My last "Food for Thought" column noted that elderly women may keep their wits longer by drinking lots of caffeine. In that study, the women derived their caffeine from coffee and tea. A new study offers some guidance for Golden Girls who eschew the standard caffeinated brews but still want to reap some benefit from the stimulant. Many soft drinks contain caffeine, but how much of a jolt they deliver is seldom printed on the label.Enter two food scientists from Auburn (Ala.) University. In the August Journal of Food Science, they report just how much caffeine they measured in a host of carbonated beverages.