Caffeine in coffee, an inconsistent “fix”

I have often wondered why, when buying coffee from the same establishment on different days, or the same size from different establishments- they do not have an equal effect on my subjective alertness.  It probably has a lot to do with internal milieu- hormones, neurotransmitters and the like, but I came across another reason: the amount of caffeine in coffee can vary quite a bit, even when bought at the same place on different days.  If fact it was pretty shocking to me, so I thought I would report it here for those who may not know.  There are also health concerns- if you are expecting a certain dose and it is actually well above it, it may cause anxiety in those prone to it, interact with medications, or negatively effect the cardiovascular system.

Prior to the study by McCusker et al. (1), it was established that preparation of coffee and the source of the coffee bean influences caffeine content dramatically.  The average value in a 150mL/5oz cup is approximately 85 mg.  But McCuster and colleagues tested specialty coffees to see how much they differed.

They tested by gas chromatography with nitrogen-phosphorous detection 20 caffeinated and 7 decaffeinated samples from coffee shops in Maryland, as well as samples of Starbucks Breakfast Blend bought over 6 consecutive days from the same shop in Florida.

Here are the results of the 20 caffeinated coffees:

As you can see, the expresso coffees varied a bit, but the brewed coffees were more dramatic- from 143 mg to 259 mg for an equal amount.  The average amount was 188 mg per 16 oz, or 59 mg per 5 oz, which is lower than the previously reported 85 mg average.

The authors note that the 7 decaf coffees had concentrations less than 17.7 mg.  They further explored decaf coffees in a subsequent study, finding up to 13.9 mg per 16 oz in specialty coffees and up to 15.8 mg/shot of decaf espresso from Starbucks, so if you are particularly sensitive to caffeine or are supposed to completely restrict it for medical reasons, this information may be relevant.

Here are the results of the Starbucks product purchased on 6 consecutive days from the same shop:

This was the most surprising to me- the same product from Starbucks varied from 259 mg to 564 mg on different days.

The authors note several variables that may influence caffeine content: variety of bean, roasting method, particle size, proportion of coffee to water, and length of brewing time. But even with standardized conditions in the Starbucks coffee, which is a “blend of Latin American coffees,” caffeine content varied by over 2-fold.

Another study by Hackett et al. (2) used a different analysis method: LC-diode array detection to test 32 coffee powder samples for caffeine content after using a microwave to extract the caffeine.

The results show again a dramatic difference in the amount of caffeine- from 5.2 to 12.2 mg/g of powder.

The simplest solution to get the most consistent amount to me would be to brew your own at home, which would give you more control over the variables that influence caffeine content.  If you are extra sensitive to caffeine, be careful when trying a new brand/product and start at a lower amount of coffee in case there is a significantly greater amount of caffeine in it.

Getting your “fix”

With such inconsistencies in dosing, I now have a reasonable explanation for why it sometimes feels like I don’t get much of a boost in alertness.  Though as a recent study suggests (3), alertness is only really returning to baseline in regular coffee drinkers.  The “fix” is only really eliminating withdrawal symptoms from acclimation to coffee’s effects.  The study, on 379 subjects, found that in habitual coffee drinkers, after doses totaling 250 mg of caffeine (not from coffee and standardized), had alertness levels only reached those of non-coffee drinkers who did not receive the caffeine.  However, they also found that people who consume little or no caffeine on a regular basis did not get a boost in alertness either.  This differs from other studies and could be because of self-report and current estimations of foodstuffs/beverages with caffeine, or compliance issues- indeed they found salivary caffeine concentrations to differ quite dramatically compared to other studies with similar consumption classifications.  They also explored some interesting genotype relationships with caffeine and anxiety, though consumption seems relatively unaffected by susceptibility to anxiety and habituation brings tolerance to these effects.  One of the associated SNPs, ADORA2A (adenosine A2A receptor gene) rs5751876, is one that I can lookup from my own 23andme data, I can see that I am (C;T), suggesting a reduced risk for anxiety to caffeine compared to (T;T).  There were several other SNPs on ADORA2A that were linked to anxiety as well.  The other major symptom besides low alertness with caffeine withdrawal is headache.

So if you’ll excuse me i’m off to refill my cup- the headache isn’t gone yet.


1. McCusker RR, Goldberger BA, & Cone EJ (2003). Caffeine content of specialty coffees. Journal of analytical toxicology, 27 (7), 520-2 PMID: 14607010

2. Hackett J, Telepchak MJ, & Coyer MJ (2008). Analysis of total caffeine and other xanthines in specialty coffees using mixed mode solid-phase extraction and liquid chromatography-diode-array detection after microwave digestion. Journal of analytical toxicology, 32 (8), 695-701 PMID: 19007523

3. Rogers PJ, Hohoff C, Heatherley SV, Mullings EL, Maxfield PJ, Evershed RP, Deckert J, & Nutt DJ (2010). Association of the anxiogenic and alerting effects of caffeine with ADORA2A and ADORA1 polymorphisms and habitual level of caffeine consumption. Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, 35 (9), 1973-83 PMID: 20520601