Coffee is among the most consumed beverages in the world, probably only next to tea. For crores of people around the world, their day begins with a cup of coffee. It is supposed to help people get over their morning blues and wake them up completely. It is also supposed to make you feel less tired at the end of a long work day. So, how does coffee work? Does it really wake us up or make us feel energized? What else does it do?
Before we dive into how consumption of coffee effects our brain and other parts of the body, a little background information will be useful. The basic unit of organization of our body is the Cell. Every cell is surrounded by a limiting perimeter made of fat molecules (technically, lipids) called the Cell Membrane. The cell membrane separates the inside of a cell from its outside. Because the contents of a cell are water soluble (our body is about 60-70% water, etc.), they cannot mix with the lipids and hence, cannot cross the cell membrane and get out of the cell.
Whenever a message has to be conveyed from outside the cell to the inside (like say, you need to die now because you have become cancerous), messenger molecules (Ligands) are released that bind to proteins embedded in the cell membrane called the Receptors. The receptors in turn activate a series of steps that bring about the desired effect (like cell death). This process is called Signal Transduction. An example of this process is given in the following representation of a signalling cascade initiated by the binding of Insulin (ligand) to it’s receptor in the cell membrane. This particular signal transduction sequence ultimately results in cell growth, synthesis of new proteins, etc. inside the cell.
The main ingredient of coffee, the one responsible for all its supposed useful effects, is a substance called Caffeine. Considering the amount of coffee consumed everyday, caffeine is probably the most widely used narcotic substance in the world. By some estimates, the average amount of caffeine consumed by a person in a day is about 200 mg and more than 1,20,000 tonnes of caffeine is consumed worldwide every year. Like any other drug, overdosing on caffeine could be dangerous and could lead to death at more than 10 grams at a time.
Like the Insulin receptor in the above image, another protein found in the cell membranes is the Adenosine Receptor. One of the places where adenosine receptor is most prominently found is the Brain cells. Here, one of the processes that the adenosine receptor participates in is the control of sleep. As the name suggests, adenosine receptor binds to a signalling molecule called the Adenosine. This molecule is accumulated in the brain during the hours of wakefulness and when it binds to the receptor, induces sleep. Caffeine found in the coffee too has the ability to bind to adenosine receptors. This prevents adenosine from binding to its receptor and sleepiness cannot be induced by the messenger even though it has accumulated.
According to a recent study from Cornell University, caffeine can mess with not only sleep but also the sense of taste. Results of these experiments showed that consumption of caffeine could significantly alter the perception of sweetness. However, there was no difference among the study participants in how they perceived salty, bitter or sour tastes. Two sets of participants were asked to drink coffee either with (caffeinated) or without (de-caffeinated or decaff) caffeine. Those in the caffeinated group felt that their drink was about 25-30% less sweet than those who had the de-caff. When the two groups were given a sugar solution after drinking the coffee, the de-caff group’s sweetness rating still was higher than that of the caffeinated group.
Along with brain, adenosine receptors are also found in the taste buds on the tongue. The researchers conclude that the caffeine in coffee binds to adenosine receptors in the taste buds, like in the brain, and suppresses the sensing of sweetness, either of the coffee itself or any other food consumed soon after. So, next time you dunk a biscuit in a cup of coffee, or tea, remember that you are not doing complete justice to its taste.
If you are interested in reading the original research article: Caffeine May Reduce Perceived Sweet Taste in Humans, Supporting Evidence That Adenosine Receptors Modulate Taste. Choo E, et. al, J Food Sci., 2017 Sep; 82(9): 2177-2182. doi: 10.1111/1750-3841.13836.
A million thanks to my friend GK who helped me access the above original research article from behind a paywall.
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