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Letters to the Editor |
-Glutamyltransferase, Fasting Plasma Glucose, and Triglycerides in the General Population
1 Sezione di Chimica Clinica, Dipartimento di Scienze, Morfologico-Biomediche, Università degli Studi di Verona, Verona, Italy
2 Sezione di Endocrinologia e, Malattie del Metabolismo, Dipartimento di Scienze, Biomediche e Chirurgiche, Università degli Studi di Verona, Verona, Italy
aAddress correspondence to this author at: Sezione di Chimica Clinica, Dipartimento di Scienze Morfologico-Biomediche, Università degli Studi di Verona, Ospedale Policlinico G.B. Rossi, Piazzale Scuro, 10, 37134 Verona, Italy. Fax 0039-045-8201889; e-mail ulippi{at}tin.it.
To the Editor:
Recent population-based epidemiologic studies have convincingly shown that serum 
-glutamyltransferase (GGT) activity is associated with many cardiovascular disease risk factors and predicts new-onset type 2 diabetes, hypertension, stroke, and myocardial infarction (1)(2). We read with interest the recent article by Lim et al.(3) on the possible interaction between GGT and obesity and its association with the risk of prevalent type 2 diabetes, findings indicating that obesity itself may not be a sufficient risk factor for diabetes when GGT concentrations approach the lower limit of the reference interval. The clinical implications of this conclusion are noteworthy because overweight-obese people with GGT concentrations at the lower limit of the reference interval (e.g., <20 U/L) would no longer be considered at high risk of developing diabetes. To further investigate the relationships among GGT, diabetes, and other major biochemical components of the metabolic syndrome in the general population, we analyzed whether serum GGT concentrations predict prevalent diabetes and hypertriglyceridemia, and whether there is an interaction between GGT and hypertriglyceridemia that affects the risk of prevalent diabetes.
We performed a retrospective analysis on the database of the laboratory information system of the clinical chemistry laboratory at the Verona University Hospital to retrieve test results for serum GGT, fasting plasma glucose (FPG), and triglyceride, which had been performed on the whole cohort of outpatients consecutively referred by general practitioners for routine blood testing in the preceding 9 months (August 2006–April 2007). Venous blood from outpatients was routinely collected in the morning from fasting individuals, and FPG, triglycerides, and GGT were assayed by enzymatic procedures on a Roche/Hitachi Modular System (Roche Diagnostics GmbH). We assessed the significance of differences and frequency distributions of values with the Kruskal–Wallis test and the 
2 test (for categorical variables), respectively. Multivariable logistic regression analysis was used to examine the interaction relationship with diabetes, as the dependent variable, predicted from triglycerides (subdivided in tertiles: <0.95, 0.95–1.47, and >1.47 mmol/L) within 3 categories of GGT (<20, 20–39.9, and 
40 U/L). Adjusting variables were age and sex. Statistical analyses were performed using the statistical package SPSS-version 12.
Cumulative results for GGT, FPG, and triglycerides were retrieved for 7267 outpatients >35 years old during a 9-month period. As shown in Table 1
, the concentrations of FPG and triglycerides markedly increased among the GGT categories. Similarly, the frequency of those with FPG 7.0 mmol/L, a cutpoint suggestive for diagnosing diabetes according to the American Diabetes Association guidelines (4), and of those with hypertriglyceridemia (1.7 mmol/L by the Third Adult Treatment Panel criteria) increased steadily across the spectrum of GGT thresholds from 16% to 31% for FPG and from 14% to 39% for triglycerides, respectively. These results remained unchanged after we adjusted for sex and age. Interestingly, as also shown in Table 1
, the age-, and sex-adjusted frequencies of diabetes significantly increased across the tertiles of triglycerides within all 3 GGT categories, thus excluding the existence of a significant interaction between GGT and triglycerides on the risk for prevalent diabetes.
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Increased concentrations of GGT, an enzyme associated with liver damage (2), are conventionally interpreted as a marker of alcohol consumption. However, as previously reported, alcohol consumption could not entirely explain the association between increased GGT concentrations and type 2 diabetes, because this association was observed even after adjustment for daily alcohol intake and existed among nondrinkers(1)(2). Moreover, it has been shown that increased GGT concentrations are associated with lipid abnormalities independently of obesity(5). In the present study, we failed to find a significant interaction between GGT and triglycerides in predicting prevalent diabetes, suggesting that the association between triglycerides and diabetes might be only marginally influenced by serum GGT concentrations. Obviously, we must be cautious in making any causal inference because of the cross-sectional nature of our study, but such findings would not be unexpected because of the intertwined and complex biological relationships linking diabetes and hypertriglyceridemia. Nevertheless, our results do not exclude the possibility that increased GGT, as a marker of fatty liver, plays an important role in the development of type 2 diabetes(1)(2).
Overall, we agree with the suggestions of Lim et al. (3) that GGT measurement may be useful in clinical settings for detecting high-risk subpopulations of type 2 diabetes and/or hypertriglyceridemia. Such individuals might benefit from a more intensive therapeutic approach to decrease their global cardiovascular risk, regardless of potential unmeasured effects of lifestyle or obesity. Conceivably, the significant association of serum GGT concentrations with FPG and triglycerides, observed in our investigation, may be biologically explained by some underlying mechanisms such as hepatic steatosis, insulin resistance, and increased oxidative stress.
References
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