Sorry for the Internets silence - apparently cable modems can just up and die on you.

We have short memories. Ages ago, a molecule that was essentially a chimera of sugar and fatty acids was found to provide essentially no nutriment whatsoever (ironically, it was hoped it would be a rich source of calories). In fact, it passed right through you. It was essentially a noncaloric fat. Bafflingly, it took ages for people to realize that noncaloric fat could be a boon to the prepared foods industry. Ten short years ago, however, Olestra was loosed upon the world.

Olestra had a dark side, of course - readers of a certain age (OK, over 20 or so) will remember the warnings on the label:

This Product Contains Olestra. Olestra may cause abdominal cramping and loose stools. Olestra inhibits the absorption of some vitamins and other nutrients. Vitamins A, D, E, and K have been added.

Oil in the stool. Not to mention preventing the absorption of countless fat-soluble antioxidants (carotenes and the like). Olestra had its day in the sun, but it's downright hard to find in some places today.

More recently, another drug, Orlistat, was released as a prescription formulation, Xenical. The idea here is that it's another fat-like molecule, also undigestible, but it induces its action by blocking lipases in the gut - you just don't digest fat as well. Everything's a little like Olestra - you just pass a good chunk of ingested fat (along with any dissolved nutrients and anal seepage).

Recently, the FDA approved half-dose Orlistat as Alli, an over-the-counter drug. Alli is apparently doing very, very well - it's really a new kind of weight-loss drug, and people are excited about the new OTC anti-lardo pill. However, it's not magic, you're supposed to limit fat (to a somewhat spartan 15g per meal - the little bags of potato chips have nearly that). Absent diet, you can expect some oily stool and less-than-stellar results.

For another take, see Derek Lowe.

Ironically, the olefin/unsaturated version of orlistat is even more effective, but it was passed over (apparently due to logistics rather than toxicity - unsaturated fats can go bad, even the magic pill kind!).