Applied nutritional investigationEffect of post-exercise caffeine and green coffee bean extract consumption on blood glucose and insulin concentrations
Introduction
Caffeine supplementation can aid in the improvement of exercise performance by increasing exercise work capacity [1], [2], [3], particularly through its effect on the neuromuscular system [2], [4], rather than on metabolism [5]. Caffeine alters metabolism through several complex mechanisms [3], [5], and may help facilitate fat loss [6] and post-exercise glycogen resynthesis [7].
Although pure caffeine supplementation may enhance performance, it also may negate the exercise-induced insulin sensitivity response to exercise training and the body's ability to tolerate elevated blood glucose levels at rest. Caffeine has been shown to reduce the beneficial effects of prior exercise on insulin sensitivity and glucose transport into skeletal muscle [8]. Additionally, caffeine has been shown to reduce the insulin-mediated whole-body glucose disposal and decrease insulin sensitivity during an oral glucose tolerance test (OGTT) in humans at rest [9], [10].
Chlorogenic acid, a natural polyphenol found in green coffee beans, has been reported to increase blood glucose disposal during an OGTT in animals [11] and in humans [12], [13]. Chlorogenic acid has been reported to inhibit hepatic glucose-6-phosphatase and consequently prevent glucose-6-phosphate hydrolysis in rats [14]. This enzyme plays a major role in the homeostatic regulation of blood glucose by catalyzing the removal of a phosphate group from glucose-6-phosphate, effectively enabling hepatic glucose release into the bloodstream. Since chlorogenic acid has been shown to inhibit glucose-6-phosphatase in the liver, it also may help spare liver glycogen during exercise. Furthermore, it was recently shown that chlorogenic acid stimulates glucose transport in rat skeletal muscle via 5′-adenosine monophosphate-activated protein kinase activation [15]. Therefore, chlorogenic acid supplementation may help spare liver glycogen, or enhance blood glucose disposal and transport into skeletal muscle in humans. As a result, there may be less accumulation of glucose in the blood after consuming green coffee bean extract with dextrose compared with consuming caffeine with dextrose.
The effect of pre-exercise caffeine consumption on glucose metabolism during exercise has been reviewed [5]. The effect of pre- and during-exercise [16], as well as post-exercise [7], [8], [17], [18] caffeine supplementation on glucose metabolism and homeostasis during an acute, passive recovery has been investigated. Ingestion of green coffee bean extract has heightened in popularity over the past several years. Purportedly, ingestion of green coffee bean extract can aid in weight loss and affect blood glucose and insulin concentrations, presumably due to its chlorogenic acid content [19]. In this respect, limited research has investigated the effect of chlorogenic acid supplementation alone on blood glucose and insulin in humans during an OGTT [12], [13]. To our knowledge, there have not been any human studies that have investigated the effects of pre- or post-exercise green coffee bean extract (50% chlorogenic acid, 3% caffeine) supplementation on blood glucose and insulin concentrations.
We hypothesized that post-exercise coingestion of green coffee bean extract with dextrose will lower blood glucose and insulin concentrations more than the coingestion of caffeine with dextrose during the post-exercise recovery period when compared with ingesting dextrose alone. Therefore, the purpose of this study was to investigate the effect of ingesting dextrose plus caffeine or dextrose plus green coffee bean extract immediately after an intense bout of submaximal cycling on blood glucose and insulin concentrations when compared with ingesting dextrose alone.
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Participants
Ten healthy men ages 19 to 34 y with a mean VO2 peak of 55.9 ± 8.4 mL·kg·min−1 (4.28 ± 0.45 L/min) and 5 ± 4 y of cycling experience participated in the study. The men were recruited by word of mouth, posted flyers, and email. They described their daily caffeine consumption using a questionnaire and were defined as regular caffeine users by the consumption of at least two caffeinated coffee or tea beverages, five or more caffeine-containing soft drinks, or a combination of beverages per week [9]
Results
All participants completed all trials. Descriptive characteristics and data collected from the VO2 peak assessment are summarized in Table 1. Average HR responses during each trial were not significantly different (P > 0.05) from each other (PLA = 167 ± 6 bpm, 91 ± 4% HR max; CAF = 165 ± 8 bpm, 90 ± 4% HR max; GCB = 163 ± 9 bpm, 89 ± 4% HR max). The average power output for each trial was 235 ± 30 W, 60% ± 2% peak power output (P > 0.05).
No statistically significant treatment effect for fasting
Discussion
In this study, the effect of post-exercise caffeine and green coffee bean extract supplementation on glucose and insulin concentrations was investigated in 10 male cyclists. The primary finding from this study was that post-exercise consumption of caffeine with dextrose or green coffee bean extract with dextrose had no significant effect on glucose and insulin concentrations when compared with each other or a placebo.
Limited research exists examining the effect of immediate post-exercise
Conclusion
Our data indicate that neither treatment exerted a statistically significant effect on blood glucose and insulin concentrations when compared with each other and a placebo. More human research is needed to determine the effect of these combined nutritional treatments and exercise on changes in blood glucose and insulin.
Acknowledgments
The authors acknowledge the men who volunteered to participate in this study. We also acknowledge the Department of Health, Exercise & Sports Sciences and the university's graduate school for administrative support. Finally, we acknowledge Hung-Sheng Hsu and Dr. Micah Zuhl for assisting in the piloting process for this research study.
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Partial funding and other support for this study was provided by the university's Academic Provost, College of Education, Graduate and Professional Student Association, Graduate School, and Department of Health, Exercise & Sports Sciences. JRB and CMM were responsible for the conception and design of the study; the generation, collection, assembly, analysis and/or interpretation of data; and drafting or revision of the manuscript. ALG and CMK, and CAC were responsible for the conception and design of the study and drafting or revision of the manuscript. ACW was responsible for generation, collection, assembly, analysis, and/or interpretation of data and drafting or revision of the manuscript. All authors approved the final version of the manuscript.