the way i understand it (and this may not be perfect or even completely correct), but the way i understand it, xenical flushes the fat right out of your body (via...well...the bathroom). I think if you eat very little, xenical does very little.
If you miss a meal or have a meal without fat, you don't need to take your dose of Xenical, it works on the food you eat. Talk with your doctor... The side effects scare me (like pooping my pants would freak me out... LOL)... here they are (so make sure you discuss any concerns with your DR...
Because XENICAL works by blocking the absorption of dietary fat, it is likely that you will experience some changes in bowel habits. These generally occur during the first weeks of treatment; however, they may continue throughout your use of XENICAL. These changes may include oily spotting, gas with discharge, urgent need to go to the bathroom, oily or fatty stools, an oily discharge, increased number of bowel movements, and inability to control bowel movements. Due to the presence of undigested fat, the oil seen in a bowel movement may be clear or have a coloration such as orange or brown.
These bowel changes are a natural effect of blocking the fat from being absorbed and indicate that XENICAL is working. They generally occur early in treatment, particularly after meals containing higher amounts of fat than are recommended. These symptoms are often temporary and may lessen or disappear as you continue treatment and keep to your recommended diet of meals containing no more than about 30% fat. However, these side effects may occur in some individuals over a period of 6 months or longer.