Cup of Joe—No Go?
With Comments from Russell H. Greenfield, MD, Medical Director, Carolinas Integrative Health, Carolinas HealthCare System, Charlotte, NC, Clinical Assistant Professor, School of Medicine, University of North Carolina, Chapel Hill, NC. Dr. Greenfield is Executive Editor of Alternative Medicine Alert.
Source: Lee SJ, et al. Caffeine ingestion is associated with reductions in glucose uptake independent of obesity and Type 2 diabetes before and after exercise training. Diab Care 2005;28:566-572.
Goal: To determine the effect of a single exposure to caffeine on insulin-mediated glucose uptake in previously sedentary men, both lean and obese, the latter with and without Type 2 diabetes mellitus (DM). Also, to see whether three months of aerobic fitness training without weight loss influences the effects of caffeine on glucose uptake in these same men.
Design: Randomized, double-blind, 17-week trial.
Subjects: Sedentary lean (n = 8, BMI < 25 kg/m2) and obese (n = 15, BMI > 27 kg/m2, 7 with Type 2 DM) Caucasian, non-smoking Canadian men.
Methods: All subjects performed two trials, one with placebo and one with caffeine, before and after exercise training. The 13-week aerobic exercise program consisted of either walking or light jogging on a treadmill at moderate intensity for 60 min five times weekly. A weight-maintenance diet was undertaken prior to and during the exercise-intervention period, and detailed food intake records were completed. After fasting for 10-12 hours overnight, baseline blood samples were obtained and subjects then took the placebo pill (dextrose) or caffeine 5 mg/kg (equivalent to 2-3 cups of coffee) with 250 cc water. An IV of normal saline with glucose, insulin, and potassium was infused to maintain plasma glucose concentration at approximately 5 mmol/L, and blood samples were obtained every 30 min for three hours. Whole body MRI was performed to determine body fat (total and visceral) and skeletal muscle mass.
Results: Caffeine ingestion caused significant reductions in glucose uptake in all groups both prior to and after the exercise program. Impairments in glucose uptake ranged from 33-37% at baseline to 23-36% post-exercise, with the highest numbers occurring in men with Type 2 DM.
Conclusions: Caffeine ingestion reduces insulin-mediated glucose uptake independent of BMI or Type 2 DM. Caffeine’s effect on glucose uptake is not attenuated through aerobic exercise that maintains body weight despite small reductions in visceral body fat.
Study strength: Thorough methodology.
Study weaknesses: Small sample; subjects were already moderate consumers of caffeine; results applicable only to white men.
Of note: Self-reported caffeine ingestion prior to the study was 1-5 cups/d; four men with DM were being treated with oral hypoglycemic agents; subjects were not permitted to use vitamins or supplements during the trial; to ensure normal muscle glycogen levels and control for known effects of exercise on glucose uptake, participants were asked to eat at least 200 g carbohydrate and to avoid strenuous exercise for at least four days prior to measurement of insulin sensitivity; it’s interesting that the placebo pill was dextrose.
We knew that: Prior studies have shown that caffeine impairs glucose tolerance and insulin sensitivity; even a single episode of exercise is normally associated with improvements in glucose tolerance and insulin sensitivity; caffeine may impact skeletal muscle adenosine receptors or cause significant increases in epinephrine; recent studies suggest that high intakes of coffee (5-6 cups/d) may actually help prevent Type 2 DM (but coffee consumption alone may underestimate total caffeine consumption, and coffee contains magnesium, which increases insulin sensitivity).
Clinical import: Data regarding preventive effects of high coffee intake are interesting, but one would be hard pressed to recommend 5-6 cups of coffee daily as a therapeutic intervention. This small study adds to data strongly suggesting that regular consumption of caffeine heightens the risk for metabolic disorders, especially in those already at risk with a BMI > 25 kg/m2. We should counsel our patients, especially those struggling to attain a healthy weight, to limit caffeine exposure, while allowing for the occasional splurge.
What to do with this article: Keep a copy on your computer.