After ingestion, caffeine is absorbed from the stomach and intestines into the bloodstream.
“The stimulant effects of caffeine are primarily associated with A1 and A2A adenosine receptors, primarily in the brain,” the report notes.
The effects of caffeine can last “for several hours,” depending on how quickly it is metabolized and broken down in the liver, and excreted in the urine.
Those with “high sensitivity to caffeine” have a slow metabolism in the liver and “high binding in the central nervous system”.
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“Even small amounts of caffeine will have a stimulating effect and higher doses can lead to sleep problems,” the researchers warned.
People considered to have “regular caffeine sensitivity” can drink two to five cups of coffee during the day without adverse effects.
This is because there is “a balance between inactivating caffeine in the liver and binding in the central nervous system.”
Some people are considered to have “low sensitivity to caffeine” because they metabolize the substance quickly in their liver.
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This means that this type of person can drink coffee before bed without disturbing their sleep.
Even if you belong to the ‘low sensitivity to caffeine’ group, the European Food Safety Authority (EFSA) recommends that you do not exceed five cups of coffee per day.
Dr Longer said: “It is common for people to ask their doctor questions, such as why they are kept awake by a cup of coffee, while their partner easily falls asleep after five cups.
“The answer is, we are all unique coffee drinkers. Our genetic makeup programs our reaction to caffeine, just as it programs our hair color and the color of our eyes.”
On average, the report pointed out that caffeine has an average half-life of four hours.
How does the liver break down coffee?
Liver enzymes are called cytochrome P450, which is responsible for breaking down caffeine in the liver.
A key cytochrome P450 enzyme is called CYP1A2, which can inactivate 95 percent of all caffeine ingested.
“The ability to produce this enzyme is encoded by the CYP1A2 gene,” the report explains.
“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 metabolize caffeine more quickly.
This means that the caffeine will have a “mild, short-lasting effect throughout the body.”
Slow metabolizers, on the other hand, will experience heightened alertness for an extended period.