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How caffeine affects the brain’s blood flow

Moderate caffeine stimulates the central nervous system enhancing alertness, cognitive function, and energy levels but how much is to much?

The most widely consumed psychoactive substance globally is caffeine. It’s found in varying amounts within coffee, tea, soft drinks, chocolates, and energy drinks.

However, chronic high-dose caffeine consumption may lead to negative health consequences. This includes increased blood pressure, vascular resistance, arterial stiffness, and cerebral vasoconstriction.

Studies suggest that regularly consuming two to four cups of caffeine daily can reduce cerebral blood flow by 22-30%.

Caffeine’s structure resembles adenosine, allowing it to bind and block adenosine receptors in the brain. This blockage increases the release of neurotransmitters linked to improved mood, energy, and concentration. Additionally, caffeine may inhibit phosphodiesterase activity, leading to higher cellular levels of cyclic adenosine monophosphate (cAMP) and potentially increased blood pressure.


An investigation was conducted to assess the influence of caffeine consumption on blood flow velocity within the middle cerebral artery of healthy young adults. The study included 45 university students aged 18-22, none identified as habitual coffee drinkers. It’s worth noting that most participants reported occasional coffee consumption (one to two cups daily) during stressful periods like exams.

Participants were randomly assigned to three groups of 15 individuals each. The low-caffeine and high-caffeine groups received capsules containing 45mg and 120mg of caffeine, respectively. The control group received a placebo – flour capsules containing no caffeine.

Transcranial Doppler ultrasonography was used to measure baseline blood flow velocity in the middle cerebral artery before and 30 minutes after caffeine administration. While velocity measurements were taken, participants completed functional tests evaluating hypo- and hyperventilation alongside three cognitive tasks assessing short-term memory, vocabulary problem-solving, and mathematical problem-solving.

Study Analysis

Analysis of pre- and post-caffeine measurements in the low-dose group identified significant reductions in average and peak systolic blood flow velocities within the middle cerebral artery during hypoventilation. Similar reductions were observed during the short-term memory and vocabulary tests. Notably, a substantial decrease in end-diastolic velocity was found during hypoventilation, hyperventilation, and the short-term memory test. Additionally, heart rate significantly decreased during hypoventilation, hyperventilation, the short-term memory test, and the math problem-solving test.

The high-dose group displayed a similar pattern. All measured parameters – mean velocity, peak systolic velocity, end-diastolic velocity, and heart rate – showed significant reductions across all evaluations after caffeine consumption. Interestingly, the control group receiving a placebo exhibited no significant changes in blood flow velocities between baseline and post-intervention measurements.

Changes during functional tests

The analysis conducted at the post-intervention time point revealed a significant increase in blood flow velocities during hypoventilation compared to baseline levels across all groups. Regarding heart rate, a significant decrease was observed in both low- and high-caffein groups.

During hyperventilation, a significant decrease in blood flow velocities was observed at the post-intervention time point in all groups compared to baseline levels. Regarding heart rate, a significant increase was observed in all groups.

Changes during cognitive tests

A significant increase in end-diastolic velocity and heart rate was observed in the high-caffeine group during the short-term memory test. In the control group, a significant increase in end-diastolic velocity was observed during the math problem-solving test.

No significant changes in blood flow velocities and heart rate were observed in the low-caffeine group during cognitive tests.

Study significance

Caffeine intake acutely influences the cardiovascular system and interferes with blood flow velocities of the middle cerebral artery in a dose-dependent manner. The study findings suggest that caffeine-mediated decrease in blood flow in the middle cerebral artery could be due to widening of cerebral arteries caused by the short-term effects of caffeine.



The objective of the present study is to evaluate whether, after caffeine ingestion, there are variations in blood velocity of the middle cerebral arteries in clinically healthy young people as well as to evaluate whether this variation is dependent on the administered dose.


We used transcranial Doppler ultrasonography to record blood velocities of the middle cerebral arteries in three groups of 15 clinically healthy young adults each: no caffeine, a45 mg, and 120 mg of caffeine groups. Transcranial Doppler ultrasonography provided simultaneous bilateral velocity of the middle cerebral arteries measurements while participants performed functional tests (hyperventilation and hypoventilation orders) and three cognitive activities (test 1, short-term memory; test 2, solving a vocabulary problem; and test 3, solving a math problem) each in 31-s tests with 1-min rests between them. Participants were assessed before and 30 min after caffeine ingestion.


There was a significant decrease in mean velocity, peak systolic velocity, end-diastolic velocity, and heart rate after high caffeine intake, except in hyperventilation, which was only observed in peak systolic velocity. With the intake of a lower dose, significant decreases were seen with hypoventilation and with test 1. In hyperventilation, there was only a significant decrease in end-diastolic velocity and heart rate; in test 2, it was found in mean velocity and peak systolic velocity; and in test 3, only in heart rate.


With this study, we conclude that caffeine influences the cardiovascular system acutely, interfering with the velocity of the middle cerebral arteries, causing its decrease. We also conclude that this acute effect causes vasodilation of the cerebral arteries, more accentuated with higher doses of caffeine.

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