Caffeine has its pros and cons, its worshippers and its skeptics. But now, researchers have, with the help of brain imaging, been able to see just what caffeine does to the human brain. Their findings open the door to a much more informed consideration of what's good and what's not so good about caffeine.

Caffeine is the most commonly consumed psychoactive substance in the world. It is found in many foods and beverages and acts as a stimulant in the body, impacting alertness, attention and cognitive performance. In the United States, 80% of adults consume caffeine on a daily basis. The average intake is 200 mg a day, the equivalent of two 5-ounce cups of coffee or four caffeinated soft drinks.

Caffeine’s biological effects are mainly due to the way it mimics adenosine, a chemical found in every cell and which plays a role in transferring energy in our bodies and brains. Our adenosine levels rise every minute we are awake. It is believed that this biochemical is important to reducing arousal and promoting sleep.

Caffeine impersonates adenosine, taking its place at A1 adenosine receptors in the brain, blocking them and preventing the feeling of tiredness associated with adenosine buildup. It appears that's how caffeinated beverages keep us alert.

In this new study 15 men consumed no caffeine for 36 hours. Then each underwent a PET scan. Next, the men were given short intravenous infusions of caffeine in increasing amounts. With the help of a radioactive tracer chemical, researchers were able to see and compare the distribution of caffeine at adenosine receptors in the brain before and after the caffeine infusion. (The image below shows the change in adenosine distribution volume in one of the subjects after a 4.1 mg/kg dose of caffeine.)

Figure 1.

Image courtesy of The Journal of Nuclear Medicine

The results showed that In most coffee drinkers, about half of the adenosine receptors in the brain may be occupied by caffeine. According to the researchers, the daily blockade of a significant number of A1 adenosine receptors most likely has long-term effects on adenosine receptor expression and availability.

It is these changes that are likely behind the health benefits linked to caffeine.

“There is substantial evidence that caffeine is protective against neurodegenerative disease like Parkinson’s or Alzheimer’s disease." said David Elmenhorst, lead author of the study. “Several investigations show that moderate coffee consumption of 3 to 5 cups per day at mid-life is linked to a reduced risk of dementia in late life.”

But don't lose sight of the fact that consuming too much caffeine, such as can happen when highly-caffeinated energy drinks are consumed, can be dangerous, causing irregular heart rhythms, high blood pressure, dehydration, and other more serious conditions. It can also disrupt sleep and the body's clock, both implicated in a number of serious health problems. It's all about moderation.

The study was published in The Journal of Nuclear Medicine.