Letters Section Editor: Stephen J. Lurie,
MD, PhD, Senior Editor.
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.
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.
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.
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).
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.
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
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.
Ärnlöv J, Vessby B, Risérus U. Coffee Consumption and Insulin Sensitivity. JAMA. 2004;291(10):1199-1201. doi:10.1001/jama.291.10.1199-b