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Original Research |
Department of Epidemiology (K.A.M., D.S., M.J.S., E.B.R.), Boston, Massachusetts
Department of Biostatistics (D.S.), Boston, Massachusetts
Department of Nutrition (M.J.S., E.B.R.), Boston, Massachusetts
Harvard School of Public Health, Department of Pathology (N.R.), Boston, Massachusetts
Channing Laboratory, Department of Medicine, Brigham and Womens Hospital (M.J.S., E.B.R.), Boston, Massachusetts
Harvard Medical School, Department of Laboratory Medicine, Childrens Hospital (N.R.), Boston, Massachusetts
Drug and Alcohol Department, Royal Prince Alfred Hospital and Departments of Medicine, Psychological Medicine and Public Health and Community Medicine, University of Sydney, AUSTRALIA (K.M.C.)
Department of Public Health, Tri-Service General Hospital, National Defense Medical Center, Taipei, Taiwan, REPUBLIC OF CHINA (N.-F.C.)
Address correspondence to: Eric B. Rimm, ScD, Department of Nutrition, Harvard School of Public Health, 665 Huntington Avenue, Boston, MA 02115. E-mail: eric.rimm{at}channing.harvard.edu
| ABSTRACT |
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Methods: A cross-sectional study of 462 disease-free men selected from the Health Professionals Follow-up Study to provide information on a range of drinking patterns. Study participants were 48 to 82 years of age who provided a blood sample and detailed information on diet, life-style and alcohol consumption patterns in 1994. Among the study participants, 267 men provided a fasting blood sample and contributed to the analyses of insulin and c-peptide.
Results: Biologic markers were not strongly related to average alcohol consumption. Compared to abstainers, differences in insulin concentrationsall statistically non-significantwere 0.06, 1.25, 1.02, and 0.12 µU/mL for consumers of <1, 11.9, 22.9, 3+ drinks per day, respectively. The frequency of alcohol consumption was inversely related to fasting c-peptide and insulin concentrations after controlling for average alcohol consumption and other potential confounding variables. Compared to men who reported consuming alcohol one to three days per week, c-peptide concentrations were 0.08 ng/mL and 0.29 ng/mL lower (p-trend = 0.04) in men who reported consuming alcohol on four to five days per week and six to seven days per week, respectively. Men who consumed alcohol on most days also had lower fasting insulin levels than more irregular drinkers (p-trend = 0.05).
Conclusions: Our results suggest that frequent alcohol consumption is inversely related to fasting c-peptide and insulin concentrations.
Key words: insulin, c-peptide, HbA1c, alcohol, diabetes, men
| INTRODUCTION |
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Previous epidemiologic studies may be limited because few biological parameters of glycemic status have been assessed and only average daily alcohol consumption was examined. To our knowledge, no study has yet reported on the relation of usual pattern of drinking, including the frequency of consumption and drinking with meals, on insulin sensitivity. More detailed measurements of glycemic control and alcohol consumption may contribute to a fuller understanding of the potential effects of alcohol.
In the current study, we examine the cross-sectional associations between drinking patterns and three markers of glycemic control (fasting insulin, fasting c-peptide and HbA1c levels) in 462 men from the Health Professionals Follow-up Study (HPFS). Average alcohol consumption was assessed with a semi-quantitative food frequency questionnaire. Unique to our study are data that further characterize typical drinking patterns with respect to drinking with meals and the regularity of consumption, which allowed us to examine the independent and joint effects of these three aspects of drinking behavior.
| MATERIALS AND METHODS |
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On average, the 18,225 men who provided blood samples were similar in age, BMI and smoking status to members of the cohort as a whole. Medians of age and BMI for the HPFS cohort and the self-selected sample of blood providers were, respectively, 62 and 61 years for age, and 25.5 kg/m2 for both groups for BMI. Of the entire cohort 6% were current smokers, compared with 5% of those who provided blood. In turn, the 468 study subjects were similar to the 18,225 cohort members who provided blood with respect to medians for age (61 years in both groups), BMI (25.0 vs. 25.5 kg/m2) and smoking status (6% vs. 5% current smoking).
Data Collection
We queried drinking patterns in questionnaires and defined them in terms of average daily alcohol consumption, the proportion of total alcohol consumed with meals and the number of drinks consumed per drinking day. Total alcohol consumption was assessed in 1994 as part of a 131-item semi-quantitative food frequency questionnaire. Average consumption of light and regular beer, white and red wine and liquor was reported as one of nine response categories ranging from less than one serving per month up to 6+ servings per day. Standard portion sizes were specified as one glass, bottle or 12-ounce can for beer (355 mL), one 4-ounce glass for wine (118 mL), and one shot of liquor (45 mL). We calculated total alcohol intake for each participant as the sum of the reported average intakes of each alcoholic beverage. In addition, we assigned gram amounts of alcohol to each beverage: 12.8 grams of alcohol for beer, 11.3 grams for light beer, 11.0 grams for red and white wine and 14.0 grams for spirits. In the 1994 follow-up questionnaire, we asked cohort members to report the proportion of total alcohol that they typically consumed with meals; categories were <25%, 25%49%, 50%74%, and 75% or more of total alcohol consumed with meals. In 1986 and 1988, cohort members had also reported the frequency with which they typically consumed alcohol per week, ranging from 07 times per week.
The validity and reproducibility of the food frequency questionnaire were assessed in a random sample of 136 participants who had returned the 1986 baseline questionnaire and a second questionnaire in 1987 [18]. Alcohol use as measured by the first and second questionnaires was compared to that measured by two one-week diet records administered six months apart between the questionnaires. The estimated average daily alcohol intakes from the 1986 (mean = 12.0 g) and 1987 (mean = 12.5 g) food frequency questionnaires were very similar to each other and to the mean daily intake from diet records (mean = 12.8 g). The Spearman correlation coefficient for alcohol consumption calculated from the 1987 questionnaire and the diet record was 0.86, and the correlation coefficient for the first and second questionnaires was 0.92. Furthermore, the correlation between HDL-C and the 1987 questionnaire assessment of average alcohol was 0.35, a value similar to what would be expected based on metabolic studies of alcohol consumption and lipids [19]. Finally, we have evidence that self-reported alcohol was inversely associated with CHD and type 2 diabetes and positively related to colon cancer and fractures, which is similar to results found in other studies where methods of reporting have varied from self-report to interview administered.
From April 1993 through August 1995, 18,225 HPFS cohort members provided blood samples for the study of biologic markers. Blood samples were collected in liquid EDTA blood tubes and returned to the laboratory, via overnight courier, on ice packs stored in Styrofoam containers. Over 95% of the samples arrived within 24 hours of being drawn. We centrifuged, aliquoted and stored the samples in liquid nitrogen (-150°C) until analysis. Based on visual inspection, less than 15% of the samples were slightly hemolyzed and very few were moderately hemolyzed (<3%), lipemic (<1%) or not cooled upon arrival (<0.5%). We measured red blood cell HbA1c by a temperature controlled HPLC method; against standards of 5.6% and 9.6%, the coefficients of variation for HbA1c were less than 2.5% (Boehringer Mannheim). Plasma insulin and c-peptide were measured with an RIA method, which has little or no proinsulin or c-peptide cross-reactivity and has a reported average CV of less than 10% (Linco Research, St. Charles, MO). Red cell levels of HbA1c were available from laboratory analysis for 466 of the 468 study subjects; plasma levels of insulin and c-peptide were available for all 468 study subjects.
This study was primarily designed to examine drinking patterns in relation to lipids and clotting factors; supplemental support to additionally measure insulin, c-peptide and HbA1c was received after blood samples had been obtained. A questionnaire was included in the blood kit that queried the date and time of the blood draw, as well as the time since last meal. A substantial number of participants (N = 199) reported having consumed a meal within six hours of the blood draw and were therefore not eligible for inclusion in the insulin and c-peptide analyses. We excluded these 199 men, and 36 who did not respond to the question, from the analyses of insulin and c-peptide. In addition, we excluded subjects with insulin levels that were implausibly high for a non-diabetic population (greater than 40 µU/mL) and all subjects whose blood was partially hemolyzed, based on visual inspection, from the insulin analyses (n = 22). After exclusions, there were 462 observations available for the analysis of HbA1c, 267 for c-peptide and 245 for insulin.
We assessed data on covariates, when possible, from the 1994 follow-up questionnaire and assigned missing covariate values the most recent, available covariate value. Other covariate information assessed in 1994 included age, self-reported diagnosis of hypertension or anti-hypertensive medication use and dietary intakes of fiber, saturated fat and polyunsaturated fat.
Statistical Analysis
We examined the population distribution of alcohol consumption and created categorical variables for each of the three alcohol drinking variablesaverage daily consumption, frequency of consumption and the degree to which alcohol was consumed with mealsto ensure adequate numbers of subjects in each category for analysis. We used linear regression models to assess the difference in each continuous biologic marker associated with a one level increase in each categorical drinking pattern variable. We used the robust variance to allow for valid statistical inference from linear regression models despite non-normality of continuous outcome variables [20]. All regression coefficients are presented with 95% confidence intervals; reported p-values were two-tailed.
We adjusted for potential confounding variablesage, BMI, physical activity, hypertension, smoking, dietary fiber, saturated fat and polyunsaturated fatin multivariate regression analyses. We also included average alcohol consumption and number of drinks consumed per drinking day in the analyses of alcohol consumed with meals and average alcohol consumption and drinking with meals in the analysis of number of drinks consumed per drinking day in an attempt to isolate the effects of each drinking variable. We considered possible effect modification by age and BMI. There were insufficient numbers of current smokers to study effect modification by smoking status in multivariate analysis. Finally, we examined the specific effects of wine, beer and spirits by simultaneously including three-level indicator variables (0 grams/day, 0 < grams/day < 15, and 15+ grams/day) for each beverage type.
| RESULTS |
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| DISCUSSION |
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Our finding that average alcohol consumption was not inversely related to insulin runs counter to several cross-sectional studies [511] and prospective cohort studies which support an inverse relation between average alcohol consumption and incident type 2 diabetes drinkers [14,21,22]. In 1995, we reported that HPFS cohort members who consumed 30.049.9 g of alcohol daily had a relative risk of diabetes of 0.61 (95% confidence interval 0.44 to 0.91) compared to abstainers over six years of follow-up [1].
In the present study, the lack of an inverse relation between alcohol intake and insulin concentrations is unlikely to be attributable to either confounding bias or measurement error. We controlled for variables believed to affect insulin levels, including BMI, physical activity and smoking. A validation study of the alcohol questions on the FFQ demonstrated similar estimated mean intakes from the FFQ and two one-week diet records [18]. Although we did not conduct a formal validation study of the insulin measure, the direction and magnitude of the associations between insulin and other covariatesincluding physical activity [23] and obesity [24]were consistent with expectation in our study population.
The oversampling of episodic and heavy drinkers from the HPFS cohort also would not explain the differences between our findings and those of other research groups. Our analyses suggested that the study sample was comparable to the cohort population with respect to several correlates of insulin sensitivity and alcohol consumption. Further, we adjusted for potential confounding variables, including characteristics of alcohol consumption, in multivariate regression models. Rather than impair the validity of this study, our sampling scheme would optimize the statistical power for examining a range of drinking patterns in this population and to control for confounding of average consumption with frequency.
Although we expected an inverse relation between insulin and average alcohol consumption, the literature has not consistently supported an inverse association. Several previous reports suggested a U-shaped association between alcohol and insulin resistance or diabetes [25,26], and still other studies have shown a positive association between alcohol consumption and insulin or glucose concentrations [13,17]. The importance of the pattern of drinking, rarely reported, may explain some of the apparent conflict in past literature on the effect of alcohol on glycemic control and on subsequent risk of diabetes. For example, the relation between average consumption and glycemic control may be confounded by other characteristics of alcohol consumption, such as frequency of consumption.
We found an inverse association between frequency of alcohol consumption and levels of c-peptide and insulin. Compared to abstainers, the mean insulin concentration was 1.62 µU/mL lower in those who consumed alcohol five to seven days a week, which would be clinically significant on a population level [27]. These results are consistent with findings from a parallel analysis in the HPFS cohort. After 12 years of follow-up, we documented 1,571 cases of type 2 diabetes mellitus among 46,892 men free of chronic disease at baseline. Men who reported alcohol consumption on 5+ days per week had a significantly lower incidence of diabetes (RR = 0.60; 95% CI: 0.430.84) compared to men who drank one to two days per week, controlling for the number of drinks consumed per day. In this analysis, average daily alcohol consumption was not strongly related to diabetes incidence once it was adjusted for frequency of alcohol consumption. That is, within strata of frequency of alcohol consumption, the risk of diabetes was relatively constant over categories of drinks consumed per drinking day [28].
There are several potential mechanisms through which alcohol may affect the insulin carbohydrate system. Alcohol acts as a preferred fuel, being metabolized in preference to other calorie sources in the body, thus reducing glucose disposal [29,30]. However, alcohol also inhibits gluconeogenesis [3133] and thus when consumed in moderation may result in better maintenance of normoglycemia. In laboratory studies, the effect of alcohol on the insulin carbohydrate system may vary both with the amount of alcohol consumed and also from the fasting to the fed state. In fasting subjects, high doses of alcohol can lead to hypoglycemia, possibly due to reduced gluconeogenesis [34,35]. But in the non-fasting state, high doses of alcohol may reduce glucose disposal causing resistance to the effects of insulin [29,36]. Higher levels of alcohol may reduce insulin binding and inhibit intracellular signaling related to insulin [37,38], but moderate levels appear to be related to enhanced sensitivity [6,7]. Our findings that frequent consumption of alcohol may be more beneficial than more infrequent consumption has not been explored in metabolic studies. Jequier hypothesized that alcohol may induce increases in energy expenditure [39,40], which, if spread over most days, could be more influential on long term glucose utilization and insulin sensitivity than infrequent consumption.
Like many other studies of this association [511], our study was cross-sectional, and determinations of causality may not be warranted. However, findings from several prospective cohort studies have supported an inverse relation between alcohol and measures of glycemic control [14,21,22]. Further, recent findings from a randomized metabolic trial showed that daily moderate alcohol consumption significantly reduced fasting insulin and increased insulin sensitivity [41]. Together, these results are strongly suggestive of cause and effect. Our study sample was small, and one possibility is that we simply lacked sufficient power to detect meaningful associations with average alcohol consumption.
Our findings demonstrate the potential importance of frequency of alcohol consumption on measures of glycemic control, specifically c-peptide and insulin, and warrant assessing not only average daily consumption, but also drinking patterns in examining the effects of alcohol on biological markers and disease outcomes.
| FOOTNOTES |
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Received March 20, 2002. Accepted October 7, 2002.
| REFERENCES |
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