Difference Between Slim Fast & Optifast

Slim Fast and Optifast have similar names but are very different approaches to weight loss. According to MayoClinic.com, Slim Fast is a meal-replacement diet in which followers consume purchased weight-management products in place of one or two meals a day. While Optifast also involves meal replacement, it is primarily a very low-calorie diet with intake restricted to between 400 and 800 calories daily. Before beginning any weight-management program, talk to your doctor about the possible advantages and disadvantages.


Slim Fast dieters consume 1,200 calories each day provided by three snacks and three meals. Two of the meals consist of 200-calorie Slim Fast meal bars or shakes, with the third being a 500-calorie meal you prepare yourself, such as grilled fish with vegetables or a salad made with lean steak. The snacks are each 100 calories and can be a Slim Fast snack bar or fresh vegetables, fruit or nuts.

Optifast is available only to obese individuals who have been approved for the program by a doctor, and can only be conducted under medical supervision. The diet begins with 12 weeks of eating nothing more than a liquid meal replacement five times a day. Over time, Optifast participants are re-introduced to solid food and receive counseling on establishing healthy lifestyle habits.

Weight Loss

Difference Between Slim Fast & Optifast

Learn More

A 2006 study published in the "British Medical Journal" demonstrated that Slim Fast was as effective as other commercial weight-management programs like Weight Watchers at promoting short-term weight loss. Another study, published in "Obesity" in 2000, indicated that Slim Fast dieters are able to keep their weight off long term as well, with the average follower losing 8.5 percent of their body weight after four years.

According to Diet.com, a study conducted by Novartis Medical Nutrition, the manufacturers of the Optifast liquid replacement, determined that the Optifast program may help participants lose an average of 50 pounds. Other studies conducted by the Syracuse University Center for Health and Behavior in the 1990s indicate that Optifast dieters can sustain long-term weight loss, but more recent research is needed.


U.S. News & World Report says that Slim Fast dieters should easily meet the dietary guidelines for nutrition recommended by the government and that the program is safe for most healthy adults. In addition, the Slim Fast program may help lower your risk of diabetes and heart disease by improving your blood sugar level, blood cholesterol total and blood pressure. The plan is simple to follow and does not require complicated food preparation.

The rapid weight loss experienced by some extremely obese people on the Optifast diet may help prevent the development of serious medical problems. Participants are required to meet with a doctor weekly and blood samples are taken throughout the course of the program to ensure that the body isn't lacking any essential nutrients.


Difference Between Optifast & Medifast

Learn More

Slim Fast can be expensive to continue for a long period of time, and, because the diet doesn't include instructions for how to fashion low-fat, low-calorie, nutrient-dense meal plans without the Slim Fast products, dieters may regain the weight they lost when they stop using the shakes and bars. Vegans and people avoiding gluten won't be able to use Slim Fast products.

Optifast requires frequent doctor's appointments and can be prohibitively expensive, especially if you don't live near a hospital or medical clinic that administers the program and you need to travel. People who live at home while on Optifast may find it hard to stick with 12 weeks of a liquid diet with other people eating food around them. Losing weight too quickly may increase your risk of gallstones and cause diarrhea, fatigue and nausea.