Ethnicity affects the postprandial regulation of glycogenolysis

Ashok Balasubramanyam1, Siripoom McKay1,2, Prashant Nadkarni1, Arun S. Rajan1, Armandina Garza3, Valory Pavlik3, J. Alan Herd4, Farook Jahoor2, and Peter J. Reeds2

1 Division of Endocrinology and 4 Section on Atherosclerosis, Department of Medicine, 2 Department of Pediatrics, Children's Nutrition Research Center, and 3 Department of Community Medicine, Baylor College of Medicine, Houston, Texas


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

We investigated the effect of nutrient intake on glucose metabolism in normal Mexican-Americans (n = 6) and European-Americans (n = 6). Subjects were studied after an 18-h fast and after 5-6 h of ingestion of hourly meals that supplied 6.35 or 12.75 µmol glucose · kg-1 · min-1. Endogenous glucose production (EGP), gluconeogenesis (GNG), and glycogenolysis (GLY) were estimated by mass isotopomer analysis with [U-13C]glucose infusions. Fasting EGP, GNG, and GLY did not differ between the groups. Food ingestion lowered the molar rate of GNG by only 31%. However, while consuming the lower quantity of nutrients, Mexican-Americans had higher plasma glucose (P < 0.05), a 39% higher rate of EGP (P < 0.05), and a 68% (P < 0.025) higher rate of GLY than the European-Americans. At the higher intake, EGP and GLY were suppressed completely in both groups. There was a linear relationship between insulin concentrations, EGP, and GLY in both groups, but the slope of the line was significantly (P < 0.05) greater in the European-Americans. We conclude that the sensitivity of GLY to nutrient intake differs between ethnic groups and that this may play a role in the increased predisposition of Mexican-Americans to type II diabetes.

Mexican-American; type II diabetes; gluconeogenesis


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

ENDOGENOUS GLUCOSE PRODUCTION (EGP) plays a dominant role in determining the hyperglycemia of type II diabetes in both the fasting (10, 15, 16, 20, 32-34, 40) and postprandial (19, 20, 34) states. The increased predisposition of certain ethnic groups to type II diabetes raises the important question as to whether genetic factors affect the regulation and rates of fasting and postprandial EGP and of its components, gluconeogenesis and glycogenolysis. One way to approach this question is to compare these rates during fasting, and the degree of their suppression during feeding, in apparently normal members of ethnic groups who collectively have either a high or a low prevalence of type II diabetes.

Despite an active and ongoing debate regarding the most appropriate way to calculate the results (31, 46), the isotopic technique of mass isotopomer distribution analysis, in which a multiply labeled tracer can be distinguished from labeled molecules of the same tracer that have been resynthesized by the subject (27, 28, 30, 45, 46), presents an attractive approach to quantifying the various glucose carbon interactions and to measuring the rate of glucose carbon cycling in vivo during fasting and feeding. We have utilized this approach in the present study, employing intravenous infusions of uniformly labeled [13C]glucose ([U-13C]glucose) to quantify total EGP, gluconeogenesis, glycogenolysis, and glucose carbon recycling in the fasting and fed states. We have compared these rates in healthy, glucose-tolerant American men of Mexican descent, who belong to an ethnic group with a high prevalence of type II diabetes, with those in normal American men of Northern European extraction, who belong to an ethnic group with a significantly lower prevalence of type II diabetes. Our initial hypothesis was that subjects of Mexican origin would have higher rates of glucose production (specifically, of gluconeogenesis) irrespective of their short-term feeding status.


    MATERIALS AND METHODS
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ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Subjects

After approval was obtained from the Baylor Affiliated Hospitals Institutional Review Board, six adult Mexican-American men and six adult European-American men were recruited for the study. The ages of the subjects ranged from 33-64 yr among the Mexican-Americans and from 35-64 yr among the European-Americans. Ethnicity was determined by the ethnic self-identification of the subject and by the ethnicity of each of his parents and grandparents. All of the Mexican-American subjects, except one, were long-term (>10 yr) residents of Houston, TX. None had a first- or second-degree relative known to have type II diabetes or a parent or grandparent who could be identified as European. The European-Americans were all of Northern European extraction and were also long-term (>10 yr) residents of Houston, TX. None had a first-degree relative known to have type II diabetes or a parent or grandparent who could be identified as Native American or Mexican-American in ethnicity.

The Mexican-American subjects were matched for age with the European-American subjects. They were also matched for body mass index (BMI), which ranged from 24-30 for the Mexican-American subjects and 23-27 for the European-Americans. (The reasons for the slight disparity in the upper end of the BMI range may be traced to the characteristic adult body habitus of persons of these ethnic groups, as described in the DISCUSSION section). All subjects had normal levels of glycosylated hemoglobin (HbA1c) and thyroid-stimulating hormone (TSH) (Table 1). Before the subjects were recruited into the study, measurements of plasma glucose were made while they were fasting at 8 AM, as well as 0.5 and 2 h after an oral load of 75 g glucose (Sun-Dex 75, Curtis Matheson Scientific, Houston, TX). All were found to have glucose tolerance within normal limits (Table 1) according to both the criteria of the National Diabetes Data Group (24) and the more recent recommendations of the American Diabetes Association (1).

                              
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Table 1.   Subject characteristics

Infusion Protocol

Each subject participated in three studies, once while fasting ("fasting study") and twice while consuming at hourly intervals a balanced protein- and lipid-containing enteral formula (Ensure, Ross Products Division, Abbott Laboratories, Columbus, OH) that provided different doses of ingested glucose: 1) 6.38 µmol · kg-1 · min-1, calculated to only partially suppress a fasting level of EGP ("low-fed study"); and 2) 12.75 µmol · kg-1 · min-1, calculated to fully suppress a fasting level of EGP ("high-fed study"). For two days before each study, the subjects consumed Ensure in an amount that supplied 35 cal · kg-1 · day-1 [22% from fat, 64% from carbohydrate (85% from glucose and 15% from fructose), and 14% from protein] and 1.2 g protein · kg-1 · day-1. The isotope infusion was performed on the third day. The fasting study commenced after a 12-h overnight fast. A blood sample was taken for measurement of the baseline labeling of plasma glucose and lactate, and the subject then received, via a superficial hand vein, a 6-h primed constant intravenous infusion of [U-13C]glucose. The [U-13C]glucose was purchased from Cambridge Isotope Laboratories (Woburn, MA). The abundance of the 13C6-isotopomer was 92%, and 7% of the labeled glucose was 13C5. For the fasting study, the target priming dose was 12.5 µmol/kg and the infusion rate was 0.20 µmol · kg-1 · min-1. Samples of blood were obtained at half-hour intervals during the infusion. The protocols for the fed studies were identical to that of the fasting study, with the exception that: 1) beginning 2 h before the start of the [U-13C]glucose infusion, and every 60 min (low-fed study) or 30 min (high-fed study) thereafter for 8 h (until the end of the 6-h infusion), the subjects consumed 30 ml of Ensure. This protocol was calculated to supply 7.5 µmol of carbohydrate · kg-1 · min-1 (6.38 µmol of glucose · kg-1 · min-1) for the low-fed study or 15 µmol of carbohydrate · kg-1 · min-1 (12.75 µmol of glucose · kg-1 · min-1) for the high-fed study; and 2) the target priming dose of [U-13C]glucose was 20 µmol/kg, with an infusion rate of 0.30 µmol · kg-1 · min-1. The measured infusion rates are shown in RESULTS (see Table 5).

Blood samples were collected in precooled vacutainers, placed on ice, and centrifuged to separate plasma. Plasma was stored at -70°C for later analysis.

Sample Preparation and Analysis

Plasma samples were acidified with an equal volume of 1 M acetic acid and applied to a Dowex 50×8 (H+) cation exchange resin. The sample front and a 2-bed volume water wash were dried and used for glucose and lactate analysis.

Mass spectrometry was performed on a Hewlett Packard 9890A gas chromatograph quadrupole mass spectrometer with helium as the carrier gas and methane as the ionizing gas. Isotopomers of glucose were measured by positive chemical ionization of their pentaacetate derivatives with a DB 29 column, which separates the acetates of glucose and fructose. Lactate was measured by negative chemical ionization of its pentafluorobenzyl derivative with a DB 25 column. We monitored ions with a mass-to-charge ratio of 331-337 for glucose and 87-90 for lactate.

Plasma insulin concentrations were measured in duplicate samples by RIA with a kit from Linco Research (St. Charles, MO). Plasma glucose concentrations were measured by the glucose oxidase reaction with a COBOS clinical analyzer. TSH was measured by a standard double-antibody radioimmunoassay. HbA1c was measured by electrophoresis.

Calculations

The crude ion abundances of all metabolite tracers were converted to fractional abundances with the matrix approach, with, as baseline, the ion spectrum of glucose isolated from each subject before the start of the [U-13C]glucose infusion.

The entry rate of glucose (glucose Ra) was calculated as
R<SUB>a</SUB> = R × <FENCE><FR><NU><AR><R><C>fractional abundance of</C></R><R><C>infused [<SUP>13</SUP>C<SUB>6</SUB>]glucose</C></R></AR></NU><DE><AR><R><C>fractional abundance of</C></R><R><C>[<SUP>13</SUP>C<SUB>6</SUB>] glucose in plasma</C></R></AR></DE></FR> − 1</FENCE> (1)
in which R is the molar rate of glucose infusion. In the fasting study,
EGP = glucose R<SUB>a</SUB> (2)
In the fed study
EGP = glucose R<SUB>a</SUB> − glucose intake (3)
There is continuing controversy regarding the calculation of the gluconeogenic rate from the recycling of 13C during a [U-13C]glucose infusion. Because at this stage we did not wish to enter into this debate, in the present paper we elected to utilize the equations of both Tayek and Katz (46) and Landau et al. (31). Both equations provide an estimate of the fractional contribution of gluconeogenesis (GNGF) to the glucose Ra, so that the molar rate of gluconeogenesis (GNGA) was calculated as GNGA = glucose Ra × GNGF (31, 46).

Glycogenloysis was then calculated by subtracting the value for GNGA from EGP as calculated with Eq. .

Statistics

The data are expressed as means ± SD. The glucose kinetic and metabolic cycling data were initially analyzed by two-way ANOVA with ethnicity and feeding status as the independent variables. Post hoc testing of ethnic differences within a given study was by grouped t-tests. A value of two-tailed P < 0.05 was taken as statistically significant. Plasma glucose and insulin concentrations were analyzed by one-way ANOVA, and a value of one-tailed P < 0.05 was taken as statistically significant.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Subject Characteristics

Table 1 outlines physical and biochemical characteristics of the subjects. Age ranges were similar for the two groups. Values for HbA1c, fasting or stimulated plasma glucose concentrations, and TSH were in the normal range for all subjects and similar for the two groups. BMI ranges overlapped, and mean BMIs were not significantly different for the two groups.

One Mexican-American subject declined to participate in the high-fed study. Therefore, the results described below are for n = 6 in each group for every study except the high-fed study, in which n = 5 in the Mexican-American group.

Glucose and Insulin Concentrations

Plasma glucose and insulin concentrations were measured in samples taken before the start of the [U-13C]glucose infusion, as well as every half hour during the infusion. The values over the last 2 h of each tracer infusion are shown in Figs. 1 and 2, and the data are summarized in Table 2. Over the last 2 h of the fasting study, the European-Americans had significantly higher plasma glucose levels than the Mexican-Americans (P < 0.05), a similar (but nonsignificant) difference having been found in the fasting samples, taken as part of the initial screening test for glucose tolerance (Table 1). In both groups, plasma glucose concentrations declined significantly over the course of the infusion in the fasting study: from 5.28 ± 0.39 to 4.94 ± 0.22 mmol/l in the Mexican-Americans and from 5.72 ± 0.44 to 5.28 ± 0.33 mmol/l in the European-Americans (Fig. 1). In contrast to the fasted state, during the last 2 h of the low-fed study infusion, the Mexican-Americans had significantly (P < 0.05) higher plasma glucose levels than the European-Americans. Although a similar difference in glucose concentrations persisted during the high-fed study, it was not significant (Mexican-Americans: 6.67 ± 0.33 mmol/l vs. European-Americans: 6.33 ± 0.33 mmol/l; P = 0.064).


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Fig. 1.   Plasma glucose concentrations during final hours of [U-13C]glucose infusions when metabolic measurements were made. A: fasting study. B: low-fed study (6.38 µmol glucose · kg-1 · h-1). C: high-fed study (12.75 µmol glucose · kg-1 · h-1). Each point is mean plasma glucose concentration ± SD. , Mexican-Americans; open circle , European-Americans.



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Fig. 2.   Plasma insulin concentrations during final hours of [U-13C]glucose infusions, when metabolic measurements were made. A: fasting study. B: low-fed study (6.38 µmol glucose · kg-1 · h-1). C: high-fed study (12.75 µmol glucose · kg-1 · h-1). Each point is mean plasma insulin concentration ± SD. , Mexican-Americans; open circle , European-Americans.


                              
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Table 2.   Plasma glucose and insulin concentrations

Figure 2 shows mean plasma insulin concentrations for each group over the last 2 h of each study, i.e., during the period when the metabolic measurements were made. Plasma insulin concentrations were maintained at a steady level in each group while fasting (within 2-3 µU/ml) and at each level of feeding (within 2-5 µU/ml). There was a trend (P = 0.062) for the Mexican-American group to have higher plasma insulin concentrations while fasting (Table 2). This ethnic difference became statistically significant in the fed state (low intake difference = 6.0 ± 2.4 µU/ml; P < 0.05; high intake difference = 6.4 ± 1.8 µU/ml, P < 0.025).

Glucose Kinetics and Metabolic Recycling

Time course of glucose labeling. Figure 3, A-C, shows the isotopic labeling of the 13C3- and 13C6-isotopomers of glucose and of the 13C3-isotopomer of lactate in the fasting (Fig. 3A), low-fed (Fig. 3B), and high-fed (Fig. 3C) studies. Between 4 and 6 h of infusion during the fasting study, the labeling of [13C6]glucose was at steady state as adjudged both by a slope that was not significantly different from zero and by a within-subject standard deviation of 3.2-5.4% of the mean value. The 13C3-isotopomers of glucose and lactate also showed little or no systematic change in their fractional abundances between 4 and 6 h. The mean values for the [M+1] to [M+3] isotopomers of glucose and lactate as well as the [M+6]glucose isotopomer enrichment are shown in Table 3 ([M+1], [M+3], and [M+6] refer to percentages of glucose molecules with 1, 3, and 6 13C-atoms, respectively, or lactate molecules with 1 and 3 13C-atoms, respectively). There were no significant differences between the fractional abundances of the two groups for any of the labeled isotopomers, except for [13C6]glucose during the low-fed study, in which the fractional abundance of [13C6]glucose was significantly lower in the Mexican-Americans (P < 0.01), indicating a significantly higher glucose Ra (P < 0.01) in the Mexican-Americans at this glucose intake. The ratio of [M+3]lactate to [M+6]glucose, a measure of the glycolytic metabolism of glucose, rose significantly when subjects were fed, from 0.36 ± 0.04 in the fasted state to a mean value of 0.49 ± 0.05 (P < 0.025) in the fed state. This was reflected in a highly significant fall (P < 0.01) in the dilution factor (D in Table 4) used in the calculation of gluconeogenesis by the Landau method. The other factor in this calculation, the contribution of recycling to glucose labeling (presented as F in Table 4), also fell highly significantly (P < 0.001) with feeding. Neither the lactate labeling nor the fractional recycling of tracer showed a significant ethnic difference.


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Fig. 3.   [U-13C]glucose, [13C3]glucose, and [13C3]lactate labeling during steady state in the final 2 h of the [U-13C]glucose infusion when metabolic measurements were made. A: fasting study. B: low-fed study (6.38 µmol glucose · kg-1 · h-1). C: high-fed study (12.75 µmol glucose · kg-1 · h-1). Fractional abundances of all isotopomers were quantitatively very similar in the 2 groups during fasting and high caloric feeding (high-fed study). Fractional abundance of [M+6] glucose was significantly lower in Mexican-Americans during the low caloric feeding (low-fed study), indicating a significantly higher rate of glucose production in this group. Filled symbols, Mexican-Americans; open symbols, European-Americans.


                              
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Table 3.   Fractional abundance of [M + 1] - [M + 6]glucose and [M + 1] - [M + 3]lactate in plasma


                              
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Table 4.   Derived factors necessary for the calculation of gluconeogenesis by Tayek and Katz (46) and Landau et al. (31) approaches

Glucose metabolism. Table 4 shows the factors used in the calculation of gluconeogenesis by the two approaches, and Table 5 shows the estimates of glucose Ra, glucose production, gluconeogenesis, and glycogenolysis. Glucose Ra in the fasted state was not significantly different between the two groups and was increased by feeding in both groups. With the lower intake, however, the increase in glucose Ra (+5.34 ± 0.71 µmol · kg-1 · min-1) in the Mexican-American group was significantly greater than that in the European-American group (+3.33 ± 0.68 µmol · kg-1 · min-1). As a consequence, the calculated EGP (7.12 µmol · kg-1 · min-1 in the Mexican-Americans and 5.14 µmol · kg-1 · min-1 in the European-Americans) differed significantly (P < 0.01) between the two groups at the lower intake. At the higher intake, glucose Ra was not only similar between the two groups of subjects but was very close to the calculated intake of glucose.

                              
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Table 5.   Ra, EGP, GNGF, GNGA, and GLY

Table 5 also shows estimated fractional gluconeogenesis (percentage of glucose Ra) and absolute gluconeogenesis (µmol · kg-1 · min-1). In the fasted state, as expected, the two methods of calculating these parameters gave different results (fractional gluconeogenesis was 70 ± 5% for the Tayek and Katz method and 45 ± 12% for the Landau method), but there was no ethnic difference. Indeed neither fractional nor absolute gluconeogenesis showed an ethnic effect at either level of feeding. Although the proportional contribution of gluconeogenesis (GNGF) to glucose Ra was significantly (P < 0.001) lower in the fed studies (-60% in both groups and by both methods), strikingly, when calculated in molar terms, feeding inhibited GNG to only a small extent (-31%) that barely achieved statistical significance. As a consequence of the higher glucose Ra accompanied by unaltered gluconeogenesis, the calculated rate of glycogenolysis during the low-fed study was substantially higher in the Mexican-Americans than in the European-Americans. Once again, both methods of calculation, although yielding different values, led to the same overall conclusion, i.e., that in the Mexican-Americans the lower rate of feeding had had virtually no effect on the contribution of glycogenolysis to glucose Ra.

Figure 4 shows the relationship between EGP and plasma insulin concentrations for the two groups. In this figure, only data for the fasting and low-fed studies are shown, because EGP in the high-fed state was not significantly different from zero in either group. In both groups there was a statistically significant linear relationship, but the slopes of the lines were different (Mexican-Americans: -0.143 ± 0.044 and European-Americans: -0.326 ± 0.130; difference in slopes P < 0.05). The y-intercepts (9.56 and 9.65 µmol · kg-1 · min-1) were not significantly different. Figure 5 shows the relationship between the calculated rate of glycogenolysis and plasma insulin concentrations in the fasting and low-fed studies in the two groups. Here, as for EGP, there was a significant negative relationship, irrespective of the method chosen to calculate the values. Moreover, the methods yielded values that differed to only a minor extent within each group. However, the slope of the line for the Mexican-Americans (-0.135 ± 0.060) was significantly (P < 0.05) different from that of the European-Americans (-0.198 ± 0.059). In contrast, the relationship between gluconeogenesis and plasma insulin concentrations was nonsignificant and showed no ethnic effect (data not shown).


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Fig. 4.   Relationship between plasma insulin concentrations and total endogenous glucose production. , Mexican-Americans; open circle , European-Americans.



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Fig. 5.   Relationship between plasma insulin concentrations and calculated rate of glycogenolysis. A: Mexican-Americans. B: European-Americans. , Glycogenolysis calculated by method of Tayek and Katz (46); open circle , glycogenolysis calculated by method of Landau et al. (31).


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Our results show first that there are significant ethnic variations in glucoregulation that are manifested by a differential impact of carbohydrate intake on the rate of glycogenolysis. Second, they show that feeding suppresses gluconeogenesis to only a relatively small extent. Importantly, we have arrived at these conclusions by applying either of two equations to our glucose isotopic data. Although, as expected from the ongoing debate regarding the relative accuracy of the two methods (31, 46), they yielded different quantitative values for the gluconeogenic rate, the results from both sets of calculations led to the same physiologically and clinically significant conclusions. In some respects this outcome is not surprising as both methods are based on the same data, the relative abundances of the M+1 - M+3 and M+6 isotopomers of glucose and of the M+1 - M+3 isotopomers of lactate.

A considerable body of evidence suggests that increased EGP is an important factor affecting the fasting hyperglycemia that occurs in persons with type II diabetes (3-5, 10, 13, 15, 16, 19, 20, 29, 34, 47). In fact, across a wide spectrum of glycemia, the rate of EGP has been shown to be linearly related to levels of fasting blood glucose (19, 26), and we found a similar significant relationship in the present study of apparently normoglycemic individuals. However, because most human beings spend the greater portion of the day in the postprandial state, it is likely that the factors that affect EGP after the ingestion of a meal are at least as significant to the development of type II diabetes and to long-term glycemic control (18, 20, 32) as those that affect fasting EGP. Indeed, Firth et al. (19) have demonstrated a relationship between fasting blood glucose levels and postprandial EGP in type II diabetes and have suggested that the inability to suppress EGP after food intake is partly related to hepatic resistance to insulin. Abnormally high rates of EGP have also been found in persons with impaired glucose tolerance (IGT), and Mitrakou et al. (33) have shown that this largely reflects a diminished suppression of EGP by insulin, coupled with early blunting of the insulin secretory response.

The present data show, for what we believe to be the first time, that there is a difference in the rate of suppression of EGP upon feeding between normal, glucose tolerant persons belonging to ethnic groups with significantly different risks for type II diabetes. Although both groups of subjects exhibited a continuous relationship between plasma insulin and EGP, the slope of the line was ~60% steeper in the European-American group than in the Mexican-American group. Thus, despite higher insulin levels after low caloric feeding, the Mexican-Americans had a higher glucose Ra than the European-Americans. These results complement studies that have shown differences in insulin secretion and sensitivity between normoglycemic African-American and white children (2), in resting energy expenditure between normal African-American and white adolescents (36), in sympathetic nervous system activity between normal Pima Indian and white adults (44), and, critically, in basal and stimulated insulin levels between normoglycemic Mexican-American and white individuals (22).

A key question is whether elevated EGP in persons with type II diabetes or IGT reflects a disorder of gluconeogenesis, glycogenolysis, or both pathways of glucose production and the extent to which these pathways are sensitive to the effects of feeding. Previous studies have emphasized the role of gluconeogenesis, in part because it is generally believed to be the major contributor to fasting glucose production. In the present study of normal individuals, even the method of Landau, which gave the lower value, indicated that gluconeogenesis contributed 46% of EGP after an 18-h fast. Numerous studies have suggested that gluconeogenesis is elevated in type II diabetes both while fasting (10, 11, 15, 16, 20, 23, 39) and in the postprandial state (19). Even so, it should be noted that Tayek and Katz (45), who showed an elevation in the fasting molar rate of gluconeogenesis in patients with type II diabetes, commented that this was not associated with an increased fractional contribution of the process but was part of an apparent upregulation of both glycogenolysis and gluconeogenesis. Furthermore, a recent study (14) suggests that in type II diabetic patients with moderately elevated fasting blood glucose levels, gluconeogenic rates are not increased compared with normal controls, despite increased lactate turnover.

Our results showed that, irrespective of the method used to calculate the values, there was little ethnic difference in the rate of gluconeogenesis. Perhaps more interestingly, feeding, even at a rate that completely suppressed EGP (as shown by the equivalence of glucose Ra and glucose intake), produced only a small (25-30%) suppression of gluconeogenesis, as shown qualitatively by the continuing production of low mass isotopomers of glucose. It appears therefore that in the fed state, hepatic glucose synthesis from C-3 precursors and its subsequent release into the circulation are balanced almost exactly by hepatic glucose utilization. This conclusion is consistent with the observations of Petersen et al. (40) who used [13C]glucose infusions to measure glucose Ra and NMR spectroscopy to measure the rate of change of hepatic glycogen.

Despite extensive data in favor of gluconeogenesis as a major contributor to hyperglycemia, glycogenolysis is also likely to contribute significantly to the abnormal EGP in type II diabetes. Moreover, glycogenolysis may be the component of EGP that is most significantly regulated by physiological levels of insulin (7). Acute changes in EGP in response to insulin, glucagon, catecholamines, and leptin, as well as the effects of hepatic "autoregulation" are predominantly due to changes in glycogenolysis (5, 21, 35, 40, 41). In insulin clamp studies, glycogenolysis is much more sensitive to insulin than gluconeogenesis (8, 9, 42), and the rate of fall of EGP during a fast parallels that of glycogenolysis, not that of gluconeogenesis (43). Finally, it has been shown that the drug metformin, which substantially decreases EGP in patients with type II diabetes, has its main effects on glycogenolysis rather than on gluconeogenesis (12).

The present results furnish additional proof of the importance of glycogenolysis to glucoregulation. The dominating influence on what appeared to be an ethnic difference in the effect of nutrient absorption and/or insulin on EGP was largely due to a less marked suppression of glycogenolysis in the Mexican-American subjects. These results demonstrate that normal, unrelated adults who belong to an ethnic group with a high prevalence of type II diabetes, even when they are healthy, glucose tolerant, and specifically selected for a low familial tendency to type II diabetes, appear to be less responsive to the suppressive effects of ingestion of a mixed meal on glycogenolysis than normal adults of an ethnic group with a substantially lower type II diabetes risk. This observation raises the possibility that the factors that regulate the rates of glycogenolysis differ significantly between different ethnic groups and contribute to increased genetic susceptibility to type II diabetes in groups at higher risk.

Mexican-Americans have a high prevalence and incidence of type II diabetes (6). Hanis et al. (24) have shown that over 50% of Mexican-American individuals older than 35 yr are directly affected by diabetes, either by virtue of their having the disease or by being a first-degree relative of a diabetic person. From the outset, the strong predisposition of the Mexican-American population toward type II diabetes and high BMI (36) was underscored by the difficulty we experienced in identifying subjects who could satisfy all our eligibility criteria. Indeed, we found that even the subjects who met the criteria of relative leanness, glucose tolerance, and absence of a family history of type II diabetes had higher insulin concentrations, as well as a tendency to higher BMI, than the age-matched European-American group. The observation of higher fasting concentrations of insulin in the Mexican-Americans subjects has been made previously in an extensive epidemiological survey in San Antonio, TX (22). The present data extend this to show that the ethnic difference in plasma insulin concentrations observed in the fasted state became progressively greater and statistically more significant as the level of glucose intake was increased. There was also a tendency toward higher circulating glucose concentrations in the Mexican-Americans upon feeding.

The increased BMI in the upper part of the range in our otherwise normal Mexican-American subjects reflects the results of extensive demographic surveys that indicate that, at comparable ages after maturity, Mexican-Americans have more upper body fat than US non-Hispanic whites and that by age 30, the average BMI of Mexican-Americans in South Texas is >30 (37). This raises the question as to whether the differences in postprandial glucose metabolism between the two groups might be explained in part by differences in body composition. We found no interindividual correlations between BMI and rates of EGP, gluconeogenesis, or glycogenolysis, and the mean BMIs of the two groups were not significantly different. However, in view of the complex relationship between ethnicity, environment, body composition, and insulin sensitivity, it would be appropriate to say that the differences we have found are related to some degree of interaction between ethnicity and environment, one manifestation of which is a difference in body habitus.

Eriksson et al. (17) have shown that a defect in nonoxidative glucose disposal (i.e., in glycogen synthesis) is perhaps the earliest defect noted in glucose- tolerant, Northern European persons at risk for type II diabetes. We have found that a defect in the suppression of glycogenolysis while feeding might prove to be an important and early marker for the predisposition of Mexican-Americans to type II diabetes. These results warrant further investigation of the regulation of glycogenolysis in this ethnic group, as well as more extensive and detailed examination of interactions between ethnicity-genotype and metabolic phenotype in type II diabetes.


    ACKNOWLEDGEMENTS

We thank Dr. Glori Chauca for recruitment of subjects, Sopar Seributra, Terry Techmanski, and the nursing and dietary staffs at the Metabolic Research Unit, Children's Nutrition Research Center (CNRC), and the Baylor Adult General Clinical Research Center (GCRC) for meticulous care of the study participants, and Dr. Lisa Hung Yu and Elizabeth Frazer for expert technical assistance in mass spectrometric analysis.


    FOOTNOTES

This work was supported by a Juvenile Diabetes Foundation Career Development Award, the Redfern Fund and a Chao Scholar Award (A. Balasubramanyam), and the United States Department of Agriculture/Agriculture Research Service (USDA/ARS) under Cooperative Agreement No. 5862-5-01003 (USDA/ARS CNRC, Department of Pediatrics, Baylor College of Medicine and Texas Children's Hospital, Houston, TX). Support was also provided by the GCRC at Texas Children's Hospital (NIH-RR-0188).

The contents of this publication do not necessarily reflect the views or policies of the USDA. Mention of trade names, commercial products, or organizations does not imply endorsement by the US Government.

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Address for reprint requests and other correspondence: A. Balasubramanyam, Division of Endocrinology, Baylor College of Medicine, Rm. 549E, One Baylor Plaza, Houston, TX (E-mail: ashokb{at}bcm.tmc.edu).

Received 3 March 1999; accepted in final form 11 June 1999.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
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Am J Physiol Endocrinol Metab 277(5):E905-E914
0002-9513/99 $5.00 Copyright © 1999 the American Physiological Society




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