Q. When studies refer to “cups” of coffee or tea, do they mean eight-ounce cups or the more typical amount found in a standard mug of coffee or tea?

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A. Jeffrey Blumberg, PhD, senior scientist in Tufts’ HNRCA Antioxidants Research Laboratory, who has studied the health effects of coffee and tea, answers: “The ‘conventional’ size for a cup of coffee in the US is six ounces and for tea it can be five or six ounces (i.e., not the standard ‘kitchen’ cup of eight ounces). In practice, these values are partly determined by manufacturers of coffeemakers/percolators and tea kettles, which define how many ‘cups’ their machines make – and they vary from brand to brand.

However, the US legal cup is 8.1 fluid ounces, the customary cup is 8 fluid ounces, the Imperial cup (UK) is 9.6 fluid ounces, the metric cup is 8.45 fluid ounces, the Japanese customary cup is 6.75 fluid ounces, and the Japanese traditional Go cup is 6.1 fluid ounces. Of course, if you are a Starbucks fan, then your serving size for a cup of coffee can be short (8 ounces), tall (12 ounces), grande (16 ounces), venti (20 ounces), or trenta (31 ounces)!”

When the Dietary Guidelines Advisory Committee concluded that drinking three to five cups a day of coffee was associated with minimal health risks (see the May 2015 newsletter), it referenced eight-ounce cups totaling up to 400 milligrams of caffeine.

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  1. Sorry, but you didn’t really answer the question. RESEARCHERS might think that a “cup” is 6 ounces or 8 ounces or whatever (your 2nd paragraph showed how unclear the measurement could be), but if a study relies on self-reported “cups of coffee,” then someone who has one 20-ounce mug of coffee with breakfast could very well report that as ONE cup of coffee — because to HIM, it’s a cup.
    How do researchers account for this? I haven’t seen studies that say exactly how they do — which makes me assume they DON’T account for it. And that, unfortunately, calls into question all the research on coffee drinking.
    Please, researchers, tell us exactly what YOU meant AND how you conveyed that to your respondents — because most surveys DON’T convey it, but rather rely on respondents’ own ideas. That’s really bad data collection. (And I clicked on the link to the 2015 newsletter, but it did not say how the researchers made sure that the RESPONDENTS knew that a “cup of coffee” meant 8 ounces. Did they do that? If so, how? Details, please!)

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