Have you ever thought about what is actually going on in your body when you throw back that energy drink or sip that coffee?
How is caffeine broken down and how does it affect your metabolism? How long does caffeine stay active in your system and why do people crave the stuff?
We’ll try to answer those questions and more in the simplest terms possible so that you can be an educated consumer when it comes to caffeine metabolism.
Caffeine From the First Sip
Caffeine easily passes through body membranes so from the first sip the caffeine is entering your bloodstream through the lining of your mouth, throat, and stomach.
It only takes 45 minutes for 99% of the caffeine to be absorbed through these membranes.
In humans, the half-life for caffeine is anywhere from 4 to 6 hours on average, which explains why the average energy drink or coffee’s effect lasts about 4 to 6 hours.
Things like age, medical conditions, and drug interaction can have an effect on the half-life.
Note: Humans also can have 3 levels of sensitivity to caffeine. This also determines how well you metabolize caffeine and to what degree its stimulating properties affect you. Read about those here.
Caffeine in the Blood Stream
While most research on caffeine has been conducted using animals, the data has been converted to show the most likely effect on the human body. As soon as the caffeine enters the body it is already being metabolized by the liver and broken down into theophylline, theobromine, and paraxanthine.
From there these chemicals travel throughout the body where they affect various body functions.
It’s in Your Genes
The speed at which caffeine is metabolized depends on specific genes. Research continues to discover gene variations that appear to be responsible for how long caffeine stays in the bloodstream.
People with a specific variation of the gene PDSS2 process caffeine more slowly than others. They, therefore, need less coffee for the same stimulant effects.
Caffeine in the Brain
The most studied of these is the way caffeine is similar to the molecule adenosine in the brain. The caffeine molecules bind to the adenosine receptors in brain cells and block adenosine from binding.
Adenosine plays a role in the sleep-wake cycle. When adenosine binds to enough receptors, it signals the brain that it is time for rest or sleep. Caffeine doesn’t replace the person’s need for sleep, but just covers up drowsiness symptoms that adenosine can no longer produce.
This also interacts with the dopamine system in the brain, which is the feel-good neurotransmitter. When adenosine is blocked by caffeine, the dopamine system works more efficiently.
Furthermore, elevated levels of adenosine in the blood cause the adrenal glands to release adrenaline. This stimulating hormone further adds to the feelings of alertness and energy.
Here’s a great TED-Ed video that explains what happens in the brain.
Caffeine’s Effects on the Body
Theophylline relaxes smooth muscles, which has been beneficial to those with asthma and is the reason why after drinking caffeine a person often feels the need to use the bathroom as it is affecting the smooth muscles of the colon. Theobromine increases the amount of oxygen and nutrients that can be used by the brain and muscles.
Caffeine causes an initial contracting of artery walls (vasoconstrictor) but then relaxes this smooth muscle which has an overall vasodilator effect on the blood vessels. In other words, it opens blood vessels to allow for greater blood flow. src.
There have been many other researched effects of caffeine covered by Caffeine Informer, but the above represents the most researched and common.
Caffeine’s Exit from the Body
The caffeine metabolites are then filtered by the kidneys and they exit the body with the urine.
Caffeine has been shown to have a diuretic effect on the body, which basically causes the body to release more water in the urine.
However, new research suggests that this is only in people who have not built up a tolerance to the caffeine molecule.
When caffeine has exited the body or has been used by the various cells of the body the person can experience a “crash” that is caused by elevated levels of adenosine flooding the brain and dopamine now being repressed. This causes feelings of tiredness and fatigue. Prolonged use of caffeine also causes withdrawal symptoms.
- Biology Online
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- Grant, D. M., Tang, B. K., & Kalow, W. (1983). Variability in caffeine metabolism. Clinical Pharmacology & Therapeutics, 33(5), 591-602. link
- Echeverri, D., Montes, F. R., Cabrera, M., Galán, A., & Prieto, A. (2010). Caffeine's vascular mechanisms of action. International journal of vascular medicine, 2010. study link