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Letters
March 10, 2004

Coffee Consumption and Insulin Sensitivity

Author Affiliations
 

Letters Section Editor: Stephen J. Lurie, MD, PhD, Senior Editor.

JAMA. 2004;291(10):1199-1201. doi:10.1001/jama.291.10.1199-b

To the Editor: Coffee consumption has been associated with a substantially lower risk of developing type 2 diabetes.1 However, despite the widespread use of coffee, there are few data on the specific effects of coffee on the 2 main causes of diabetes, ie, insulin resistance and defective insulin secretion. We investigated the association between coffee consumption and both insulin sensitivity and insulin secretion in a sample of elderly Swedish men without diabetes.

Methods

We reanalyzed cross-sectional data collected between 1990 and 1994 from the Uppsala Longitudinal Study of Adult Men (ULSAM). A dietitian instructed all participants to record their dietary intake using a 7-day precoded food diary. Coffee and tea consumption were recorded 6 times daily (breakfast, lunch, supper, between meals, and in the evening). Amounts of sugar, cream, and milk used in coffee, as well as of cookies, cakes, and biscuits consumed with coffee, were also recorded at these occasions. Daily intakes were calculated using a computer program and the Swedish National Food Administration database (SLV Database, 1990). Participants also reported their leisure-time physical activity on a standardized questionnaire.

Insulin sensitivity index was determined by hyperinsulinemic euglycemic clamp.2 Insulin secretion was measured as the early insulin response (EIR) (ie, 30-minute increment of insulin per 30-minute increment of glucose) during an oral glucose tolerance test. We used linear univariate and multiple regression models to test the relationships between these 2 outcomes and the diet and lifestyle variables. Statistical analyses were carried out using JMP 3.2 (SAS Institute Inc, Cary, NC); P<.05 was used to determine statistical significance.

All participants had previously provided written informed consent. The study was approved by the ethics committee of Uppsala University.

Results

Data on insulin sensitivity, insulin secretion, and coffee consumption were available for 1088 of the original 1221 participants (89%). Participants with type 2 diabetes were excluded, leaving 936 participants for analysis. Baseline characteristics of the sample at the time of data collection are presented in Table 1.

Table 1. Baseline Characteristics (N = 1088)
Image description not available.

Coffee consumption was not significantly associated with insulin sensitivity in univariate analysis (Table 2). Because intake of sugar, milk, pastries, and alcohol was not normally distributed after logarithmic transformation, we decided to dichotomize these variables according to dietary guidelines of the Swedish National Food Administration (Table 1). After adjustment for tea consumption, amounts of sugar and cream used in coffee, amounts of cookies, cakes, and biscuits consumed with coffee, alcohol consumption, body mass index, physical activity level, and smoking status, we found that a 1-cup increase of coffee per day was associated with 0.16-units higher insulin sensitivity (Table 2). Further adjustment for age, daily intake of dairy products, fat, protein, carbohydrates, total energy intake, and waist girth, as well as inclusion of participants with diabetes, did not substantially change the results (data not shown). Notably, both consumption of tea and use of sugar were found to be independently associated with insulin sensitivity (Table 2).

Table 2. Univariate and Multivariate Linear Regression Analyses of the Relation Between Coffee Consumption and Insulin Sensitivity Index
Image description not available.

No associations were found between coffee consumption and EIR even after the above multivariate adjustment, indicating there was no effect of coffee consumption on insulin secretion (univariate regression coefficient, 0.25; P = .33).

We also found significant univariate correlations between coffee consumption and several factors associated with an unhealthy lifestyle: daily total energy intake (P<.001), amounts of sugar and pastries consumed with coffee (P<.001), and body mass index (P = .02). Furthermore, smokers had a higher coffee consumption compared with nonsmokers (3.8 [SD, 1.6] vs 3.3 [SD, 1.7] cups/d, respectively; P<.001). We found no significant relationship between coffee consumption and level of physical activity.

Comment

As in previous studies,1 we found that excessive coffee drinking was associated with other variables that suggest an unhealthy lifestyle. After controlling for these variables, however, we found that both coffee and tea consumption were related to improved insulin sensitivity. Given that caffeine has been reported to impair insulin action,3 our data suggest that other substances in these drinks are responsible for our findings. For instance, both coffee and tea contain phenolic compounds with antioxidant activity,4,5 which is relevant because oxidative stress might promote insulin resistance.2 It is possible that antioxidants in coffee could improve insulin sensitivity, given that insulin sensitivity has been reported to be associated with activity of antioxidants.6 Long-term consumption of coffee might also result in acquired tolerance to caffeine, which could explain a different metabolic effect compared with short-term use of caffeine.3

As use of sugar was independently associated with decreased insulin sensitivity, sugar may negate the benefits of coffee. However, it is also possible that such moderate amounts of sugar merely reflects other unmeasured aspects of an unfavorable lifestyle, rather than an effect of sugar itself.

Our study has several limitations. First, the cross-sectional design does not enable us to draw conclusions regarding cause and effect. Furthermore, as we only examined white men of a similar age, this study may have limited generalizability to women, and to other age and ethnic groups. However, the ULSAM cohort appears particularly suitable to investigate the present aim, as it is the largest sample in which insulin sensitivity had been examined directly, together with a reliable method used for determining coffee consumption and additives.

The proposed antidiabetogenic effect of coffee1 seems to involve improved insulin sensitivity rather than improved beta-cell function. Thus, it could be relevant to perform controlled trials in insulin resistant individuals to investigate a potential insulin-sensitizing effect of coffee.

Funding/Support: This study was sponsored by the Dr P. Håkanssons Foundation, the Emma Petterssons testamente, the Royal Scientific Society Foundation (Kungliga Vetenskapssamhällets fond), the Swedish Medical Research Council, the Swedish National Association against Heart and Lung Disease (Hjärt-Lungfonden), the Thuréus Foundation, and the Uppsala Geriatric Research Foundation.

Role of the Sponsor: None of the funding sources had any involvement in the collection, analyses, and interpretation of data; in the writing of the report; or in the decision to submit the paper for publication.

References
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Greer F, Hudson R, Ross R, Graham T. Caffeine ingestion decreases glucose disposal during a hyperinsulinemic-euglycemic clamp in sedentary humans.  Diabetes.2001;50:2349-2354.PubMed
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Bruce CR, Carey AL, Hawley JA, Febbraio MA. Intramuscular heat shock protein 72 and heme oxygenase-1 mRNA are reduced in patients with type 2 diabetes: evidence that insulin resistance is associated with a disturbed antioxidant defense mechanism.  Diabetes.2003;52:2338-2345.PubMed
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