Q. Can you tell me how the so-called "fat blockers" work and what kind of short- and long-term effects they have on the body?
November 21, 2001
There are at least two "fat blockers" that are commercially available in the United States. One, Xenical (also known as orlistat) is an prescription drug approved by the U.S. Food and Drug Administration that reduces fat absorption by preventing an important set of enzymes (called lipases) from working in the gastrointestinal tract. Lipases break down large fat particles into smaller, more easily absorbable forms. Most studies have shown about a 30 percent drop in fat absorption with Xenical. Xenical is used by some weight-loss clinicians as a part of an overall weight-reduction program. While there are no severe side effects associated with its use, Xenical produces a range of gastrointestinal side effects including nausea, vomiting, intestinal gas, diarrhea, abdominal pain and increased bowel movements. Because of the way Xenical works, it also can block the absorption of fat-soluble vitamins and produce a vitamin deficiency. Because of these side effects, Xenical should be used only by people who have a significant weight problem.
Obesity usually is defined by the body mass index (BMI). The BMI is calculated by looking at both height and weight. Click here to calculate your own BMI. People with a BMI of greater than 30 are considered to be obese. As an example, if you were 5 feet, 5 inches tall, you would need to weigh 180 pounds to meet the criteria for obesity. Only people with a BMI greater than 30 should be treated with Xenical. I should point out that the impact of Xenical on weight loss is not dramatic. In the clinical studies, a total of 57 percent of people in a weight-control program with Xenical managed to lose 5 percent of their total weight (that's 9 pounds in someone weighting 180 pounds) at the end of a year as compared to 31 percent of those only using a weight-reduction program.
A number of companies sell products containing a drug called "chitosan" as fat blockers. Chitosan is a by-product of shellfish and has been used in a number of industries, including the woolen and pharmaceutical industries. Unlike Xenical, chitosan theoretically works by binding to fat particles, making them less likely to be absorbed. A study was recently published looking at the effects of both Xenical and chitosan in reducing fat absorption. The study was conducted by the manufacturers of Xenical but was published in a peer-reviewed obesity journal. Twelve healthy volunteers were given a standard diet (2500 calories) containing 30 percent fat and then took either Xenical or chitosan. There was a significant increase in the amount of fat in the stool of the volunteers during the Xenical study period but almost no observed increased in the amount of fat in stool during the chitosan study period. That means that chitosan did not effectively block fat from being absorbed.