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Ask Tufts Experts December 2016 Issue

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?

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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