Coffee, Not Caffeine; Tea, Not Tannins
Members of the Church of Jesus Christ adhere to a code of health first outlined in an 1833 revelation to the prophet Joseph Smith. He taught the saints that "hot drinks are not for the body or belly" (Doctrine and Covenants 89:9). Subsequent revelations have clarified this injunction by identifying coffee and tea as the "hot drinks" referenced. Some well-meaning members have interpreted this focus on coffee and tea as a condemnation of caffeine because that is one compound which both drinks share, but coffee, not caffeine, is the banned substance--and with good reason.
While caffeine may be harmful, studies have shown that coffee contains other substances which impair human health. According to a 1997 article published in The American Journal of Clinical Nutrition,
"the polyphenols (tannins) in coffee bind to iron in the intestinal lumen, forming an insoluble complex and thereby inhibiting iron absorption" (168).
A second article, published in the journal Critical Reviews in Food Science and Nutrition during the year 2000, confirms that tea likewise inhibits iron absorption and that these two drinks are associated with anemia and other related nutritional disorders. I note this not to add identify other potentially harmful substances (tannins!) in these two beverages but to note that, diverting as it might be to uncover the "reason" for a given commandment, the most important principle is obedience. Focusing on our own interpretations of the law (caffeine is bad!) might lead us to rationalize disobedience (decaffeinated coffee can't be bad . . .) and suffer the consequences (decreased iron absorption) we don't always forsee.
Latter-day Saints don't abstain from coffee and tea because those drinks contain caffeine or tannins; we abstain because God commanded us to.
Comments
But then the implication becomes: proof of God would be illogical commandments. Which doesn't make a whole lot of sense either.
There was a conference talk, ohhh... 5 or 6 conferences ago about the Word of Wisdom. I've tried to find it, but had little success so far. But in it he approaches the law from a direct health angle - that yes, it's a commandment of course, but that its purpose is health. Which I know everyone here agrees with. But I actually like that emphasis first and foremost that God's commandments are logical. And of course, he then asks us to follow them.