Author: Neil Majithia
Institution: Virginia Commonwealth University
Date: November 2007
Whether it is a steaming mug of morning Joe or an afternoon pick-me-up soda, the world is addicted to caffeinated comforts. According to a study conducted by New Scientist magazine, 90% of North American adults consume some form of caffeine on a daily basis, making this legal, psychoactive substance the world's most widely used drug. Its widespread use, coupled with its lack of nutritional value, has attracted the condemnation of many dietary purists who brand caffeine as some sort of "demon compound." But to what extent is draining that café latte from Starbucks an exercise in sinfulness? While existing research offers conflicting opinions, the current consensus seems to be chanting "everything in moderation."
Caffeine is a chemical compound found in the beans, leaves, and fruits of over 60 known plants. In its natural environment, it acts as an organic pesticide, protecting vegetation from insects with parasitic tendencies. When consumed by humans, the stimulant operates on the central nervous system (CNS) to produce an energizing effect that, among other things, wards off lethargy and promotes mental acuity. Caffeine's "buzz," which is unregulated by governments worldwide, is responsible for the success of household commodities such as coffee, soda, energy drinks, and even certain pills.
Competitive Inhibition: Caffeine's Modus Operandi
When you feel sleepy it is because of molecules called adenosine floating around in your brain. Adenosine binds to adenosine-receptors on neurons, facilitating the slow-down of your brain's signaling functions, and inducing a sensation we term "drowsiness." Adenosine concentration is highest in the brain when you have expended a lot of adenosine triphosphate (ATP),your body's energy currency,so it makes sense that you feel tired after working all day or running a marathon.
Structure of Adenosine
Caffeine acts as an antagonist. It is similar to adenosine (fig 2) in structural arrangement to confuse nerve cells, which are unable to distinguish between the two molecules. This is why caffeine works so well as a stimulant on the central nervous system. Images courtesy of Biological Magnetic Resonance Data Bank and Ben Best
Now let's bring caffeine into the equation. The key to caffeine's stimulating capabilities lies in its chemical structure, which is markedly similar to that of adenosine. To a nerve cell, the two compounds are virtually indistinguishable, and thus, will allow either caffeine or adenosine to bind to the adenosine-receptors.
In scientific lingo, this process is called competitive inhibition because the caffeine is inhibiting the effects of adenosine by "competing" for its binding sites. It might help to think of this chemical phenomenon as a molecular game of musical chairs in which the adenosine and caffeine molecules are competing for "seats" on nerve cells. Probability suggests that the greater the caffeine to adenosine ratio, the more spaces caffeine will occupy, and the larger the impact of that afternoon espresso will be.
But what's the big deal if caffeine steals some seats from adenosine? The answer lies in the different ways that the two molecules act on the nervous system. When caffeine occupies the binding sites on nerve cells, it does not precipitate the slow-down that results when adenosine binds to the nerve,instead of slowing down, the cells speed up and fire their signals rapidly. The pituitary gland, a hormone-secreting gland that lies at the base of the brain, senses all the neuron firing going on, and assumes that the brain is in a state of emergency. In order to aid the body in this apparent molecular pandemonium, the pituitary gland stimulates the sympathetic nervous system (SNS) by releasing epinephrine (more familiarly known as adrenaline). As the "fight or flight" branch of the nervous system, the SNS is responsible for gearing the body up to deal with stressful situations by dilating the pupils, increasing heartbeat, and facilitating release of sugar by the liver to allow for ample energy. All of these effects work together to produce the signature buzz that has won caffeine so many fans.
The pituitary gland is an endocrine gland that is responsible for regulating homeostatic processes such as body growth and blood pressure. It also stimulates the body in response to the binding of caffeine on nerve cells. Image courtesy of Yahoo! Health
Conventional logic tells us that having your brain in a prolonged state of emergency can't be good for your health, and this perhaps explains some of the criticism caffeine has endured during recent years. However, as with most health-related issues, conventional logic isn't enough to draw definitive conclusions.
Impact of Caffeine on Health
When taken in moderation, the effects of caffeine are quite well known to most people. In less than one hour after consumption, an adequate dose might result in heightened alertness and newfound energy, along with an increased capacity for both mental and physical labor. As an individual continues to use caffeine, however, his or her body will gradually become more tolerant to the substance, and the aforementioned effects will be less pronounced. The phenomenon of tolerance can be explained biologically by the growth of additional adenosine receptors in the brain to adapt to the prolonged presence of caffeine. This makes the body more sensitive to adenosine, reducing the impact of caffeine in the process.
More significant, still, are the withdrawal effects experienced by tolerant users due to the increased number of adenosine receptors. If there is no caffeine in the system, more adenosine is able to bind to the large number of receptors, creating an exaggerated impact on the body.
For example, adenosine naturally widens blood vessels and increases blood flow in a process called vasodilation. With more adenosine setting off signaling cascades, there is an extreme widening of the vessels, particularly of those leading to the brain where excess blood accumulates and causes headache or nausea.
Because caffeine is known to stimulate production of serotonin, a neurotransmitter that regulates mood, anger, and aggression, the fall in serotonin following caffeine withdrawal can trigger unpleasant behavior such as irritability, anxiety, and lack of concentration. The manifestation of these symptoms can force the user to consume more caffeine just to alleviate the distress, temporarily providing a fix, but clearly only avoiding the root of the problem. In the more severe cases, this vicious cycle can lead the user on the road to clinical depression.
The vast majority of caffeine consumers never experience these kinds of problems because their intake of the compound isn't significant enough to cause complication. As we discussed before, caffeine consumption in moderation is regarded as a safe, and possibly even a healthy habit. While the data is still a bit scanty and inconclusive, existing research suggests that caffeine can help improve memory, ward off Parkinson's disease, as well as reduce the risk of heart disease. Caffeine's role in producing these beneficial effects is still poorly understood and is undergoing more extensive investigation.
The moral to learn from the caffeine story is that most things you put into your body can neither be all good or all bad. The body is formed in a very intricate manner, with many elaborate connections running between the various organ systems. This complexity of nature explains why virtually everything we consume has primary, secondary, and even tertiary effects. It is no different with caffeine; as a psychoactive substance that keeps the world running, it comes with good, bad, and potentially even ugly attributes, depending on how you use it.
References:
1) Bolton, Ph.D., Sanford; Gary Null, M.S. "Caffeine: Psychological Effects, Use and Abuse". Orthomolecular Psychiatry. 1981. 10 (3): 202211.
2) Fisone, G; Borgkvist, A; Usiello, A. "Caffeine as a psychomotor stimulant: mechanism of action". Cell and Molecular Life Sciences. April 2004. 61 (78): 857-72.
3) Lovett, Richard. "Coffee: The demon drink?". New Scientist. 24 September 2005.
4) Nehlig, A; Daval, JL; Debry, G. "Caffeine and the central nervous system: Mechanisms of action, biochemical, metabolic, and psychostimulant effects". Brain Res Rev. May-August 1992. 17 (2): 139-70.
Written by Neil Majithia
Reviewed by Antje Heidemann
Published by Pooja Ghatalia.