High-dose caffeine improves reaction time but increases nervousness

A recent study in Spain found that an intake of 6 mg/kg of caffeine improved participants’ reaction times in a series of reaction tasks. It also made participants more nervous, alert, and active. The research was published in the European Journal of Nutrition.

Caffeine is a natural stimulant found in coffee, tea, chocolate, and many energy drinks. It works by blocking adenosine, a neurotransmitter that promotes sleep, leading to increased alertness and wakefulness. Caffeine is absorbed quickly into the bloodstream and typically begins affecting the body within minutes.

Caffeine is widely used to improve focus, reaction time, and reduce fatigue. In moderate doses, caffeine is considered safe, but excessive intake can cause side effects such as jitteriness, increased heart rate, and insomnia. It is one of the most widely consumed psychoactive substances globally. Consequently, caffeine plays a significant role in cultural practices worldwide and the daily routines of many people.

Study author María Ramírez-delaCruz and her colleagues aimed to determine the acute effects of different doses of caffeine (0, 3, or 6 mg/kg) on cognitive performance in physically active, healthy individuals. They hypothesized that caffeine would increase cognitive performance and that 3 mg/kg would be the optimal dose to achieve this effect.

The study participants included 29 healthy young adults, 15 of whom were men. The participants’ average age was 22 years. All participants completed three trials. In each trial, they received a capsule with a different dose of caffeine adjusted to their body weight. In one trial, the dose was 3 mg/kg; in another, it was 6 mg/kg; and in the third, the capsule contained no caffeine. Participants did not know which dose was in the capsule they consumed, and this information was also unavailable to the researchers working with them during the experimental sessions. The order of doses (which dose was given in each trial) was randomized.

In each session, after consuming their designated capsule, participants completed tests of reaction time (on the Dynavision™ D2 Visuomotor Device), visual acuity related to hand-eye coordination and anticipation (the Bassin Anticipation Timer), sustained attention (the “Go/No-Go Test” and “Eriksen Flanker Test”), and memory (using lists of words created by the study authors for participants to memorize).

The results showed that reaction time improved after the intake of 6 mg/kg of caffeine, while the 3 mg/kg dose had no effect. Additionally, there were no effects of caffeine on anticipation, sustained attention, or memory performance.

After receiving the 6 mg/kg dose, participants also exhibited increased nervousness and activeness (i.e., they were more alert, had higher energy levels, and became more active).

“The acute intake of 6 mg/kg of caffeine was effective in improving RT [reaction time] in the test used on the Dynavision™ D2 visuomotor device. In addition, the 6 mg/kg dose of caffeine augmented the occurrence of side effects, mainly increased activeness and nervousness,” the study authors concluded.

The study sheds light on the effects of caffeine on cognitive performance. However, it should be noted that the study was conducted on a very small group of participants, all of whom were young adults. Effects on other demographic groups might not be identical.

The paper, “Effects of different doses of caffeine on cognitive performance in healthy physically active individuals,” was authored by María Ramírez-delaCruz, Paula Esteban-García, Pablo Abián, Alfredo Bravo-Sánchez, Inés Piñas-Bonilla, and Javier Abián-Vicén.