"Some people get jumpy after drinking a single cup of coffee, while others can gulp down a Venti Americano without feeling a thing. Part of that variability is due to the development of tolerance by regular coffee drinkers; but there are genetic differences in how people metabolize caffeine as well."
“Coffee, I Must Have Coffee…”
http://spittoon.23andme.com/2011/04/18/coffee-i-must-have-co...
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"Caffeine Consumption "
https://www.23andme.com/health/caffeine-consumption/
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"Caffeine Metabolism"
https://www.23andme.com/health/Caffeine-Metabolism/
"Some people get jumpy after drinking a single cup of coffee, while others can gulp down a Venti Americano without feeling a thing. Part of that variability is due to the development of tolerance by regular coffee drinkers; but there are genetic differences in how people metabolize caffeine as well."
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http://www.snpedia.com/index.php/Caffeine