Following ingestion, caffeine is absorbed from the stomach and intestines into the bloodstream.
“The stimulating effects of caffeine are associated mainly with A1 and A2A adenosine receptors, mostly in the brain,” the report noted.
Caffeine’s effects can last for “several hours”, depending on how quickly or slowly it’s metabolised and broken down in the liver, and excreted via urine.
Those with a “high sensitivity to caffeine” have a slow metabolism in the liver and “high binding in the central nervous system”.
“Even small amounts of caffeine will cause a stimulating effect and higher doses may cause sleep problems,” warned the researchers.
People considered to have a “regular sensitivity to caffeine” are able to drink between two to five cups of coffee during the day without adverse reactions.
This is because there’s “balance between caffeine inactivation in the liver and binding in the central nervous system”.
Some people are considered to have a “low sensitivity to caffeine” because they quickly metabolise the substance in their liver.
This means this type of person can drink coffee before bedtime without their sleep being disturbed.
Even if you do fall into the “low sensitivity to caffeine” group, the European Food Safety Authority (EFSA) recommends no more than five cups of coffee daily.
Dr Longer said: “It’s common for people to ask their doctor questions such as why they are kept awake by one cup of coffee, while their partner easily falls asleep after five cups.
“The answer is that we are all unique coffee drinkers. Our genetic make-up programmes our reaction to caffeine, just as it programmes our hair colour and eye colour.”
On average, the report pointed that caffeine has an average half-life of four hours.
How does the liver break down coffee?
Liver enzymes are called cytochrome P450, which are responsible for breaking down caffeine in the liver.
One key cytochrome P450 enzyme is called CYP1A2, which can inactivate 95 percent of all ingested caffeine.
“The ability to produce this enzyme is coded for by the CYP1A2-gene,” explained the report.
“Different people have different versions of the CYP1A2-gene, and these genetic variations determine how active the CYP1A2-enzyme is in each person.”
Those with a “very active CYP1A2-gene” will metabolise caffeine more quickly.
This means caffeine will have a “shorter-lasting and mild effect throughout the body”.
Slow-metabolisers, on the other hand, will experience increased alertness for an extended amount of time.
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