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(Circulation. 2001;103:357.)
© 2001 American Heart Association, Inc.
Clinical Investigation and Reports |
From the Department of Biological Sciences (D.J.F.), University of Durham, Durham, UK; Robertson Centre for Biostatistics (J.N., I.F.), University of Glasgow, Glasgow, UK; Department of Pathological Biochemistry (N.S., C.J.P., J.S., A.G.), Glasgow Royal Infirmary University NHS Trust, Glasgow, UK; Department of Clinical Biochemistry (R.D.G.N.), Dryburn Hospital, North Durham Healthcare NHS Trust, Durham, UK; Department of Medical Cardiology (S.M.C., A.R.L., P.W.M.), Glasgow Royal Infirmary, Glasgow, UK; Department of Medicine (C.I.), Dumfries and Galloway Royal Infirmary, Dumfries, UK; and University Department of Medicine (J.H.M.), Glasgow Royal Infirmary, Glasgow, UK.
Correspondence to Dr Allan Gaw, Department of Pathological Biochemistry, Glasgow Royal Infirmary University NHS Trust, Glasgow G31 2ER, UK.
| Abstract |
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Methods and ResultsOur
definition of diabetes mellitus was based on the American Diabetic
Association threshold of a blood glucose level of
7.0 mmol/L.
Subjects who self-reported diabetes at baseline or had a baseline
glucose level of
7.0 mmol/L were excluded from the analyses. A total
of 5974 of the 6595 randomized subjects were included in the analysis,
and 139 subjects became diabetic during the study. The baseline
predictors of the transition from normal glucose control to diabetes
were studied. In the univariate model, body mass index, log
triglyceride, log white blood cell count, systolic blood pressure,
total and HDL cholesterol, glucose, and randomized treatment assignment
to pravastatin were significant predictors. In a multivariate model,
body mass index, log triglyceride, glucose, and pravastatin therapy
were retained as predictors of diabetes in this
cohort.
ConclusionsWe concluded that the assignment to pravastatin therapy resulted in a 30% reduction (P=0.042) in the hazard of becoming diabetic. By lowering plasma triglyceride levels, pravastatin therapy may favorably influence the development of diabetes, but other explanations, such as the anti-inflammatory properties of this drug in combination with its endothelial effects, cannot be excluded with these analyses.
Key Words: diabetes mellitus prevention lipids trials risk factors
| Introduction |
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Pravastatin is an HMG Co-A reductase inhibitor that has significant effects on the plasma lipid and lipoprotein profile, lowering total and LDL cholesterol and triglyceride levels and raising HDL cholesterol levels.6 7 8 In addition to these lipid effects, pravastatin has been shown to have a range of other antiatherothrombotic effects, including the restoration of endothelial function9 and anti-inflammatory effects.10
Pravastatin therapy has been shown unequivocally to reduce cardiovascular risk in a series of large-scale, randomized, controlled clinical trials.6 7 8 In a diabetic/glucose-intolerant subgroup analysis of one study, pravastatin significantly reduced coronary events, but no attempt was made to examine the effects of pravastatin on glucose control over time.11
The West of Scotland Coronary Prevention Study (WOSCOPS)
database provided an opportunity to study prospectively the effects of
pravastatin therapy on the risk of developing diabetes in subjects with
follow-up ranging from 3.5 to 6.1
years.6 We used the American
Diabetes Association (ADA) definition of diabetes mellitus (fasting
blood glucose level of
7.0
mmol/L)12 and were able to
study the major baseline predictors of the loss of glucose control in
this patient group. In addition, we examined whether pravastatin
influenced the subsequent development of diabetes
mellitus.
| Methods |
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Laboratory Methods
Glucose was measured according to an automated
enzymatic method, and plasma lipids and lipoproteins were measured
according to the protocols of the Lipid Research
Clinics.14 Blood counts,
including white blood cell count (WBC), were performed with a Coulter
STKR or S+1 automated cell counter.
Definition of Diabetes
Fasting venous blood glucose measurements were
performed at baseline and at 6-month intervals throughout WOSCOPS. We
used the ADA definition of diabetes mellitus, which requires a fasting
blood glucose level of
7.0 mmol/L. Two important additional criteria
were imposed. Although all glucose measurements were intended to be
performed on fasting samples in WOSCOPS, inevitably in such a large
group during 5 years, some patients may have failed to fast. Because a
single, nonfasting glucose measurement may result in the
misclassification of a subject as diabetic, a minimum of 2 glucose
measurements of
7.0 mmol/L were required. In addition, because we
were primarily interested in examining subjects who experienced
significant deterioration in their glucose control, a further restraint
was incorporated into the definition, whereby one glucose measurement
must be
2.0 mmol/L above baseline. The value of 2.0 mmol/L was chosen
to represent an average increment from a normal glucose of
5.0
mmol/L to a diabetic value of 7.0 mmol/L.
Inevitably, this strategy restricted the number of subjects classified as newly diabetic but increased our level of confidence that the subjects labeled in this study as newly diabetic were truly thus. In addition, individuals who had been newly prescribed hypoglycemic agents (oral hypoglycemic agents or insulin) during the study were accepted as having become diabetic.
Importantly, because this analysis was designed to examine
the development of new diabetes, subjects who were self-reported
diabetics at baseline (76 subjects) or had baseline glucose level of
7.0 mmol/L (an additional 72 subjects) were excluded. In addition,
473 subjects with insufficient on-treatment glucose measurements to
allow classification of diabetes according to the above definitions
were excluded.
Statistical Methods
Data are summarized as mean (SD) for continuous
variables and number of subjects (percent) for categorical variables.
Cox proportional hazards models were fitted to identify predictors of
transition to diabetes, both univariately and
multivariately.15 Subjects
time to becoming glucose intolerant was taken as the 6-month visit at
which they first had
2 postrandomization glucose measurements of
7.0 mmol/L and
1 postrandomization glucose measurement of >2.0
mmol/L above the baseline glucose level or as the postrandomization
visit at which they first indicated taking hypoglycemic drugs. Due to
nonattendance at visits or end of study (with varying length of
follow-up), subjects time to becoming glucose intolerant was censored
at the last 6-monthly visit at which their glucose level was measured.
Lifestyle, lipids, and other coronary heart disease risk factors at
baseline were considered. The multivariate model contained all of the
covariates, regardless of statistical significance, to allow the effect
of each covariate in the presence of all other, possibly confounding,
covariates to be assessed. Glucose was modeled after transformation
into quintiles, and plasma triglyceride and WBC were log
transformed.
| Results |
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2 postrandomization blood glucose measurements and were neither
self-reported diabetics nor had elevated fasting blood glucose levels
(
7.0 mmol/L) at baseline
(Table 1
|
Univariate Predictors of Diabetes
As shown in
Table 2
, body mass index (BMI), HDL cholesterol, log
triglyceride, total cholesterol, log WBC, baseline glucose, and
systolic blood pressure were all univariate predictors of the
development of diabetes. Age, alcohol intake, and smoking status were
not significant predictors of development of diabetes. Interestingly,
pravastatin treatment itself significantly influenced development of
diabetes (hazard ratio 0.70, 95% CI 0.50 to 0.98;
P=0.036).
|
When baseline blood glucose levels were divided into 5
quintiles as shown in
Table 2
, highly significant differences in the risk of
developing diabetes mellitus were observed among these. As might be
expected, baseline glucose is a strong predictor of developing new
diabetes with levels in the top quintile (>5.0 mmol/L) conferring a
13-fold greater risk than the bottom quintile in the univariate
analysis and an almost 10-fold greater risk in the multivariate
analyses.
Kaplan-Meier plots of the development of diabetes according
to median baseline glucose, BMI, log triglyceride, and treatment
assignment are shown in the
Figure
.
|
Multivariate Predictors of Diabetes
In the multivariate Cox model, baseline BMI, log
triglyceride, and baseline glucose remained significant predictors, but
systolic blood pressure and log WBC were no longer statistically
significant
(Table 2
). Pravastatin therapy also remained a significant
predictor with a multivariate hazard ratio of 0.70 (95% CI 0.50 to
0.99,
P=0.042).
| Discussion |
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|
|
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The definition of diabetes mellitus according to the ADA
requires a fasting blood glucose of
7.0 mmol/L. We have used this
cutoff to define diabetes, but to avoid the mislabeling of patients as
diabetic on the basis of single nonfasting samples, we used more
rigorous additional criteria by requiring two blood glucose levels of
7.0 mmol/L and one level of
2.0 mmol/L than baseline. The finding
that BMI and plasma triglyceride, both well-recognized risk factors for
insulin resistance and glucose
intolerance,5 were strong
independent predictors of the loss of glucose control in this group is
reassuring.
In our multivariate analysis, baseline WBC failed to reach
statistical significance
(P=0.063) as a predictor of the
loss of glucose control. The importance of baseline WBC as a predictor
of the loss of glucose control has, however, been established by
others4 and potentially lies
in its association with low-grade chronic inflammation. Only very
recently has it been appreciated that inflammatory markers linked with
insulin resistance associate with the development of diabetes in
adults.4 16 The
mechanism by which inflammation leads to glucose intolerance and
ultimately diabetes is unknown, but proinflammatory cytokines, such as
tumor necrosis factor (TNF)-
, and leptin may produce insulin
resistance by influencing the function of the insulin
receptor17 18 or
impairing insulin action and inhibiting insulin
secretion.19
Previous investigations into the effects of pravastatin on glucose tolerance have been equivocal. Most studies have been performed in small groups for a short duration and have yielded inconclusive results.20 21 In the present study, the benefits associated with pravastatin therapy are long term, and it should be noted that the subjects studied in WOSCOPS, on average, had normal triglyceride levels at baseline. The beneficial effects of pravastatin on glucose-intolerant subjects have been shown in a subgroup analysis of the CARE trial,11 in which significant reductions were observed in cardiovascular risk. The mechanism by which pravastatin may reduce a subjects risk of developing diabetes mellitus is not clear from these analyses. One possibility that cannot be discounted with the present analysis is the impact of pravastatin on vascular events that lead to a secondary reduction in the need for the use of cardiovascular drugs that influence glucose control, such as thiazide diuretics and ß-blockers. The WOSCOPS database has information on concomitant medication but has insufficient resolution with respect to specific drugs to answer this question. Drug use at baseline was virtually identical between the placebo and pravastatin groups, and it may be argued that active treatment would actually increase the use of drugs such as thiazides and ß-blockers by reducing the fatal event rate in the study.
We may, however, speculate that three known effects of pravastatin therapy may play a primary role, either individually or in concert in the development of diabetes.
First, the triglyceride-lowering effect of pravastatin therapy may be important over the long term in reducing the risk of the development of insulin resistance. It has been known for many years that elevated triglyceride levels are associated with the development of diabetes.5 In WOSCOPS, pravastatin therapy reduced triglycerides by an average of 12%.6 Interestingly, when the on-treatment triglyceride values were put into the stepwise multivariate model, pravastatin therapy was no longer a statistically significant, independent predictor (data not shown). This, however, cannot be interpreted as necessarily explaining the pravastatin effect. On-treatment triglyceride levels, calculated as the average of the 6- and 12-month postrandomization values, are not independent of pravastatin therapy. Indeed, as stated earlier, pravastatin significantly decreases the plasma triglyceride level. The data are therefore compatible with a treatment effect mediated through a change in plasma triglyceride levels but are not necessarily explained by this. It should also be noted that other lipid-lowering drugs, such as fibric acid derivatives, which have a greater lowering effect on plasma triglyceride than do statins, do not appear to improve insulin resistance.22 This would suggest that triglyceride lowering per se does not explain the observed effect.
Second, the anti-inflammatory effects of pravastatin may be
key. Pravastatin has been shown to reduce circulating levels of the
cytokines interleukin-6 and
TNF-
.23 TNF-
and
interleukin-6 are known to inhibit lipoprotein lipase
activity24 and to stimulate
lipolysis in adipose
tissue.25 Indeed, it has
been postulated that these cytokines, derived in part from adipose
tissue, may be possibly responsible for the metabolic syndrome
associated with insulin
resistance.16 The
anti-inflammatory properties of pravastatin may therefore interrupt the
natural progression from central obesity to insulin resistance mediated
by the adipose tissuederived cytokines.
Finally, another effect of pravastatin that has been consistently demonstrated is the effect on endothelial function.9 26 27 This may be explained in part by the lipid-lowering effects of this statin, as dyslipidemia is known to impair endothelium-mediated vasodilatation.28 Impaired endothelial function has recently been shown to result in diminished capillary recruitment and in turn to correlate with the degree of insulin resistance.29 By restoring endothelial function, pravastatin may significantly influence selective tissue perfusion and thereby beneficially affect glucose and insulin transport.
These 3 mechanisms may all be operative, and together they may explain this important new finding that pravastatin therapy may reduce the propensity of subjects within WOSCOPS to develop diabetes. However, there may be other direct or indirect effects of pravastatin therapy on glucose control that have yet to be unraveled. Regardless of the mechanism or mechanisms, the prevention or delay of the onset of diabetes may contribute significantly to the observed cardiovascular benefits of pravastatin therapy.
These findings are generated from a post hoc analysis of WOSCOPS. As such, we must emphasize that our results should be treated as hypothesis generating and should now be confirmed in a prospective manner in other statin trials, such as the ongoing Prospective Study of Pravastatin in the Elderly at Risk (PROSPER).30
| Appendix 1 |
|---|
|
|
|---|
Data and Safety Monitoring Committee
Michael F. Oliver (chairman), Anthony F. Lever, Byron
W. Brown, John G.G. Ledingham, Stuart J. Pocock, Basil M.
Rifkind.
Cardiovascular End-Points Committee
Stuart M. Cobbe (chairman), Barry D. Vallance, Peter
W. Macfarlane.
Adverse Events Review Board
A. Ross Lorimer, James H. McKillop, David
Ballantyne.
Data Center Staff
Liz Anderson, David Duncan, Sharon Kean, Audrey
Lawrence, June McGrath, Vivette Montgomery, John
Norrie.
Population Screening
Melvyn Percy.
Clinical Coordination, Monitoring, and
Administration
Elspeth Pomphrey, Andrew Whitehouse, Patricia
Cameron, Pamela Parker, Fiona Porteous, Leslie Fletcher, Christine
Kilday.
Computerized ECG Analysis
David Shoat (deceased), Shahid Latif, Julie
Kennedy.
Laboratory Operations
M. Anne Bell, Robert Birrell.
Company Liaison and General Support
Margot Mellies, Joseph Meyer, Wendy
Campbell.
| Acknowledgments |
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| Footnotes |
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Received May 25, 2000; revision received August 3, 2000; accepted August 8, 2000.
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