Mind and body: how caffeine affects you

As soon as you take that first sip, caffeine begins to work it's magic but how does it make you feel awake?
05 February 2016

Interview with 

Dr Thomas Krieg, Cambridge University


From bean to cup and then cup to consumption: what happens when we have Girl drinking teacaffeine? Dr Thomas Krieg put Graihagh's body to the test when three espressos and explained the effects to Chris Smith...

Thomas - The main thing, the main effects we are talking about here are the effects of the brain.  So you ingest the coffee and the caffeine goes into the bloodstream and it's soluble in oil and water - that means it can easily cross the blood brain barrier.  And in the brain it has effects, as everywhere else in the body, of blocking the adenosine receptors.  There are many variations of adenosine receptors and caffeine pretty much blocks all of them. But what happens in the brain is that the effect of these adenosine receptors is, if they are active, they make you drowsy, and by blocking these receptors you're obviously more alert and that is the main effect.

Chris - If every organ in the body has got these or every cell in the body has these adenosine receptors, are the effects only confined to the brain then - one would think not?

Thomas - No, there are many effects all over the body.  Another prominent effect is, obviously, on the heart where the heart rate gets increased and the force of the heart and, therefore, the blood pressure increases as well.

Chris - So caffeine has a generally whole body enlivening effect?  You're going to stimulate the brain's wakefulness system because you're inhibiting what would normally make you feel sleepy and, at the same time, all the body cells are having these adenosine receptors blocked leading to an increase in their metabolic effort and how active those cells are.

Thomas - Yes, that's correct.

Chris - What about the fact that people say well, you know, we should try and keep our heart rate low and we should try and keep our blood pressure low. If it's going to increase the action of your heart and make your blood vessels constrict, isn't that bad?

Thomas - Yes, that's very tricky to investigate first of all because we have to distinguish between a chronic effect and an acute effect. So, a lot of things which are obviously good for you, like sport, increases your heart and your blood pressure acutely but, in the long term, it has a beneficial effect.  What is helpful in this case is to look at diseases that are directly linked to situations like high blood pressure, for example strokes, and there are big studies out there now where people were followed on over many, many years and it turned out that they do not have an increased rate in stroke.  In fact, in some studies they even have less risk of stroke.

Chris - So you're saying I can drink my way to fitness?

Thomas - Not quite but almost, yes.

Chris - One of the things that people often say about coffee and therefore caffein, is it makes them very jittery.  And I know friends who say I have one cup of coffee and I can't do anything the rest of the day and, in fact, there was a sign on the wall in a lab I worked in Australia and it said "do stupid things more quickly with caffeine."  I'm not one of those people but I know many who are.  Why is that?

Thomas - The biggest things you have to look at here is that all receptors in the body can be downregulated when they are chronically activated and the same is true with adenosine receptors.  So, if you take coffee in on a regular basis, the effects go away - so that is one thing.  The second thing is that caffeine will be broken down in the liver by certain enzymes - they are called cytochromes.  And the levels of cytochromes are highly different between individuals, so one might have very high levels of cytochromes, another one might have almost none and that means that caffeine will be broken down at different speeds between different individuals.

Chris - With that in mind then, how long will it be between having an intake of caffeine and it finally being washed out from the body, on average - how long does it last

Thomas - The half life of caffeine in itself is about 5-7 hours but the effect, you would probably not see it after like 3 or 4 hours.

Chris - We might have a long night ahead of you Graihagh.  Shall we find out what your heart rate and blood pressure is doing....

Graihagh - Why not because we measured it...

Chris - Because you've now had quite a big dose in the kitchen there with Peter.

Graihagh - Yeah I have, I've had a couple of espresso's worth, and we measured my blood pressure and heart rate before the show to get a good idea if it would change at all.

Chris - That will be accurate won't it - just before an invigorating radio performance.  What was it before the show?

Thomas - Yes.  Graihagh was kind of relaxed with 133 over 88 and a heart rate of 75.  Now we will measure again and see if there's an increase.  Obviously, this is not a scientific study we're doing here.

Graihagh - Yes, there's only one of me and, I imagine, as you said, lots of people have lots of different reactions to coffee so you know...

Chris - Here we go - we're pumping up the cuff.  So what would you anticipate then Thomas?  What would you expect the effect of coffee to have been on a young fit female?

Thomas - I would not expect much of an effect; maybe a slight increase but then again, it's very hard to tell with this setting here of a radio show whether this is really due to the coffee and...

Chris - Or just to the adrenalizing effect of the stimulating content on this programme - is that what you're trying to say?

Thomas - Yes, that's what I'm trying...

Graihagh - Precisely - I think that's exactly what he was trying to say.  And also there's the running back and forth from the kitchen as well.  Lots of moving around.

Chris - Excercise is good for you see.

Thomas - And it increases the blood pressure as well.  Okay now - your number now is 141 over 88 with a pulse of 82.  So slightly higher than before but not much.

Chris - I'm also slightly nervous now about occupational health grounds because what this proves is that doing an invigorating radio programme gives staff hypertension and tachycardia  So, could that be grounds for suing?

Thomas - I doubt it, I doubt it.


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