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| Date: 04-15-2015 | HC# 031561-518 |
Re: Post-exercise Consumption of Caffeine and Green Coffee Bean Extract Has No Effect on Blood Glucose or Insulin Concentrations
Beam
JR, Gibson AL, Kerksick CM, Conn CA, White AC, Mermier CM. Effect of post-exercise caffeine and green coffee bean extract
consumption on blood glucose and insulin concentrations. Nutrition. February
2015;31(2):292-297.
Coffee (Coffea
arabica, Rubiaceae) is consumed throughout the world, and caffeine
supplementation, a compound found in coffee, has been shown to improve exercise
performance. However, it has also been reported to decrease the
exercise-induced insulin sensitivity and glucose transport. Chlorogenic acid, a
polyphenol compound found in green coffee beans, has been previously shown to have
beneficial effects on glucose metabolism, such as elevated glucose disposal in
humans, increased glucose delivery to skeletal muscle in vivo, and inhibition
of liver gluconeogenesis. These mechanisms may result in the modulation of
blood glucose concentrations. This randomized, double-blind, placebo-controlled
clinical trial investigated the impact of ingesting dextrose together with
either caffeine or green coffee bean extract (CoffeeGenic®; Life
Extension; Fort Lauderdale, Florida) on glucose and insulin concentrations
following a bout of submaximal cycling exercise.
This
study enrolled 10 healthy male subjects ranging from 19 to 34 years old with an
average maximum oxygen consumption (VO2 peak) of 55.9 ± 8.4
ml/kg/min and 5 ± 4 years of cycling experience. Subjects consumed caffeine
regularly, with the weekly consumption of 2 coffee or tea (Camellia sinensis, Theaceae) drinks with caffeine, 5 or more soft
drinks with caffeine, or any combination of these. Those under 18 or over 44
years old or with systemic or chronic diseases were excluded. Cycling
experiments were conducted in triplicate following an overnight fast (12 hours),
with a 1-2 week separation between each experiment. Subjects cycled for 30
minutes at 60% of peak power output or highest workload of exercise, and upon
completion consumed 75 g of dextrose (NOW® Foods; Bloomingdale, Illinois) along with either 5
mg/kg body weight of caffeine (NuSci Institute & Corp.; Walnut, California),
10 mg/kg body weight of CoffeeGenic, or 5 mg/kg body weight of dextrose as a
placebo.
VO2
peak was calculated at baseline with a cycling test. The protocol consisted of "70
[sic] W" and was then ramped up
by 30 or 35 W/min. Heart rate and gas exchange were monitored, and when cycling
decreased to 50 revolutions per minute, the test was stopped. Subjects were
told to maintain fasting 12 hours before experimental tests (water excepted),
record diet and exercise within 24 hours of the tests, maintain the same diet
before each test to ensure consistency, maintain regular caffeine consumption, and
refrain from alcohol usage and any extreme exercise (> 80% of maximum heart
rate) within 24 hours of the test.
Following
the collection of a resting blood sample, a 5-minute warmup at 50% peak power
was conducted. Subjects then cycled for 30 minutes at 60% peak power, and heart
rate was measured. Blood was collected again after exercise was completed, and
subjects were administered treatments or placebo along with the 75 g of
dextrose mixed with 500 ml of water at room temperature. During the first hour
following treatment, blood was collected from subjects in a sitting position
every 15 minutes. Blood was collected every 30 minutes during the second hour. Blood
samples were used for the measurement of glucose and insulin concentrations. Water
consumption was allowed as needed, and cycling was done in a room maintained at
20-23°C with 20-30% humidity.
All
subjects completed the study. The average age of subjects was 26.5 ± 5 years,
average weight was 77.6 ± 13.3 kg, average body mass index was 24.0 ± 4.3 kg/m2,
and average VO2 peak was 55.9 ± 8.4 ml/kg/min. During each test, the
average power expended was 235 ± 30 W or 60% ± 2% of peak power expenditure. The
heart rates among subjects were not significantly different. No differences
were observed in glucose or insulin concentrations among groups, including no
significant effect of time × treatment or area under the curve (AUC).
This
study did not observe any significant effects on blood glucose or insulin
concentrations as a result of green coffee bean extract or caffeine
consumption. This is in agreement with previous studies that examined the
effect of caffeine and carbohydrate consumption after exercise and also found
no effect. However, the authors admit to several limitations, including the
small sample size, caffeine content in the green coffee bean extract (~12 mg
per 400 mg capsule), and the timing of treatment administration. The authors
also mention that measurement of glycogen stores in skeletal muscle samples
would have strengthened the results. Other stated confounding factors include
variation of impacts on fitness among individuals and effects of dextrose
consumption on glucose and insulin concentrations; however, as all subjects
consumed dextrose, presumably any effects would be uniform among groups.
Additionally, further research is needed to establish if a more real-life test
protocol, such as removal of the 12-hour fast prior to extreme exercise, would
offer different results. In conclusion, neither treatment resulted in a
statistically significant effect on glucose or insulin concentrations within 2
hours following exercise.
—Amy C. Keller,
PhD