Caffeine is known to increase performance, especially in the afternoon and evening, but how does it affect oxidative stress, cognition, and psychological state?

The study

This randomized crossover trial enrolled 13 male recreational runners. Average age: 21 years. Average caffeine intake: <28 mg/day.

The participants received caffeine (3 mg/kg of body weight) or a placebo before a 3 km race. After a 1-week washout period, they switched groups and repeated the trial.

Before and after each competition, the researchers administered cognitive and psychological tests and took blood samples.

The results

Caffeine improved 3 km performance by 1.1% (−6.62 seconds). With regard to cognition, it improved attention by 15.6% and reaction time by 5.9%. With regard to psychological state, it increased good feelings by 15.7% while decreasing stress by 17.6% and pain by 11.3%.

The race increased oxidative stress, but this increase was less in the caffeine group than in the control group for all four markers: malondialdehyde (+9.6% vs. +10.3%), superoxide dismutase (+6.5% vs. +7.6%), glutathione peroxidase (+8.8% vs. +12.2%), and reduced glutathione (−10.1% vs. −17.6%).


Eight ounces of brewed coffee, 1.5 ounces of espresso, and 16 ounces of black tea deliver approximately 100 mg of caffeine. Note, however, that this number can vary greatly based on the amount and caffeine content of the beans or leaves used to brew your cup. You can find other numbers — for coffee, tea, and other drinks — in our “How much caffeine is too much?” article, here.

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This Study Summary was published on August 3, 2021.