.5-fold increase in FPI concentration compared with lean individuals, and the FPI concentration rose progressively with increasing BMI (Table 1, Fig. 1C). The plasma insulin concentration is the main regulator of HGP (9,10). Thus, the rise in FPI with increasing BMI leads to a progressive decrease in HGP from quartiles 1–4, and HGP was strongly and inversely correlated with the FPI (r = –0.34, P < 0.0001). Obesity per se seems has a small effect on FPG. Consistent with this, the inverse relationship between FPG and glucose clearance also was observed in lean subjects (BMI <27 kg/m2). Further, the contribution of BMI (multivariate analysis) to the increase in FPG was much smaller compared with the contributions of glucose clearance and HGP (see equation 1), indicating that the primary determinants of FPG are the HGP and tissue glucose clearance. Moreover, the impact of obesity to increase the FPG is due primarily to the decrease in tissue glucose clearance. We previously have shown that the basal insulin secretion rate increases with the increase in FPG in the nondiabetic range (14). However, when the insulin secretory rate (ISR) was related to FPG, the ratio of ISR to FPG remained constant across the entire nondiabetic range of FPG levels (14). These results indicate that 1) although glucose-stimulated insulin secretion is markedly impaired with increasing FPG, basal insulin secretion is not affected by fasting hyperglycemia, and 2) fasting hyperinsulinemia is a compensatory β-cell response to fasting hyperglycemia. The resultant fasting hyperinsulinemia that accompanies fasting hyperglycemia inhibits HGP and explains the present observation that HGP declines as FPG increases within the nondiabetic range. Thus, the decrease in HGP associated with the increase in BMI can be viewed as a compensatory physiological response to fasting hyperglycemia, which aims to ameliorate the rise in FPG. However, the increased hepatic insulin resistance in obese subjects, which also strongly correlates with increasing BMI, renders the liver more resistant to the action of insulin and results in an incomplete suppression of HGP.
We previously have shown that the increase in insulin secretion rate in response to the rise in FPG peaks at an FPG concentration of
A decrease in non–insulin-dependent glucose clearance previously has been reported in subjects with type 2 diabetes (16). During the postabsorptive state, Insulin-stimulated glucose uptake in skeletal muscle correlated well with non–insulin-dependent glucose clearance during the fasting state. Thus, it is possible that the same defect responsible for the impairment in insulin-stimulated glucose uptake could explain the decrease in basal glucose clearance, since the majority (>80%) of glucose disposal during the euglycemic insulin clamp occurs in muscle (21). In summary, the results of the present study demonstrate that the decrease in non–insulin-dependent glucose clearance is the primary factor that contributes to the increase in FPG concentration within the nondiabetic range. The decline in basal HGP observed with rising BMI is explained by the increase in FPI concentration that represents a compensatory response to the obesity-related insulin resistance.
Published ahead of print at http://care.diabetesjournals.org on 13 November 2007. DOI: 10.2337/dc07-1593. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Received for publication August 13, 2007. Accepted for publication November 6, 2007.
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