The details of this study have been previously described . Briefly, eligible participants were overweight (body mass index 25–35 kg/m2), nonsmoking men and women aged 18 years or older who habitually consumed coffee (at least two cups per day). Exclusion criteria included the presence of diabetes, heart disease, stroke, hypertension, alcoholism or substance abuse, abnormal hepatic or renal function, gastro-esophageal reflux disease, a medical history of ulcers, or women planning a pregnancy or breastfeeding. Exclusions were made for individuals on medications for chronic health conditions.
Sixty-five adults were screened of which 11 were ineligible and 9 withdrew from the study prior to randomization. Three individuals did not continue the study after the baseline visit and were not included in the current analysis. The final study population included 14 men and 28 women. The study was approved by the institutional review boards of the Beth Israel Deaconess Medical Center and the Harvard School of Public Health and all participants provided written informed consent. The clinical trial registration number is NCT00305097.
After two weeks of caffeine abstention, participants attended the baseline visit in the morning after fasting overnight for at least 12 hours. Participants were randomized to either caffeinated coffee, decaffeinated coffee, or no coffee (control) treatment groups. Treatment assignments for the coffee arms were blinded to the study participants, investigators, and laboratory staff. Participants in the coffee treatment groups were given five two-gram portions of instant coffee per day (caffeinated or decaffeinated Nestlé’s Taster’s Choice®) to be mixed with approximately 6 ounces of boiling water and consumed with every meal and mid-morning and mid-afternoon. Non-caloric sweetener and non-dairy creamer were also provided. Participants in the control group were instructed to drink the equivalent amount of water at the same intervals throughout the day.
At baseline, week 4, and week 8, the study visits included a physical examination, anthropometric measurements, and a fasting blood draw. Sex hormone-binding globulin and all other endogenous sex hormones were measured via Access chemiluminescent immunoassay (Beckman Coulter, Fullerton, CA). Free testosterone and free estradiol were calculated using the Sodergard formula which is based on the law of mass action and assumptions of equilibrium binding .
All analyses were performed separately for men and women. Using general linear models, we evaluated the change from baseline in SHBG, testosterone (total and free), estradiol (total and free), testosterone to estradiol ratio, and DHEAs regressed on treatment group as a main effect with baseline log values of the dependent variable and age as additional covariates. Both covariates were grand mean centered to improve the interpretability of the estimates. Because SHBG and sex hormones did not follow a normal distribution, the variables were log-transformed and subsequently back-transformed to yield geometric means. Differences between caffeinated and decaffeinated coffee compared with the control group were based on linear contrasts. The adjusted geometric means with standard errors were reported by treatment, and 95% confidence intervals (CI) were computed. In addition, we calculated the difference between the treatment groups versus control for change from baseline. This yielded a ratio (or percentage when subtracting the value one and multiplying by 100), given the principles of logged numbers.
Statistical significance was evaluated at an alpha level of 0.05. The Statistical Analysis System version 9.1.3 was used for all analyses (SAS Institute, Cary, NC).