Fat binders are said to bind to the fat you eat thus making it indigestible so that rather than settle on your hips it passes right through you. Sounds too good to be true? Read this to find out.
The main active ingredient in some fat-binding supplements, such as XLS Calorie Reducer and Formoline L112, is chitosan, a polysaccharide compound derived from the shells of crustaceans. Other supplements, such as XLS Medical, Proactol and Lipobind, contain litramine, which is a natural fibre extracted from the cactus Opuntia ficus-indica. Both chitosan and litramine work by binding to the fat you eat, limiting the quantity of fat that can be digested. Undigested fat is then transported through the gut before the body has had a chance to extract any calories from it. Fats contain the most calories per grams compared to carbohydrates and proteins. Junk foods, in particular, tend to be very high in fat and, therefore, calories. Consequently, in theory, taking a fat-binding supplement could help you lose weight. In addition, chitosan and litramine are rich in soluble fibre, which may delay gastric emptying, creating a sense of being full and thus reducing appetite.
Other fat-binding supplements, such as Xenical and Alli, contain orlistat. Orlistat works in a different way to chitosan and litramine. It actually binds to lipase, an enzyme that helps digest fat, rendering it less effective and limiting the amount of fat that can be broken down, absorbed or stored. Xenical is only available on prescription, whereas Alli is available from pharmacists to people with a BMI of 28+. At the standard prescription dose of 120mg three times daily before meals, orlistat prevents approximately 30% of dietary fat from being absorbed, and about 25% at the standard over-the-counter dose of 60 mg.
Orlistat has been shown to be effective in promoting weight loss, although results are modest.  Data from clinical trials suggest that people given orlistat in conjunction with a low-calorie diet lose about 3 kilograms in 1 year. Orlistat also confers modest improvements in blood pressure and LDL-cholesterol (the bad cholesterol), and appears to prevent progression to type 2 diabetes in people with impaired glucose tolerance, whether due to weight loss itself or to other effects. In fact, in a large randomized controlled trial, orlistat was found to reduce the incidence of diabetes by nearly 40% in obese people. 
On the other hand, a review of clinical trials performed with dietary chitosan over a minimum of four weeks, body weight, blood pressure and cholesterol were improved only in some low-quality trials, indicating a minor effect on body weight. Other higher quality trials indicated no significant effect of chitosan and no clinical justification for advising overweight patients to take chitosan supplements. 
The official litramine website  presents data from four studies to support its fat-binding properties and one for its weight-loss claims. However, no clinical studies supporting the weight-loss action of litramine in humans have been published.
Overall, orlistat has a good safety profile, and serious adverse events (e.g. reports of severe kidney and liver injury) are rare.  However, gastrointestinal side effects, such as flatulence, oily discharge from the rectum and diarrhea after consumption of a high-fat meal, are very common. It is, therefore, recommended that daily intake of fat is spread evenly over three main meals since having a meal high in fat with orlistat will lead to an increase in gastrointestinal side effects. 
Fat-binding supplements in general decrease the absorption of fat-soluble vitamins and other fat-soluble nutrients. For that reason, a multivitamin tablet containing vitamins A, D, E, K, and beta-carotene should be taken once a day, at bedtime, when using such supplements. 
If you think you can have your cake and eat it, you may want to think again. Fat-binding supplements are only somewhat effective if you combine them with a low-fat healthy diet and exercise regime. So if you are looking to lose just a few pounds, I would suggest you skip the fat blockers altogether. Not only will you be saving yourself some money you may also be sparing yourself some very unpleasant and potentially embarrassing side effects.
If you have a BMI of 30+ (or 28+ and associated with high blood pressure, heart disease, or diabetes) you should consult your doctor to discuss your suitability for orlistat or other weight-reducing treatments.
You can learn more about the effects of sugar on our interactive online Nutrition and Weight Management course.