On my first week of Tony Ferguson, I stuck to the diet pretty strictly. Apart from a couple of days where I had four cups, I stuck to the maximum of three cups of tea a day (preferring that to one cup of coffee or five cups of green tea), and just barely managed to survive without coffee.
This week, however, when I went to the chemist to weigh in I asked the TF consultant why caffeine was restricted on the diet. She answered in a less-than-convincing way that it is because caffeine decreases your metabolism. She explained this with hand movements that swirled around her stomach region and there were a lot of "ums" punctuating the explanation, so I was left unconvinced. She was also about 12. I immediately went out and bought a small skinny cappuccino.
That was Monday, and I have now had a coffee every day bar Thursday this week. Which, coincidentally, is the only day I haven't had a TF "munch" bar for lunch, which, while filling(ish), is very dry. So this, on the left, is what my in-a-rush lunch is. And I have already had my three cups of tea for the day.
So obviously I don't believe this whole thing about caffeine slowing down your metabolism. I've done what every good librarian does first and Googled, read some stuff on Wikipedia, did some database searching (yay for working in an academic library!), and have pretty much ascertained that caffeine increases (not decreases) your metabolism, at least in the short term, but has little or no effect on weight loss. A lot of weight loss pills include caffeine but there's no conclusive evidence that it makes any difference whatsoever. Also, there are a lot of idiots on the teh interwebs.
Most of the stuff on the web is hearsay and companies spruiking their own wares - apparently on the Atkins diet (another ketosis-based weight loss program, but they get BACON! Jealous!) they say you shouldn't drink coffee - you should get your caffeine from their own product instead - from a discussion list:
Atkins prefers people get their caffeine from ATKINS ACCEL rather than from coffee...I think the rea$on$ $peak for themselve$.
I'd be interested to hear other's opinions too - and if you can find any hard research that shows whether caffeine has any actual effect on metabolism, dieting, etc, let me know. Everything I found on WoK and ScienceDirect was WAY over my head.