Answer by Steven Fowkes:
The short-term effect is acidification, and the long-term effect is alkalinization. The acidification effect can last for two hours. The aklinization effect lasts for the rest of the day 8-12 hours).
I suspect that the biphasic effect is caused by caffeine and other methylxanthines, which rev up aerobic metabolism by mitochondrial uncoupling, which produces a thermogenic effect (the direct diversion of energy production into heat). When this wears off, the more basic alkalinization effect of coffee is uncovered. Most herbal, vegetable, fruit preparations are alkalinizing.
It is interesting to note that the structure of the classic office workday is neatly arranged into two-hour segments to facilitate maintenance of the short-term coffee effect. Work for two hours, take a coffee break. Work for another two hours, take a coffee break and eat lunch. Work for another two hours for the next coffee break. Work for another two hours, and then get off work, go home and drink more coffee.
The alkalizing or acidifying effect of a food or beverage has little, if anything, to do with its pH when you eat it. Lemon juice is exceedingly tart, yet is alkalizing to the body. Seaweed is not tart, but is even more alkalizing than lemon juice. Flax oil is neither tart or alkaline, but it is strongly acidifying.