The dose makes the poison: Coffee consumption, physiology and health impact


As my alarm goes off at 4 am, I dread the day ahead. However, after countless sleepless nights since moving into a new city, I need to wake up early for my M3 orientation. Fortunately, I have caffeine at my disposal. So, reaching for my cup, I became inspired for my next wellness article, “Oh Sweet, Sweet nectar of the Gods.” For this article, I will start by sharing some statistics about coffee, followed by coffee processing, physiology, and, lastly, effects on the body.

In 2020, data from the national coffee association revealed that coffee consumption is up by 5% in the USA since 2015. The average American coffee drinker drinks over 3 cups of coffee a day. With coffee being a stimulant, it is no surprise that it is a favorite among physicians and medical students. Interestingly, one study reported that in-patient physicians were more likely to drink coffee and energy drinks than out-patient-based physicians. During orientation, the surgery director gave a piece of wisdom to the class of 2023. “Drink espresso. Less volume, more caffeine, less need to use the washroom after scrubbing in.”

Coffee Processing and Physiology

Given the high consumption rates of coffee, I want to start by appreciating the tremendous processing it must go through before we consume it. Coffee is derived from the Coffea (genus) shrub; the two most common species being canephora and arabica. First, coffee berries are handpicked, where the flesh is removed, and the seeds are left to ferment and dry. At this stage, the coffee is known as green coffee. Starbucks and other chains have started serving cold brews of coffee at this stage.

Interestingly, green coffee has the highest caffeine content. Second, comes the roasting stage, which impacts the amount of caffeine content and taste of coffee. The longer that coffee is roasted, the more moisture is lost and the less dense it becomes. As coffee is roasted, starches are broken down to simple sugars, high heat causes the breakdown of caffeine, and oils begin to develop. The oils contribute to coffee’s famous aroma. Finally, these beans are ground and brewed as they make their way into our cups. (Note: this is a very brief description, which does not cover decaf coffee)

Coffee is a stimulant, which has unique effects on the human body. Much of this content regarding coffee physiology shall be derived from a review by McLellan et al. 2016 and a sports podcast for those ortho heads: First, coffee is considered a xanthine derivative with three methyl groups attached (scientific name-1,3,7-trimethylxanthine). This structure is similar to adenosine, explaining coffee’s action as an adenosine antagonist, meaning to inhibit the actions of adenosine at an adenosine receptor (figure 1). There are four adenosine receptors. By inhibiting different subtypes of adenosine receptors, caffeine can cause different effects. For example, adenosine receptors in the brain block the release of serotonin, dopamine, glutamate, and other neurotransmitters (less in the synapse). Caffeine blocks adenosine’s actions, thus increasing the amount of neurotransmitters in the synapse, explaining caffeine’s effects. For example, increased dopamine leads to an increased perception of reward. By altering glutamate levels, caffeine can even alter the seizure threshold. However, it is less straightforward than I am making it to sound since neurotransmitters can cause different effects in different brain regions.

Figure 1: The structure of caffeine vs. adenosine

Peripherally, coffee mainly acts as a sympathetic stimulant, see Figure 2. One mechanism is by stimulating your adrenal glands to secrete catecholamines which act on various organs in the body. Finally, the effects of caffeine vary, depending on individual caffeine metabolism. For example, metabolism differs between naïve or experienced caffeine consumers. Finally, the dose/timing of caffeine intake impacts metabolism. Literature suggests that absorption takes approximately 45 minutes, peak serum caffeine occurs after 15 minutes to 2 hours following ingestion, and finally, half-life ranges from 2.5-4.5 hours.

Figure 2: Impact of Coffee on the body (Van Dam et al., 2020)

Specific Effects

Coffee, while used as a stimulant, impacts our health more than we realize. A recent umbrella review by Poole et al. (2017) looked at the risks and benefits of coffee consumption based on the findings of over 200 meta-analyses. Coffee consumption was analyzed in the following conditions: high vs. Low consumption, any vs. none, and having an extra cup of coffee per day. Overall, coffee consumptions appeared to reduce the risk of all-cause mortality, cardiovascular mortality, and cardiovascular disease. Coffee consumption was also suggested to correlate with a reduced risk of cancer. These findings have been echoed in other studies. For example, a study in 2016 by Liebeskind et al. described the “coffee paradox.” In this study, high rates of coffee consumption were found to have a reduced risk of stroke, even in those who smoked. 

Finally, a recent review published in the New England Journal of Medicine summarizes some of the consistent findings of coffee consumption and its effects on the human body (Van Dam et al., 2020). In the CNS, caffeine:

  1. Reduces fatigue, increases alertness, and improves vigilance (Note: caffeine does not compensate for chronic sleep deprivation!).
  2. Improves pain tolerance.
  3. Increases anxiety when >200 mg is consumed in one sitting or >400 mg is consumed in a day.

Caffeine withdrawal presents with headache, fatigue, and depressed mood 1-2 days after cessation of coffee consumption. Withdrawal effects last between 2-9 days. In addition, coffee toxicity (1.2g or higher) can lead to altered thought and speech, anxiety, insomnia, dysphoria, and cardiovascular toxicity—more on cardiovascular toxicity in peripheral effects. I briefly mention it here, as it is part of the toxicity presentation. 

Peripherally, coffee intake increases epinephrine release by stimulating the adrenal glands and subsequently increases blood pressure transiently, as tolerance develops over time. Coffee intake (non-toxic levels) can reduce the risk of cardiovascular disease (see coffee paradox above). Coffee may potentially improve metabolism and reduce appetite, thus causing minimal effects on weight loss. Coffee may also decrease insulin sensitivity with short-term use (long-term use counteracts these effects). Furthermore, breakdown products of coffee may act as an antioxidant and protect against reactive oxidative species (ROS). Finally, coffee has been reported to reduce the risk of mortality from any cause. 


Coffee is a staple among many households, including our patients. Though used as a stimulant, coffee can have many physiological effects, many being beneficial. However, there can be too much of a good thing. Too much coffee can increase the risk of agitation, anxiety, insomnia, and arrhythmias. Coffee is a tool, but it is our job to use it wisely and educate patients that may be at risk of too much caffeine consumption.

References and Further Reading

  1. de Melo Pereira, G. V., de Carvalho Neto, D. P., Júnior, A. I. M., do Prado, F. G., Pagnoncelli, M. G. B., Karp, S. G., & Soccol, C. R. (2020). Chemical composition and health properties of coffee and coffee by-products. In Advances in food and nutrition research (Vol. 91, pp. 65-96). Academic Press.
  2. International Coffee Organization. The Current State of the Global Coffee Trade. Coffee Trade Stats. (2016). Retrieved from:
  3. Kummer, C. (2003). The joy of coffee: the essential guide to buying, brewing, and enjoying. Houghton Mifflin Harcourt.
  4. Liebeskind, D. S., Sanossian, N., Fu, K. A., Wang, H. J., & Arab, L. (2016). The coffee paradox in stroke: Increased consumption linked with fewer strokes. Nutritional neuroscience, 19(9), 406-413.
  5. McLellan, T. M., Caldwell, J. A., & Lieberman, H. R. (2016). A review of caffeine’s effects on cognitive, physical, and occupational performance. Neuroscience & Biobehavioral Reviews, 71, 294-312.
Cite this article as: Brenda Varriano, Canada, "The dose makes the poison: Coffee consumption, physiology and health impact," in International Emergency Medicine Education Project, September 6, 2021,, date accessed: December 11, 2023

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