It's strange that the article hints at genetic differences in caffeine metabolism and sensitivity ("Some people will get edgy from a weak cup of tea. For others, a double espresso is required to get them into the shower in the morning.") but doesn't follow through on the implications. For example, my 23andMe results indicate that I'm a "fast caffeine metabolizer", which is associated with a neutral to slightly lower risk of heart attack due to caffeine consumption, whereas slow caffeine metabolizers have a higher risk. In other words, it's clear that the answer to the question "How Much Caffeine Is Too Much?" is "It depends on your genes."