> If allocated to caffeinated coffee consumption, patients were encouraged to drink at least 1 cup of caffeinated coffee (or at least 1 espresso shot) and other caffeine-containing products every day as per their usual lifestyle

I tried to skim to figure out how much caffeine/ml was actually in the drinks, but seems the researchers don't know themselves either? Wouldn't there be a huge difference depending on the beans, how it's made and so on? 1 espresso can be made very strong with packed coffee, or it can be made very weak, "1 cup of caffeinated coffee" basically says nothing at all, unless I'm missing the true definition elsewhere in the paper.

James Hoffman did some interesting videos on that -- espresso is a more efficient extraction method than Aeropress, but less complete than a very high water to coffee ratio pour over for the same weight of ground coffee. Extraction of caffeine is very directly tied to contact time and temperature, so two double shots of espresso (40g coffee) can actually be less caffeine extracted than a single large pour over (25g coffee) when the pour over is upwards of 22% extraction (typical of 20:1 water to coffee ratios and modern zero-bypass brewers). Similarly, there are large differences in beans, so 22% extraction on weak caffeine beans might be way less than 18% extraction on much higher caffeine beans. This is most obvious in the fact that Robusta has much higher caffeine on average than Arabica. Also, 22% extraction has a lot more unpleasant bitter compounds, not just more caffeine, so it's not my preference for taste in general.

I researched this a bit and found there really is no standardized "cup of coffee" for research purposes. Even for volume, I've seen it range from 6 to 12 fl oz. The main mechanisms of action are caffeine and flavonoids, and there's so much variation across beans and brewing methods that you'd think researchers would try to include that in their data to normalize it.

They were advised to drink at least one cup per day IIUC / maintain their current lifestyle. So there is no consistent amount, but it’s still meaningful vs being advised not to have any caffeine.

It's probably whatever "1 cup" means to the patient. The researchers would want them to stick to their status quo for the best results.

For example, 1 cup of coffee for my wife involves a blend of coffee and espresso beans with no adders, but 1 cup to her dad means lighter roast bean with milk. Both options have different caffeine contents and nutritional values.

There’s an average amount in the average cup of coffee.

A weak espresso sounds like you're just making a small amount of American/German coffee. How could that be passed of as an espresso? It's not just a measure of volume.

> "1 cup of caffeinated coffee" basically says nothing at all

Frankly, no. 1 cup of caffeinated coffee is on average significantly more caffeine than 0 cups, which is all you need for sufficient n. The only confounding variable to worry about here is whether caffeine is entering the diets of the two experimental groups at different rates via other channels (e.g., are the people who are forbidden to drink coffee drinking more tea to compensate?), though even this effect, if present, would likely tend to decrease the separation of the outcome distributions and is thus a low threat to validity.

People obsess over precision when it is often one of the least important aspects of an experiment. The outcomes of studies like this are in practice absolutely dependent on quality and rates of compliance. Choosing an uncomplicated treatment that is easy and natural for subjects to comply with is not merely acceptable but in fact a massively better design than complex protocols involving precisely measured quantities of lab-grown coffee in vials that can be misplaced, forgotten, deliberately shunned, etc. Also in this case the treatment corresponds closely to the real-world situation that the research is attempting to learn about.