Caffeine & Calcium
David asked if caffeine reduced calcium absorption from food. Well, if it does it is only a small effect. One review paper suggests that just a teaspoon (5 mL) of milk would offset any reduced absorption of calcium, so if you have milk in your tea and coffee (average 30 mL) then you are ahead in the calcium stakes. Sometimes you will hear that caffeine is associated with osteoporosis (brittle bones). This is not the case in those getting enough calcium each day, according the latest review papers.
As we have hinted in the past, caffeine is pretty innocuous and is certainly not the demon you heard about in the 1980s and 1990s.