Regular dark chocolate consumption’s reduction of oxidative stress and increase of free-fatty-acid mobilization in response to prolonged cycling


Journal Title (Medline/Pubmed accepted abbreviation): Int J Sport Nutr Exerc Metab.
Year: 2011
Volume: 21
Number: 2
Page numbers: 113-123
doi (if applicable): N/A

Summary of Background and Research Design

Background: Physical activity increases production of reactive oxygen species (ROS), which can lead to oxidative stress, acute-phase inflammatory responses, and possible acceleration of muscle-fatigue development. A number of studies have demonstrated that supplementation with antioxidants can reduce the magnitude of oxidative stress and inflammatory response that can occur after prolonged intense exercise. Dark chocolate (DC) is one of the richest dietary sources of antioxidant polyphenols. There are few studies, however, investigating the potential of dark chocolate to modulate exercise-induced oxidative stress and exercise performance. Dark chocolate also contains caffeine and theobromine, which may also have potential ergogenic effects.

Hypothesis/purpose of study: The study aims were to examine the effects of a 2-week period of regular DC consumption on the responses of selected plasma markers of oxidative stress, hormones, and cytokines to a single bout of prolonged exhaustive exercise.

Subjects: Twenty healthy males participated in the study (mean age, 22 ± 4 yr; body mass, 74.6 ± 8 kg; maximal oxygen consumption [VO2max], 53.1 ± 7.0 mL/kg/min; power output at VO2max, 300 ± 30 W). Participants engaged in regular physical activity for a minimum of 2 hours, 3 times/week prior to participating in the study.

Experimental design: Single-blind, randomized, counterbalanced, crossover

Treatments and protocol: All participants completed 4 exercise bouts: 2 preliminary trials for VO2max determination and familiarization, and 2 main trials. Trials were separated by 1 week, and there was a 2-week washout period between the main trials. The main trials consisted of steady-state cycling at 60% VO2max for 1.5 hours with an increase of exercise intensity to 90% VO2max for 30 seconds every 10 minutes, followed by a ride to exhaustion at 90% VO2max. In the 2 weeks before exercise, participants consumed 40 g of DC or an isocarbohydrate-fat control cocoa liquor-free chocolate (CON) twice daily and once 2 hours before exercise. Venous blood samples were taken immediately before exercise, after steady-state cycling, after cycling to exhaustion, and after 1 hour of resting recovery. Blood was analyzed for plasma markers of oxidative stress, hormones, and cytokines.

Summary of research findings
  • F2-isoprostanes, a marker of lipid peroxidation, were significantly lower with DC (P < .001) at exhaustion and after 1 hour of recovery vs CON.
  • Oxidized low-density lipoproteins were significantly lower with DC (P < .001) before and after exercise, and at exhaustion vs CON.
  • DC was associated with ~ 21% greater rises in free fatty acids during exercise vs CON (main effect of trial, P = 0.038).
  • Changes in circulating glucose, insulin, glucagon, cortisol, interleukin (IL)-6, IL-10, and IL-1ra were not affected by ingestion of DC.
  • Time to exhaustion at 90% VO2max did not differ significantly between trials (398 ± 204 sec and 374 ± 194 sec for DC and CON, respectively).

Interpretation of findings/Key practice applications

The results of this study suggest that regular intake of DC is associated with reduced oxidative stress markers and increased mobilization of free fatty acids after exercise. However, intake of DC had no effect on the plasma concentration of measured hormones or cytokines, circulating numbers of total leukocytes or neutrophils, or on exercise performance. Although currently there are limited data in human studies to suggest that antioxidant supplementation enhances exercise performance by preventing oxidative mechanisms and muscle fatigue, there continues to be a need for evaluating sources that can provide improved antioxidant capacity.
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