The effect of coffee consumption on insulin sensitivity and other biological risk factors for type 2 diabetes: a randomized placebo-controlled trial
Authors: Christian Darimont
Chieh Jason Chou
Bee-Choo Tai
Derrick Johnston Alperet
Eric Yin-Hao Khoo
Salome Antonette Rebello
Rob M van Dam
Zoey Tay
Shahram Emady-Azar
E-Shyong Tai
Sharna Si-Ying Seah
Journal: American Journal of Clinical Nutrition (2020)
Discipline: CLINICAL MEDICINE
Keywords: adult-onset diabetes mellitus; coffee; humans; hyperinsulinemic euglycemic clamp; insulin resistance
Affiliations: NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore (NUS), Singapore., A*STAR Graduate Academy, Agency for Science, Technology and Research (A*STAR), Singapore., Saw Swee Hock School of Public Health, National University of Singapore, Singapore., Department of Nutrition, Harvard T.H. Chan School of Public Health, Harvard University, Boston, MA, USA., Department of Medicine, Yong Loo Lin School of Medicine, National University of Singapore, Singapore., Division of Endocrinology, National University Hospital, National University Health System, Singapore., Investigational Medicine Unit, Yong Loo Lin School of Medicine, National University of Singapore, Singapore., Clinical Development Unit, Nestlé Research, Lausanne, Switzerland., Nestlé Institute of Health Sciences, Nestlé Research, Lausanne, Switzerland., NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore (NUS), Singapore

Abstract:  Background: In observational studies, coffee consumption has been consistently associated with a lower risk of type 2 diabetes mellitus. Trials examining the effect of coffee consumption on glucose metabolism have been limited by the use of surrogate insulin sensitivity indices, small sample sizes, lack of blinding, and short follow-up duration. Objectives: We aimed to overcome limitations of previously conducted coffee trials in a randomized placebo-controlled trial of the effect of coffee consumption on insulin sensitivity. Methods: We conducted a 24-wk randomized placebo-controlled trial in 126 overweight, non-insulin sensitive (HOMA-IR ≥1.30), Chinese, Malay, and Asian-Indian males and females aged 35-69 y. Participants were randomly assigned to receive 4 cups of instant regular coffee (n = 62) or 4 cups of a coffee-like placebo beverage (n = 64) per day. The primary outcome was the amount of glucose metabolized per kilogram of body weight per minute (Mbw) assessed during steady-state conditions with a hyperinsulinemic euglycemic clamp. Secondary outcomes included other clamp-based insulin sensitivity measures, biological mediators of insulin sensitivity, and measures of fasting glucose metabolism. Results: Coffee consumption did not significantly change insulin sensitivity compared with placebo (percentage mean difference in Mbw = 4.0%; 95% CI: -8.3, 18.0%; P = 0.53). Furthermore, no significant differences in fasting plasma glucose (2.9%; 95% CI: -0.4, 6.3%; P = 0.09) or biological mediators of insulin resistance, such as plasma adiponectin (2.3%; 95% CI: -1.4, 6.2%; P = 0.22), were observed between coffee and placebo groups over 24 wk of intervention. Participants in the coffee arm experienced a loss of fat mass (FM) (-3.7%; 95% CI: -6.3, -1.1%; P = 0.006) and reduction in urinary creatinine concentrations (-21.2%; 95% CI: -31.4, -9.5%; P = 0.001) compared with participants in the placebo arm over 24 wk of intervention. Conclusions: Consuming 4 cups/d of caffeinated coffee for 24 wk had no significant effect on insulin sensitivity or biological mediators of insulin resistance but was associated with a modest loss of FM and reduction in urinary creatinine concentrations.This trial was registered at clinicaltrials.gov as NCT01738399. Registered on November 28, 2012. Trial sponsor: Nestlé Research, Lausanne, Switzerland. Trial site: National University of Singapore. 

Retracted: No

PubPeer comments:
 This paper concludes  Coffee consumption was associated with a modest loss in body FM compared with the placebo beverage, and this potential impact on adiposity warrants confirmation in additional trials . It is very hard to see how further investigation can be justified in this arena - while there was a single positive (p 0.05) result indicating that coffee was superior for fat mass, there were no benefits seen in this study for a large number of other indicators of obesity (including weight, FFM, waist circumference etc). Given that there were 31 outcomes tested in total, and there was essentially no difference between the coffee and no coffee groups for all but 2 of the outcomes - urinary creatinine and fat mass - it seems much more likely that the few results achieving statistical significance were due to chance alone. For example, it should be noted that coffee appeared to increase heart rate and fasting glucose, with heart rate almost meeting the arbitrary 0.05 cutoff for significance . A more reasonable interpretation, surely, is that coffee has no benefit for reduction in adiposity, as demonstrated by the lack of any difference between the coffee and no-coffee groups on most of the outcomes measured. A further point is that the acknowledgements section of the study claims that  The funder had no decisive role in the design, implementation, analysis, and interpretation of the data . It is hard to see how this can be true, given that three of the authors of the paper were employed by the funding body. ,