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Caffeine and the Human Body

Please note this information is not medical advice, and for advice on your specific needs you should always consult your medical practitioner.

Caffeine is considered the most extensively used psychoactive drug in the world [1], altering brain activity and mood. It is found naturally in coffee, tea, and cocoa. Caffeine is also available in some soft drinks and over-the-counter medications. Caffeine is mainly added to the diet through roasted coffee beans and tea leaves. 71% of dietary caffeine is consumed in the form of coffee [2]. Brewed coffee typically contains 56–100 mg of caffeine per 100 ml [1]. It is estimated that about 80% of the world's population consume a caffeinated product daily [2], while an average Australian consumes 210mg caffeine per day [3].

Caffeine in the human body

Once ingested, caffeine is completely absorbed into the blood from the gut. Caffeine concentration reaches its peak level in the blood within 1-1.5 hours after consumption [1]. Caffeine dissolves readily in water and lipids making it rapidly distributed throughout the body. Caffeine can pass through the blood-brain barrier, cross the placental membrane and pass into the amniotic fluid, and even enter into the fetus [1]. Caffeine is also found in breast milk and semen [1]. Caffeine is metabolised almost completely in the liver and has a relatively long half-life of 3-7 hours [4]. Half life determines how long caffeine is circulating in the body before being completely metabolised. This half life can vary depending on several factors such as age, pregnancy, gender, and smoking habits [4].

Caffeine—mode of action

Caffeine stimulates the central nervous system through various mechanisms. Caffeine inhibits the action of adenosine, a neurotransmitter that promotes sleep, by binding to adenosine receptors in the brain. In addition, caffeine also promotes the secretion of adrenalin, a hormone that primes the nervous system for “fight or flee” response, thus increasing the alertness. Caffeine also stimulates the release of dopamine, a neurotransmitter affecting the mood [5].

Caffeine—side effects

Although caffeine acts to promote alertness and wakefulness in moderate amounts, too much caffeine can potentially lead to health hazards. Studies have linked too much caffeine consumption to conditions such as high blood pressure [6], inconsistent heart rate [7], anxiety [8], panic disorders [9], insomnia [10], and bladder instability [11]. For women who has pre-existing bladder problems, consuming even a moderate amount of caffeine may lead to unstable bladder conditions [11].

Caffeine is associated with a risk of spontaneous abortions. Therefore, pregnant women are advised to limit their caffeine intake [12], [13]. Caffeine is also known to pass into breast milk. Thus, it can cause symptoms such as fussiness and irritability in breastfed infants when breastfeeding mothers consume too much caffeine [14].

Prolonged consumption of caffeine in high doses—more than 500-600mg per day (equivalent to four to seven cups of coffee)—can result in a condition called caffeine dependence [15]. Termination of caffeine intake in such people can result in caffeine withdrawal symptoms including headache, weakness, drowsiness, fatigue and decreased alertness [16]. These symptoms can occur within 12 to 24 hours of withdrawal and can last for about a week.

Caffeine, consumed in moderate amounts, can make you feel energised and elevate your mood, however too much caffeine will cause a host of undesirable side effects. For those who like to watch their caffeine consumption, decaffeinated beverages such as decaf coffee or decaf soft drinks can be a safer option.


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[2] M. A. Heckman, J. Weil, and E. G. de Mejia, “Caffeine (1, 3, 7-trimethylxanthine) in foods: A comprehensive review on consumption, functionality, safety, and regulatory matters,” J. Food Sci., vol. 75, no. 3, pp. 77–87, 2010, doi: 10.1111/j.1750-3841.2010.01561.x. https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1750-3841.2010.01561.x

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[7] P. J. Green and J. Suls, “The effects of caffeine on ambulatory blood pressure, heart rate, and mood in coffee drinkers,” J. Behav. Med., vol. 19, no. 2, pp. 111–128, 1996, doi: 10.1007/BF01857602. https://link.springer.com/article/10.1007%2FBF01857602

[8] M. Bruce, N. Scott, P. Shine, and M. Lader, “Anxiogenic Effects of Caffeine in Patients With Anxiety Disorders,” Arch. Gen. Psychiatry, vol. 49, no. 11, pp. 867–869, Nov. 1992, doi: 10.1001/archpsyc.1992.01820110031004. https://jamanetwork.com/journals/jamapsychiatry/article-abstract/495937

[9] D. S. Charney, G. R. Heninger, and P. I. Jatlow, “Increased Anxiogenic Effects of Caffeine in Panic Disorders,” Arch. Gen. Psychiatry, vol. 42, no. 3, pp. 233–243, Mar. 1985, doi: 10.1001/archpsyc.1985.01790260027003. https://jamanetwork.com/journals/jamapsychiatry/article-abstract/493529

[10] T. Roehrs and T. Roth, “Caffeine: Sleep and daytime sleepiness,” Sleep Medicine Reviews, vol. 12, no. 2. W.B. Saunders, pp. 153–162, Apr. 01, 2008, doi: 10.1016/j.smrv.2007.07.004. https://www.sciencedirect.com/science/article/abs/pii/S1087079207000937

[11] L. A. Arya, D. L. Myers, and N. D. Jackson, “Dietary caffeine intake and the risk for detrusor instability: A case-control study,” Obstet. Gynecol., vol. 96, no. 1, pp. 85–89, Jul. 2000, doi: 10.1016/S0029-7844(00)00808-5. https://www.sciencedirect.com/science/article/abs/pii/S0029784400008085

[12] L. Dlugosz, K. Belanger, K. Hellenbrand, T. R. Holford, B. Leaderer, and M. B. Bracken, “Maternal caffeine consumption and spontaneous abortion: A prospective cohort study,” Epidemiology, vol. 7, no. 3, pp. 250–255, 1996, doi: 10.1097/00001648-199605000-00006. https://www.jstor.org/stable/3702858?seq=1

[13] A. J. Gaskins, J. W. Rich-Edwards, P. L. Williams, T. L. Toth, S. A. Missmer, and J. E. Chavarro, “Pre-pregnancy caffeine and caffeinated beverage intake and risk of spontaneous abortion,” Eur. J. Nutr., vol. 57, no. 1, pp. 107–117, Feb. 2018, doi: 10.1007/s00394-016-1301-2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5332346/

[14] “Breastfeeding and maternal caffeine consumption | Australian Breastfeeding Association.” https://www.breastfeeding.asn.au/bfinfo/breastfeeding-and-maternal-caffeine-consumption  (accessed Apr. 04, 2021).

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[16] L. M. Juliano and R. R. Griffiths, “A critical review of caffeine withdrawal: Empirical validation of symptoms and signs, incidence, severity, and associated features,” Psychopharmacology, vol. 176, no. 1. Springer, pp. 1–29, Oct. 21, 2004, doi: 10.1007/s00213-004-2000-x. https://link.springer.com/article/10.1007/s00213-004-2000-x