The material I've been reading is generally cynical at best about Xenical.
The most it can apparently do is prevent the body from absorbing about 1/3 of the fat calories consumed. It then causes the body to excrete this fat in the form of an oily substance by way of the bowels. This apparently does not happen at the preferred times in the privacy of one's bathroom, but can make a sudden appearance at any time.
This sort of embarrassing unpleasantness then forces the patient to learn to consume very little fats or oils to avoid the results.
Sure, the idea of a medication absorbing 1/3 of the fats and oils one eats sounds good -- but it seems like a rather unsatisfactory way to "teach" someone not to eat them in the first place.
The body is still able to absorb 2/3 of the fats and oils consumed while on Xenical, and it does not do anything to prevent carbohydrates (or proteins) from being digested.
Even if someone ate a very high calorie high fat
diet it would only do "some" good. For example, if someone ate the very bad diet of 3,000 calories, and 50% of those were fat calories, then Xenical would only be blocking 500 calories. (3,000 x 50% = 1,500, then 1/3 of 1,500 = 500)
But if one has the problem of bowel leakage, one is much less likely to go out and walk or do other excercise, and would really have a depressing effect on how one feels about herself. It doesn't take much in the way of excersise to burn off 500 calories.
From what I have read, Xenical is most effective for people who have physical limitations which prevent them from getting out very much, and who have 100+ lbs to lose.
This is only from what I've read about it, and it of course does not apply to everyone, but is based on various doctors' and patients' experiences.