Effect of endurance training supplemented with green tea extract on substrate metabolism during exercise in humans


Journal Title (Medline/Pubmed accepted abbreviation): Scand. J. Med. Sci. Sports
Year: 2011
Volume: 21
Page numbers: 598-605

Summary of background and research design:

Background: Endurance training has been shown to improve the body’s ability to burn fat.  Also, in mice, long-term consumption of green tea extract (GTE) has also shown to increase the body’s ability to burn fat. 

Hypothesis: Does GTE supplementation enhance the improvement of fat utilization that is gained from endurance training?

Subjects: Twelve healthy men, age 23 ± 2 yrs old

Experimental design: randomized, double-blind, placebo-controlled

Treatments:  Green tea extract (GTE): 340 mL of beverage containing 572.8 total catechins, or a placebo (contents not disclosed).  Both beverages contained about 77 mg of caffeine.

Protocol:  Initially, participants were evaluated for VO2peak (also known as VO2max), ventilation threshold (thought of as “endurance”), maximum heart rate, respiratory exchange ratio (RER) and VO2drift using a stationary bicycle. The participants were then instructed to consume one serving of either the GTE or a placebo every day during the training period.  As part of the endurance training program, the participants cycled at 60% of their VO2peak for 60 min/day, 3 days/wk, for 10 wks.  Workload was adjusted as VO2peak changed with training.  After training, they were then evaluated for the same exercise parameters.  Both before and after the training period, blood samples were acquired at rest, 30 min after the commencement of cycling, and 30 min after cycling was completed.  Blood was analyzed for glucose, glycerol (a breakdown product of triglycerides, i.e. fat), and free fatty acids.
 


Summary of research findings:
  • The average workload was similar between the 2 groups during the training sessions.
  • From a dietary analysis, there were no significant differences in nutrient intake between the two groups including total catechins, except for the test beverage.  Diets also did not change throughout the course of the supplementation period.
  • VO2max and ventilation threshold increased after training with no differences between groups.
  • Heart rate and respiration rate during exercise were slower after training compared to before training for both groups.
  • The RER did not change between before and after training for the placebo group, suggesting that they were burning energy from carbohydrates, fats, and proteins similarly at both evaluations.  However, the RER was significantly lower after the training period for those in the GTE group (0.816 ± 0.006 after vs. 0.844 ± 0.005 before training).  This suggests that they burned more energy from fat then carbohydrate at the latter exercise evaluation.
  • Total carbohydrate oxidation decreased after training compared to before training.  Carbohydrate oxidation decreased more in the GTE group compared to the placebo group.  Accordingly, fat oxidation was greater in the GTE group than the placebo group after training, but these values were not significantly different from each other or from the pre-training values.  (There may have been a net decrease in total energy burned during exercise as exercise tolerance increased.)
  • There were no differences in lactate, glucose, glycerol, or free fatty acid levels between the 2 groups.

Key practice applications:

GTE supplementation in conjunction with exercise training slightly increased fat oxidation compared with training without GTE supplementation.


Limitations:

The authors suggest that an increase in exercise with GTE supplementation increases the rate of fat burning, which in turn can promote weight loss.  The increase in aerobic capacity of the subjects during the supplementation period implies that the subjects were not involved in regular endurance training before this study, so it would be expected that the additional exercise (without compensatory increase in energy consumption; the authors showed no increase in food intake) would promote weight loss.  On average, the subjects did not lose weight or alter their BMI.  This observation is puzzling and not discussed in the article.
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