Effects of Coffee Consumption on Fasting Blood Glucose and Insulin Concentrations

Abstract

H igher habitual coffee consumption was associated with higher insulin sensitivity (1) and a lower risk for type 2 diabetes (2-6) in diverse populations. In contrast, short-term metabolic studies showed that caffeine intake can acutely lower insulin sensitivity (7-9) and increase glucose concentrations (10-15). Randomized intervention studies are needed to examine whether tolerance to these acute effects develops after longer-term consumption (16). We therefore examined the effects of coffee and caffeine on fasting blood concentrations of glucose and insulin over 2-4 weeks in two crossover studies in healthy volunteers. RESEARCH DESIGN AND METHODS-The studies were approved by the TNO Nutrition and Food Research Medical Ethics Committee, and all participants gave informed consent. The trials were originally designed to study the effects of coffee and caffeine on plasma concentrations of homocysteine, and the study designs have been reported in detail previously (17,18). Participants were regular coffee consumers (more than five cups/day) and did not have known diabetes. The first study was a 4-week cross-over study that compared the effects of regular paper-filtered coffee consumption with that of coffee abstinence. A total of 40 volunteers used 1 l of coffee (70 g coffee grounds) for 4 weeks and abstained from coffee for 4 weeks in random order. Fourteen participants did not complete the trial because of nausea and restless-ness (n 7), possible susceptibility to adverse effects of caffeine intake (n 3), or reasons unrelated to treatment (n 4). Thus, 26 participants were included in the analysis. The second study had a Latin-square design with three treatments given in random order for 2 weeks each: caffeine (a total of 870 mg in six capsules), regular paper-filtered coffee (52 g ground coffee/day in 0.9 l), and placebo (six capsules containing cellulose). Of the 54 volunteers , 6 subjects withdrew because of severe headaches (n 2), study-related illness (n 1), or reasons unrelated to treatment (n 3). For the current analyses , we excluded participants because of missing blood samples (n 1), not completing the whole caffeine intervention (n 1), or who were clear outliers for an insulin concentration (n 1). Thus, 45 subjects were included in the analysis. Caffeine-containing products (other than those provided) were prohibited during the entire trial. Venous blood samples were collected after an overnight fast. Plasma glucose concentrations were measured using the glucose hexokinase method. Serum insulin concentrations were measured using an immunoradio-metric assay (Medgenix Biosource Diag-nostics, Fleuris, Belgium). In study 1, treatment responses were compared using paired t tests. In study 2, we tested for overall treatment effects using ANOVA. All reported P values were two sided, and P values 0.05 were considered statistically significant. RESULTS Study 1 Of the participants that completed the study, 62% were women, mean (SD) age was 37 12 years, and mean BMI was 23 3 kg/m 2. After 2 weeks, coffee consumption tended to lead to higher fasting glucose concentrations, but no appreciable effect was observed after 4 weeks (Table 1). Fasting insulin concentrations, measured only after 4 weeks, were higher after the coffee period than after the no coffee period (Table 1). Tests for carry-over effects did not indicate that these existed (insulin: P 0.79; glucose: P 0.27). Study 2 Of the participants that completed the study, 56% were women, mean age was 40 14 years, and mean BMI was 24 3 kg/m 2. Fasting glucose concentrations were similar after the caffeine, coffee, and placebo period (Table 1). Compared with the placebo period, fasting insulin concentrations tended to be higher after the coffee and caffeine periods (Table 1). CONCLUSIONS-We found that high coffee consumption for 4 weeks increased fasting insulin concentrations compared with coffee abstinence. Consumption of somewhat weaker coffee and caffeine intake were nonsignificantly associated with higher fasting insulin concentrations. No substantial effects of coffee or caffeine on fasting glucose concentrations were observed. The increased fasting insulin concentration after high coffee consumption in our study probably reflects decreased insulin sensitivity. In short-term metabolic A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances.