(From Dr. David Williams – a nutrition newsletter doctor that I respect very much.)

“Over the years, various substances have been touted as being the real magic bullet, bringing on weight loss with little or no effort required on the part of the user.

One of the most notorious was amphetamine, or “speed.”  Sold under various names, it actually was quite effective, because it worked on both sides of the weight equation.  It increased BMR, and it suppressed appetite, so people ate less.  Unfortunately, amphetamine also increases the risk of heart damage, interferes with sleep, and is addictive.

Then about 20 years ago, the “fen-phen” drug combination became popular.  This regimen consisted of fenfluramine (a drug related to the SSRI antidepressants, but one which oddly enough has a depressing effect) and phentermine (a stimulant related to amphetamine). This pairing of an “upper” with a “downer” seemed to produce steady, reliable weight loss in patients, even those who had had difficulty losing weight in the past.

Unfortunately, fen-phen carried its own set of side effects: heart valve problems, and an irreversible condition called primary pulmonary hypertension, in which fluid builds up in the lungs and causes difficulty breathing.  The latest drug to be put forward as the answer to weight loss is orlistat, sold by prescription as Xenical and over the counter as allî.  (The only difference is dosage; Xenical is 120 mg three times daily and allî is 60 mg three times daily.)

Orlistat works by blocking an enzyme called lipase that digests fat in the small intestine.  The theory is that, by blocking the digestion and absorption of fat, the user will reduce the amount of calories that they’ll take in from a given meal.  Critics have commented on the fact that, by blocking the absorption of fat, you’re also blocking the absorption of fat-soluble vitamins such as A, D, and E.

The most ironic side effect hasn’t received much publicity, however.  Your body relies on internal signals to regulate practically every process.  For example, an increased level of carbon dioxide in your blood triggers an automatic breathing response, to exchange that carbon dioxide for fresh oxygen from the lungs.  The digestion of fat in the small intestine sets off the release of compounds that signal fullness, known as satiety, and so regulate appetite.

When orlistat blocks fat digestion, it also blocks the signals that tell you when you’ve had enough to eat. According to the authors of one study, “the increase in energy intake approximates the energy lost due to fat malabsorption.”  In other words, the drug did what it was supposed to do (block fat digestion), but the side effect completely undid any benefit.  (Br J Nutr 03;90:849–852)