Diabetes 53:2855-2860, 2004 © 2004 by the American Diabetes Association, Inc. Elevated Alanine Aminotransferase Predicts New-Onset Type 2 Diabetes Independently of Classical Risk Factors, Metabolic Syndrome, and C-Reactive Protein in the West of Scotland Coronary Prevention Study
1 Division of Cardiovascular and Medical Sciences, University of Glasgow, Glasgow Royal Infirmary, Glasgow, Scotland, U.K
We examined the association of serum alanine aminotransferase (ALT) with features of the metabolic syndrome and whether it predicted incident diabetes independently of routinely measured factors in 5,974 men in the West of Scotland Coronary Prevention Study. A total of 139 men developed new diabetes over 4.9 years of follow-up. ALT, but not aspartate aminotransferase, levels increased progressively with the increasing number of metabolic syndrome abnormalities from (means ± SD) 20.9 ± 7.6 units/l in those with none to 28.1 ± 10.1 units/l in those with four or more (P < 0.001). In a univariate analysis, men with ALT in the top quartile (ALT 29 units/l) had an elevated risk for diabetes (hazard ratio 3.38 [95% CI 1.995.73]) versus those in the bottom quartile (<17 units/l). ALT remained a predictor with adjustment for age, BMI, triglycerides, HDL cholesterol, systolic blood pressure, glucose, and alcohol intake (2.04 [1.163.58] for the fourth versus first quartile). In stepwise regression, incorporating ALT and C-reactive protein (CRP) together with metabolic syndrome criteria, elevated ALT ( 29 units/l), and CRP ( 3 mg/l) predicted incident diabetes, but low HDL cholesterol and hypertension did not. Thus, elevated ALT levels within the "normal" range predict incident diabetes. The simplicity of ALT measurement and its availability in routine clinical practice suggest that this enzyme activity could be included in future diabetes prediction algorithms.
The role of the liver in the pathogenesis of type 2 diabetes is attracting increasing interest. In a recent study (1), directly determined liver fat content was shown to correlate with several features of insulin resistance in normal weight and moderately overweight subjects independent of BMI and intra-abdominal or overall obesity. However, direct measurements of liver fat require ultrasound, computed tomography scan, or proton spectroscopy, and such techniques are unlikely to be recommended for this purpose in routine clinical practice. Fortunately, circulating concentrations of a number of variables appear to give insight into the extent of liver fat accumulation. Among these are -glutamyltransferase, alanine aminotransferase (ALT), and aspartate aminotransferase (AST). Of these three, ALT is the most specific marker of liver pathology and appears to be the best marker for liver fat accumulation (2). In addition, circulating concentrations of plasminogen activator inhibitor-1 may give insight into the extent of liver fat content (3) but, unlike ALT, its measurement is perhaps not as simple, standardized, or routinely available in laboratories.
In light of the above observations, it is of interest that ALT has been shown to predict incident type 2 diabetes in two prospective studies (4,5). Ohlson et al. (4) determined risk factors for diabetes in 766 men, 47 of whom developed diabetes over 13.5 years of follow-up. They reported that elevated ALT predicted diabetes independently of classical predictors inclusive of BMI, blood pressure, triglycerides, and family history. However, diabetes ascertainment was not uniform in their study, and the potential utility of ALT to predict diabetes was not examined in any detail. Vozarovoa et al. (5) examined the ALT levels in a cohort of 370 Pima Indians with normal glucose tolerance, 63 of whom developed diabetes over an average follow-up of 6.9 years. In their analyses, individuals in the top decile for ALT ( We had the opportunity to address the predictive power of ALT for new-onset diabetes in the West of Scotland Coronary Prevention Study (WOSCOPS) in more detail and attempted to address many of the above issues in this study. In particular, we were able to examine whether ALT predicts new-onset diabetes independently of classical predictors, alcohol intake, and markers of inflammation. We were also able to address whether elevated ALT predicts new-onset diabetes independently of the metabolic syndrome, previously shown to predict new-onset diabetes in WOSCOPS (9). Finally, due to the larger size of WOSCOPS, it was possible to better approximate the level of ALT above which increased risk for diabetes is evident. We believe our study enhances knowledge on the prediction of incident diabetes by ALT, with potential implications for clinical practice and the development of future algorithms to predict diabetes.
The design of WOSCOPS has been described in detail (1012). Briefly, 6,595 moderately hypercholesterolemic men (LDL cholesterol 4.56.0 mmol/l and triglycerides <6.0 mmol/l) with no history of myocardial infarction were randomized to 40 mg pravastatin daily or placebo and followed for an average of 4.9 years.
A battery of risk factors and other demographic variables were assessed at baseline (1012), and, of particular relevance to this study, fasting glucose measurements at six monthly visits were recorded throughout the study, enabling us to determine the transition to and timing of diabetes development. We excluded men with frank diabetes (72 subjects with self-reported diabetes and 76 who had a baseline blood glucose
Laboratory analyses and determination of alcohol intake.
Statistics.
A total of 139 men (2.33%) developed diabetes over an average follow-up of 4.9 years. Their baseline characteristics are given in Table 1. Notably, mean ALT was 18% higher in those who subsequently developed diabetes (P < 0.001).
In the correlation analysis (Table 2), elevated ALT was most strongly associated with BMI, triglycerides, fasting glucose, and diastolic blood pressure (all r > 0.10, P < 0.001). AST was less strongly associated with these variables. Both elevated ALT and AST were weakly associated with excessive alcohol intake, but neither was related substantially to CRP. Consistent with the above observations, ALT increased progressively with increasing number of metabolic syndrome abnormalities ranging from 20.9 ± 7.6 units/l in those with no abnormalities to 28.1 ± 10.1 units/l in those with four or more (P < 0.0001 for trend) (Fig. 1), whereas AST did not change significantly (P = 0.27).
ALT versus classical predictors. In a univariate analysis, ALT as a continuous variable was associated with risk of diabetes. A 5-units/l increment had a hazard ratio (HR) of 1.25 (95% CI 1.151.34, P < 0.0001). Men in the fourth quartile for ALT, but not those in the second or third quartiles, had a significantly increased risk for diabetes relative to men in the first quartile (Table 3). The relationship was clearly evident in the Kaplan-Meier curve (Fig. 2). In a multivariate analysis, after adjusting for other measured baseline parameters (including BMI, triglycerides, HDL cholesterol, blood pressure, glucose, and alcohol intake), men in the fourth quartile for ALT continued to have a significantly elevated risk for diabetes (HR 2.04 [1.16330.508]) (Table 3). Moreover, the predictive ability of ALT persisted when the fourth quartile was compared with the risk in men in the other three quartiles combined (HR 1.72 [1.202.48]) (Fig. 3). Similarly, an ALT cutoff of 29 units/l improved prediction of diabetes when combined with a fasting glucose cutoff of 6.1 mmol/l (data not shown). By contrast, AST did not predict incident diabetes in univariate or multivariate analyses (data not shown).
ALT versus metabolic syndrome classification. Elevated ALT continued to predict incident diabetes (HR 2.08 [95% CI 1.482.93] for ALT 29 versus ALT <29 units/l) even when subjects were categorized as having or not having the metabolic syndrome (Fig. 3). Similarly, further addition of a CRP cutoff 3 mg/l into the latter analysis did not alter the ability of ALT to predict incident diabetes. Finally, we performed a stepwise multivariate analysis of diabetes predictors, with all individual metabolic syndrome criteria cutoffs, a CRP cutoff of 3 mg/l and an ALT cutoff of 29 units/l, entered into the model. In this case, both ALT and CRP continued to independently predict diabetes but low HDL cholesterol and blood pressure did not enter the final model (Table 4).
Pravastatin effect. All measurements in this analysis, except for repeated fasting glucose concentrations, were made before randomization to active therapy or placebo and are thus unaffected by treatment allocation. Since we have previously shown that pravastatin use did influence progression to diabetes (13), we included treatment allocation in the multivariate analysis and, in addition, noted that the ALT incident diabetes association was not dissimilar (P = 0.89) in men allocated to pravastatin or placebo.
Our study adds new information to the concept of ALT as a predictor of diabetes. First, a large sample size allowed us to identify the level of ALT above which the risk of diabetes is evident. Men with baseline ALT levels 29 units/l had more than three times the risk for diabetes than men with ALT <17 units/l. Second, we were able to exclude alcohol intake and CRP, a robust biomarker of low-grade inflammation, as potential confounders. Finally, we had prior data on metatolic syndrome in this population and again were able to adjust for this in our analysis. Significantly, ALT continued to predict type 2 diabetes independently of all the above factors whether we compared risk in the fourth versus the first quartile or examined risk in those above or below the 75th percentile for ALT (i.e., 29 units). Moreover, elevated ALT and CRP continued to predict, quite independently, incident diabetes, whereas low HDL cholesterol and blood pressure did not in a stepwise regression analysis that considered all individual metabolic syndrome criteria. Given the simplicity of ALT measurement and its universal standardization and availability in routine clinical practice, these novel observations indicate the potential for an ALT cutoff to be considered in diabetes prediction algorithms. Our observations perhaps also add further support for the role of the liver in the pathogenesis of type 2 diabetes. Why should elevated ALT predict type 2 diabetes? There is now good evidence that elevated ALT, even within the normal range, correlates with increasing liver fat (2). Moreover, the condition of nonalcoholic fatty liver disease is now well recognized, and an elevated ALT is a principal diagnostic feature (14,15). With respect to diabetes risk, it is therefore likely that elevated liver fat is part of the pathogenic mechanism. In line with this, Seppala-Lindross et al. (1) elegantly demonstrated that elevated liver fat in nondiabetic men with average BMI is linked to insulin resistance independently of total adiposity. These prior observations in turn explain why elevated ALT predicted decreasing hepatic insulin sensitivity independent of total adiposity and an increase in hepatic glucose output in a study of Pima Indians (5). Of more recent interest, an inverse correlation between ALT levels and adiponectin concentrations has been demonstrated (16). This observation is relevant since low adiponectin predicts incident diabetes in prospective studies (1719) and may do so in part by enhancing hepatic fatty acid oxidation and thereby lessening fat accumulation and ALT levels. It would be clinically important in future studies, therefore, to compare ALT and adiponectin as predictors of diabetes in prospective cohorts. Why then does the liver accumulate fat? One possibility is simply excess flux of fatty acids to liver from abdominal or visceral fat depots (20). However, others (2) suggest that increased liver fat content may relate better to dietary fat intake. Further possibilities include excessive intravascular lipolysis of triglyceride-rich lipoproteins or indeed impaired free fatty acid clearance. Whether acquired or genetic defects in hepatic ß-oxidation are involved in liver fat accumulation requires direct examination.
Regardless of the mechanisms for fat accumulation, our results offer some potential clinical interest. Firstly, our data suggest that levels of ALT, even within the currently acceptable normal range, may indeed be prognostic with respect to the development of diabetes. A level of Given the increasing incidence of obesity and therefore the likelihood for diabetes worldwide, there is great interest in the development of predictive algorithms for type 2 diabetes. In this respect, it is of interest that the National Cholesterol Education Programdefined metabolic syndrome criteria have been shown to predict incident diabetes in different populations (9,21). However, we previously suggested that the National Cholesterol Education Program criteria could be modified to improve its prediction of diabetes and that differing definitions are likely to better predict risk for coronary heart disease (9). Our findings that elevated ALT and CRP continued to predict incident diabetes in stepwise regression analyses, whereas low HDL cholesterol and blood pressure did not, perhaps indicate the potential future use of ALT cutoffs in this respect. Future studies in other populations should address this potential in greater detail. The strengths of our study have been indicated above and include its larger sample size and inclusion of alcohol intake, CRP, and metabolic syndrome in analyses. Moreover, our definition of diabetes used the American Diabetes Association criteria and was consistent and validated in prior analyses (6,9,13). We acknowledge that the current study represents a post hoc analysis of men with elevated cholesterol and that men with levels of ALT >70 units/l were excluded from WOSCOPS. However, since cholesterol and LDL cholesterol are not predictive of diabetes and since others have shown higher ALT levels to predict diabetes, we feel such limitations are not significant concerns. Clearly, our results were obtained in a cohort of men and are not applicable to women. We also acknowledge that we used a modified version of the American Diabetes Association criteria to predict diabetes but feel that our conservative approach (which may have contributed to only 2.33% developing diabetes) increased rather than decreased our confidence in the diagnosis of new-onset type 2 diabetes. Finally, although these results were conducted in the context of a statin trial, and statins can transiently raise transaminases, it is important to note that ALT and AST concentrations used herein were measured before randomization and treatment allocation and that further statistical analysis indicated no heterogeneity in the ALTincident diabetes findings dependent on treatment allocation. In conclusion, we have shown that elevated ALT within the "normal" range predicts diabetes independently of classical predictors, CRP, and the metabolic syndrome in middle-aged Caucasian men of average BMI. In this respect, it is noteworthy that ALT measurement is automated, internationally standardized, and universally available, unlike many other proposed novel markers of diabetes. Therefore, we believe that the results of our study have potential implications for clinical practice or development of future algorithms to predict diabetes. The results may also add some support for the notion that liver fat accumulation is important in the pathogenesis of type 2 diabetes.
I.F. has received speaker honoraria and research/grant support from Bristol-Myers Squibb and Sankyo and speaker and committee honoraria from AstraZeneca. S.M.C. has received honoraria and grant/research support from AstraZeneca. Address correspondence and reprint requests to Dr. Naveed Sattar, University Department of Pathological Biochemistry, Glasgow Royal Infirmary, Glasgow G31 2ER, Scotland, U.K. E-mail: nsattar{at}clinmed.gla.ac.uk Received for publication April 30, 2004 and accepted in revised form July 21, 2004
Abbreviations: ALT, alanine aminotransferase; AST, aspartate aminotransferase; CRP, C-reactive protein; WOSCOPS, West of Scotland Coronary Prevention Study
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