The mean age of participants was 40 years and the mean BMI 29.5 kg/m2
. Table shows the baseline characteristics of the participants according to group allocation. Except for the lower IL-6 and CRP concentrations in the caffeinated coffee group, there were no substantial differences between the groups. Correlation coefficients for adiponectin, CRP, IL-6 and fetuin-A concentrations with markers of glucose homeostasis at baseline are shown in Table . Forty-one participants (91%) completed the 8-week intervention period. The intervention did not result in significant differences between treatment groups in body composition, diet, physical activity, and sleep duration (Additional file 1
, table s1
). To assess compliance, caffeine concentrations were measured in non-fasting blood collected during the 6-week visit. Participants were unaware that caffeine was being measured at this visit. Caffeine concentrations were 3.1, 0.3, and 0.2 μg/mL for the caffeinated coffee, decaffeinated coffee, and no coffee group, respectively (p
-value < 0.0001) and similar differences were found for caffeine metabolites (Additional file 1
, figure s2
Baseline characteristics and clinical measurements for study participants by treatment group
Correlation coefficients for adiponectin, C-reactive protein, interleukin-6, and fetuin-A concentrations and markers of glucose metabolism at baseline
Table shows the differences in biomarkers between treatment groups at the end of the intervention adjusted for baseline values and change in weight. Compared with consuming no coffee, consumption of caffeinated coffee and decaffeinated coffee was not significantly associated with changes in glucose AUC (caffeinated: -0.78 mmol/L; 95% CI: -2.52, 0.95; decaffeinated: 0.47 mmol/L; 95% CI: -1.40, 2.33) or 2-hour plasma glucose (caffeinated: -0.41 mmol/L; 95% CI: -1.64, 0.83; decaffeinated: 0.79 mmol/L; 95% CI: -0.58, 2.16).
Risk factors for type 2 diabetes by coffee treatment group at Week 8a
As compared with consuming no coffee, consumption of caffeinated coffee significantly increased adiponectin (difference in change from baseline 1.44 μg/mL; 95% CI: 0.23, 2.66) and IL-6 (60%; 95% CI: 8, 138), whereas the decrease in fetuin-A (-11%; 95% CI: -27, 9) was not statistically significant. Changes after decaffeinated coffee consumption as compared with no coffee were in the same direction and reached significance for fetuin-A (-20%; 95% CI: -35, -1), but not for adiponectin (0.91 μg/mL; 95% CI: -0.42, 2.25) and IL-6 (37%; 95% CI: -8, 103). We did not find significant differences for CRP and measures of insulin resistance or insulin secretion.
In a secondary analysis evaluating blood pressure and blood lipids (Table ), participants using decaffeinated coffee had a reduction in blood pressure as compared with those consuming no coffee, which was statistically significant for diastolic blood pressure (systolic: -3.1 mmHg, 95% CI: -11.6, 5.3; diastolic: -5.7 mmHg, 95% CI: -10.9, -0.6). For caffeinated coffee, a small non-significant increase in blood pressure was found as compared with no coffee (systolic 1.2 mmHg, 95% CI: -6.5, 8.9; diastolic: 0.4 mmHg, 95% CI: -4.3, 5.2). There were no significant differences between treatment groups for high-or low density lipoprotein cholesterol, or fasting triglyceride concentrations.
The changes from baseline at Week 4 and Week 8 for adiponectin are shown in Figure . In an exploratory analysis using the complete study population regardless of treatment group, changes in adiponectin levels during the trial were significantly associated with changes in 2-hour plasma glucose (β = -0.37 mmol/L, s.e. = 0.13, p-value = 0.009) (Additional file 1
, figure s3
) and glucose AUC (β = -0.40 mmol/L, s.e. = 0.20, p-value = 0.043). Changes in IL-6 and fetuin-A were not significantly associated with changes in measures of glucose metabolism.
Figure 1 Adjusted percentage change from baseline in adiponectin at Week 4 and Week 8 by treatment group. Adjusted mean percent change estimates were determined from analysis of covariance models which included treatment as a main effect, and baseline value and (more ...)
During the 2-week caffeine washout period prior to randomization, 15 occurrences of headaches were reported and after the baseline visit two occurrences of headaches were reported (one in the decaffeinated group and one in the control group). There were a total of four other adverse events reported during the trial that were non-serious and consisted of a urinary tract infection and gastroesophageal reflux among participants assigned to decaffeinated coffee, and a hypoglycemic event and cold sores among participants assigned to caffeinated coffee.