In response to a couple of question about the implications of long-term caffeine intake, I’d thought I’d throw out a couple of findings.
I recently wrote about a study that localized the receptors underlying the arousing effects of caffeine. (A2a receptors, in cells located in the shell of the nucleus accumbens). It’s only natural then to wonder what effect chronic caffeine intake might have on these receptors (and elsehwere in the brain).
That study didn’t look at chronic effects. But back in 1996, Glass et al. found that chronic caffeine consumption increased the global expression of adenosine receptors in the brain , suggesting that this increase was to compensate for caffeine’s antagonistic effects. Withdrawal from caffeine is, at least in part, likley related to a hypersensitivity to adenosine due to this increased number of adenosine receptors. The headaches that accompany caffeine withdrawal are thought to be related to the fact that adenosine is a known vasodilator and the increased receptor density + withdrawal of caffeine from the system leads to a significant drop in blood pressure.
A couple other interesting notes in regards to long-term effects of caffeine:
The Good News
Some case control studies have shown lower incidents of Parkinson’s disease in coffee drinkers vs. non-coffee drinkers, although this finding has not always been replicated. The correlation, when it has been found, was strongest in heavy consumers. (This is certainly a finding I would love to be true!) More evidence in support of these findings come from mouse studies showing that physiological doses of caffeine were able to reduce one of the major toxic factors associated with Parkinsons (MPTP-induced dopaminergic toxicity). It’s been suggested that caffeine may offer neuroprotective effects in the brain via action occurring at A2a receptors, which are the same receptors responsible for the arousing effect of caffeine (and which are also co-localized with dopamine D2 receptors). Additional support for this idea comes from studies in which mice who had their A2a receptors knocked out showed reduced MPTP-induced injury compared to wild types. How this all might be happening on a mechanistic level, however, is not well known.
The less good (but not totally bad) news:
Unfortunately, it seems that acute doses of caffeine often cause a rise in systolic and diastolic blood pressure, increase in catecholamine release and vasodilation (wideneing of blood vessels). However, some studies have shown this effect occurs primarily in non-regular consumers of caffeine. Many studies have shown either slight increases or no difference in blood pressure for regular users of caffeine. In fact, several large scale studies have found that heavy, regular use is protective against heart disease. (yes!) The findings are quite contradictory.
So what to make of all this? Is heavy coffee drinking bad or good for you?
There’s no simple answer to that question. But the paradoxical findings suggest that different individuals have varying levels of risk. And it’s likely that genetics play a significant role.
If one thinks of coffee as a drug, then the notion that the benefits of heavy coffee consumption might outweigh the risks seems very counterintuitive. That is, due to the brain’s propensity to maintain homeostasis, drug taking, either legal or illegal, usually involves some significant cost benefit analysis, a trade off between the good (the high, buzz, relief from psychic or physical pain) and the bad (side effects, withdrawal, expense, long-term effects on health, etc…) Yet, the evidence on long-term caffeine intake seems to put it in a disctinctive class of its own.
I was in Italy once and chatted with an extremely energetic and sprightly 93-year old man.
I asked him what was the secret to his longevity and good health. He said, “five espressos a day.” Anecdotes aren’t very informative in an empirical sense, of course, but, nonetheless, the old codger may have been on to something.*
*However, in addition to the five espressos, he’d also smoked a pack a day of American Winstons and was convinced that one of the other secrets to his good health was that he never switched brands.
Have you ever wondered why, and exactly where in the brain, coffee (or any caffeinated product, for that matter) is able to exert its arousing effects? Well, wonder no longer, because an international team of researchers from Japan, China and the US, have located the primary neurons upon which caffeine works its magic (Lazarus 2011).
It was previously known that caffeine wakes you up through inhibiting activity at adenosine A2a receptors (adenosine is an inhibitory neuromodulator involved in regulating the sleep-wake cycle). However, it was not known exactly where in the brain the receptors that exerted this effect are located.
How did they do it?
The researchers utilized a method whereby the gene that codes for A2a receptors (A2aRs) is marked such that they can be deleted, but only in a specific regions of the brain. Using a rat model, the team utilized these gene deletion strategies and found that when they knocked out A2aRs in the shell of the nucleus accumbens, rats no longer experienced the arousing effects of caffeine.
How does this work?
Adenosine activates A2a receptors in the nucleus accumbens shell, activation of which receptors inhibit the arousal system. That is, the more adenosine activation there is, the sleepier an organism becomes. Caffeine, which binds to these same receptors and blocks adenosine from exerting its activity there, essentially disinhibits the arousal system, promoting wakefulness. (Amazingly, based on similarities between the brains of mice and men, the area of the human brain in which caffeine acts to counteract fatigue is approximately the size of a pea.)
What does this mean in practical terms? (or, in other words, why should we find this so cool?)
Well, for one, it gives us a more specific mechanistic explanation for the arousing effects of caffeine. It says that in order for caffeine to work, it not only has to be effective as an A2aR antagonist, but that excitatory A2aRs on nucleus accumbens shell neurons must be tonically activated by endogenous adenosine. This is especially important in consideration of individual differences in the subjective effects of caffeine.
What if A2aRs are more densely packed in the shell of your nucleus accumbens than in mine? Might you be more sensitive to the effects of caffeine than me? That certainly seems likely. And the reason that one person might over or underexpress these receptors vs. another seems to be related to variation in the gene that produces those receptors (the gene knocked out in the rat study described above). In fact, we’ve already have evidence that this is the case. Past studies have shown genetic variations in genes coding for A2aRs were associated with greater sensitivity to caffeine and sleep impairment (Retey 2007), and greater anxiety after caffeine (Childs 2008). This study refines the existing model and should inspire, and lead to more accurate interpretation of, future genetics studies.*
*Other significant genes that underly individual differences in the subjective effects of caffeine include CYP1A2, or cytochrome enzyme P-450 1A2, which is associated with caffeine metabolism, and those coding for dopamine D2 receptors.
Lazarus M, Shen HY, Cherasse Y, Qu WM, Huang ZL, Bass CE, Winsky-Sommerer R, Semba K, Fredholm BB, Boison D, Hayaishi O, Urade Y, & Chen JF (2011). Arousal Effect of Caffeine Depends on Adenosine A2A Receptors in the Shell of the Nucleus Accumbens. The Journal of neuroscience : the official journal of the Society for Neuroscience, 31 (27), 10067-10075 PMID: 21734299
Childs E, Hohoff C, Deckert J, Xu K, Badner J, de Wit H (2008) Association between ADORA2A and DRD2 polymorphisms and caffeine-induced anxiety. Neuropsychopharmacology. 33:2791– 2800
Retey JV, Adam M, Khatami R, Luhmann UF, Jung HH, Berger W, Landolt HP (2007) A genetic variation in the adenosine A2A receptor gene (ADORA2A) contributes to individual sensitivity
to caffeine effects on sleep. Clin Pharmacol Ther. 81:692–698
My wife, who has been blogging for about a year, told me that this was a phase that a lot of newbie bloggers go through. That is the somewhat pathological obsession that I was quickly developing for checking my blog stats. I’d been blogging for a few weeks, promoting through the usual channels, when I started getting a wee bit of traffic. It was quite rewarding to know that people out there were somehow making it to the site, even if many weren’t actually reading. Never having experienced the sensation of distributing my writing publicly, let alone to a potentially unlimited and worldwide audience, I’d developed quite an addiction to checking my numbers.
One morning, shortly after putting up a post, my stats when through the roof. I didn’t think the post was anything special but it was generating tons of traffic. A quick check of the stats revealed why. Mark Morford, a columnist for the SF Gate (the online home of the San Fransisco Chronicle) had written about my summary of the study in his weekly online column and linked to my site. Over the course of the next several hours, this link brought in about 1500 visitors (approximately 1450 more than I was getting per day at the time).
I was pretty happy for the readership; that is, until I went to Morford’s column and read his summary of my summary. So, what had I written about? I’d dashed off a summary of a Danish meta study that was attempting to establish mortality rates for drugs such as heroin, cocaine, amphetamine, marijuana and ecstasy. Here’s what I wrote about the ecstasy findings (If you would like to read the full article, go here.):
“6. Ecstasy (MDMA) users did not show increased mortality rates. (However, it’s possible that a low number of deaths from MDMA contribute to low statistical power).”
And later, in the closing paragraph:
“Conclusions that can be drawn from this report? … Ecstasy is unlikely to kill you on its own, but that’s not to say it won’t do some long-term damage if abused…”
I think it was a reasonably accurate, if extremely simplified, version of the findings.
Here’s how Morford wrote it up:
“In loosely related news — assuming you like to view the world that way and really, why wouldn’t you — the other universally acclaimed wonderdrug known as ecstasy (MDMA) has been proven once again to have no real side effects, doesn’t make you want to kill yourself and doesn’t increase mortality rates overall, especially if used in relative moderation and not like some panicky teen raver or Burning Man first-timer who has no clue what he’s doing and shouldn’t be left alone in Drunken Barbie Camp with all those glow sticks, fake fur and baggies of little magic pills.
Sadly, a new Danish study shows that pot users suffer a mortality rate about five times higher than the norm (your mileage, and possible explanations, may vary). Cocaine and meth, six times. Heroin and related injectables are, as you might expect, off the charts. But ecstasy, well, it just keeps being proven to be not so bad in the slightest, and actually might, just might be one of the most remarkably safe, effective, enlightening drugs ever invented. Good thing it’s still illegal.”
HUH? Christ, I’d even provided a link to page outlining the negative consequences of taking the drug! What an interesting interpretation. I suppose I shouldn’t have been surprised that things were taken out of context and the message twisted. That’s par for the course on the internet. I suppose what really got me, though, was the possibility that some percentage of Morford’s readership was provided, inadvertently by me, with scientific justification to go out that night and do ecstasy or at least encouraged to believe it’s not a harmful drug when, in fact, it is quite harmful both in the short and long term.
Worse yet, is that when I initially went to the SF Gate to read Morford’s piece, for some reason, I couldn’t locate it. I read through a couple of his old columns (some of which cited scientific findings accurately and fairly, albeit in a very casual style) and dropped him an email thanking him for the link and praising his writing, without actually reading his summary of my post. Admittedly, I was a little drunk on the heavy influx of blog traffic and assumed it was probably just a simple sentence or two. It wasn’t until later that I found it and realized that my science journalism cherry had been popped and then some.
Obviously, this is but a tiny drop in the ocean of (mis)information transmitted daily over the interwebs. Yet, its a reminder to be extra careful of how one presents scientific findings and to keep an eye our for how others might be (ab)using these writings to support their own agendas.
I’d be curious to hear others’ stories of f’ed up reinterpretations of their writings…
Google, are you reading my mind?
One interesting aspect of having a blog is checking out the search terms that people used to land at one’s site. It’s often difficult to figure out why a particular and seemingly unrelated term might bring someone this way.
But one recent search seems to have transcended the blog and gone straight into my brain-o-sphere, into the existential recess where some of my darker thoughts about grad school are stored:
Google, you know me so well. Now stop it, you’re freaking me out.
Research has shown that people’s names influence what professions they choose to enter; for example, men named Dennis are overrepresented among dentists and men named Raymond are overrepresented among doctors who specialize in radiology.
I wonder if guilt about his name is what drove the third author below to study the ill effects of cigarette smoking …
I’m working on a slew of new posts but wanted to just throw this out there. Reading through a paper this morning, I was struck by the fact that for the 4th time this week, I’ve come across a study that reports on or mentions some non-signifiant statistical trend in the data (the latest I’ve come across touts a p value of .07). Why are these trends reported at all? They’re very misleading and most certainly only reported when they suggest support for a given hypothesis (I haven’t noticed too many papers reporting trends that would go against the central hypothesis). Why set a threshold at all if you’re going to report stats that exceed it? Am I off base here?