SlimFast, originally spelled Slim-Fast, is the trademarked brand name of both a line of diet products and a weight-management program. Slim-Fast Foods, the manufacturer of the diet products, was acquired by Unilever N.V., a company headquartered in the United Kingdom, in 2000. SlimFast diet shakes are perhaps the best-known products in the line, which also includes snack bars and cookies. The SlimFast diet plan is sometimes categorized together with other plans based on liquid diet products as a liquid meal replacement or LMR diet. LMR diet products themselves are a major business in the United States, reported in 2017 to account for over $3 billion in consumer purchases each year. The demand for LMRs is expected to increase due to growing concern about overweight among American consumers.


Although SlimFast as a specific product line was introduced only in 1977, LMR products as a type have been on the North American market since 1960, when Mead Johnson, a company better known as the maker of such baby foods as Pablum and Dextri-Maltose, introduced a liquid diet formula called Metrecal. Metrecal was packaged in 8-ounce cans, each containing 225 calories' worth of product. The dieter was supposed to drink four cans daily, for a total of 900 calories.

Metrecal itself was a rebranded food product originally designed for hospital patients or other people unable to digest solid foods. Named Sustagen, the liquid meal substitute consisted of a mixture of skimmed-milk powder, corn oil, and soybean flour, supplemented with vitamins and minerals. When Mead Johnson found that patients reported feeling comfortably full on Sustagen and were satisfied with it as the equivalent of a meal, the company decided to rename the product Metrecal and market it as a diet food in 1960. In the mid-1960s, the company introduced Metrecal cookies, nine of which made a meal, as an alternative to the liquid formula.


Slim-Fast* product

Calories per serving

Protein (g)

Carbohydrates (g)

Fat (g)

Cholesterol (mg)

Sodium (mg)

Potassium (mg)

Fiber (g)

Original shake









Advanced nutrition shake









Advanced energy shake









Original nutrition bar









Advanced nutrition chips









Crisps and snack bites









Amounts vary with product flavors

In the early 2000s, Slim-Fast lost some of its popularity due to widespread interest in the Atkins diet. The company replaced the sugar in its original liquid formula with Splenda, an artificial sweetener, and added an additional gram of fat to the formula in order to help dieters feel fuller longer. Another modification to the earlier formula was increasing the proportion of insoluble dietary fiber, which also increases the dieter's feeling of satiety (feeling full). The new line of Slim-Fast LMRs was called Slim-Fast Optima Hunger Control Shakes. In addition, the company added several lines of specialized diet products for dieters with lactose intolerance; dieters interested in a low-carbohydrate weight-control plan; and dieters who prefer a high-protein diet.


As of 2018, SlimFast products are marketed in Canada, the United Kingdom, France, Germany, Iceland, Ireland, and Latin America as well as in the United States.


In 2009, SlimFast reduced the number of its offerings and created the “3-2-1” plan as a modification of its original liquid meal replacement structure. This plan is still featured on the company's website as of 2018. The products advertised for the plan include shakes (ready to drink or powdered); smoothies (also ready to drink or powdered); cookies; chips and “snack bites”; and snack bars. The products include a line of high-protein shakes and smoothies called Advanced Nutrition, which contain more protein than the original SlimFast shakes. Another new product line called Advanced Energy shakes and smoothie mix was introduced at the end of 2017; it claims to contain “as much caffeine as a cup of coffee.”

The SlimFast products marketed in the United Kingdom differ from the American lines in two respects: they include porridges and noodles (100 calories each) to be cooked in a microwave, and the Advanced shakes are called Advanced Vitality rather than Advanced Energy. In addition, the British products come in some flavors that are not available in their American counterparts.

The 3-2-1 Plan refers to three snacks, two shakes, and one “sensible meal of your choice.” The sensible meal should total around 500 calories—although the British version of the website states 600 calories. The dieter is instructed to substitute SlimFast products for two meals per day; enjoy three 100-calorie snacks in between meals; drink plenty of water; and add 30 minutes per day of physical exercise to his or her lifestyle. Because men need more calories than women do, the SlimFast website advises men to add 200 calories to each meal replacement: “Use 2 scoops [of powder] instead of one when you're making a smoothie, eat 2 bars or cookies, or add a mini-meal of your own.”

The dieter does not have to give up coffee, tea, or other low-calorie caffeinated beverages. Dieters can arrange these meals, snacks, and meal combinations in any daily pattern that works for them, which allows them to make eating out with friends their daily meal rather than avoiding social occasions that involve eating. A more recent recommendation involves what might be called psychological self-care: “You deserve 30 minutes of me-time every day. Make the time for a daily walk, or pick a fun new activity to try!” This recommendation is intended to reduce the sense of deprivation that many people feel when starting a weight-reduction diet.

The SlimFast website offers recipe plans, “tips and tricks,” meal planners, and similar resources to help users stick to the diet plan. SlimFast also added a YouTube channel to its online website in 2006. As of early 2018, the channel has 1,723 subscribers. It includes recipe videos as well as information about the Advanced Energy line and other new products as well as customer success stories.


The SlimFast diet plan and the various SlimFast products are intended for weight reduction (at a moderate rate approved by most health professionals) or weight maintenance. Some people also use them as convenient and easily portable meal or snack substitutes when hiking or traveling.


The SlimFast diet plan has several advantages:

A unit of food energy. In nutrition terms, the word calorie is used instead of the scientific term kilocalorie that represents the amount of energy required to raise the temperature of one liter of water by one degree centigrade at sea level. In nutrition, a calorie of food energy refers to a kilocalorie and is therefore equal to 1,000 true calories of energy.
An elastic protein found in wheat, barley, rye and some other grains that gives cohesiveness to bread dough. Some people are allergic to gluten and cannot digest products containing these grains.
A sugar found in milk and milk products that produces lactic acid during the process of fermentation. Some people cannot digest lactose and must avoid products containing milk.
Liquid meal replacements (LMRs)—
A general term for prepackaged liquid shakes or milk-like drinks intended to substitute one or more meals a day as part of a weight-loss regimen or source of nutrition for people who cannot eat solid foods. The U.S. Food and Drug Administration (FDA) does not have an official definition of “meal replacement” as of 2018.
The quality or state of feeling comfortably full. It is sometimes used as a criterion for evaluating people's satisfaction with diets or diet products.
A blended beverage resembling a milkshake in texture but often made with nondairy ingredients. SlimFast and other diet product companies market prepackaged smoothies as well as shakes.

Another benefit of SlimFast products is that they are supplemented to provide sufficient intakes of minerals and vitamins when consumed in the recommended amounts; thus micronutrient deficiencies are unlikely to be a problem despite reduced energy/calorie intakes.


Dieters with type 2 diabetes or high blood pressure should consult their doctors before starting the SlimFast plan (or any other weight-reduction diet), as weight loss may require adjustments in their insulin and blood pressure medications.

People on special diets may need to make some adjustments if they follow the SlimFast plan:


No major health risks have been reported from use of either the diet plan or SlimFast products when used as directed.

Research and general acceptance

SlimFast has been studied more often than most comparable diet products. The SlimFast website includes a list of 51 clinical studies of meal replacement diets published in medical journals. A careful reading of the list indicates, however, that some of the studies go back to 1999; most were published between 2000 and 2005; and the most recent study cited was published in 2011. The chief drawback reported, as with all weight-reduction programs, is patient compliance. About 40% of the subjects in one early clinical study of SlimFast were excluded from the second stage of the study because they were judged noncompliant. This rate, however, is no higher than the noncompliance rate of subjects on other weight-reduction regimens. An associated problem with meal replacement diets is that they do not necessarily change eating habits, so when they are stopped, weight regain can occur. One group of Australian researchers reported in 2017 that attrition rates are high on meal replacement diets, “suggesting that many consumers find dietary changes required by these programs unsustainable.”


Other recent research studies that are not listed on the SlimFast website include a Texas-based study of the effects of meal replacement diets on food cravings and why these diets reduce food cravings more effectively than standard calorie-reduction diets. The researchers used functional magnetic resonance imaging (fMRI) to study the regions of the brain affected by the meal replacement diets. Another study done in Maryland reported that meal replacement diets can be effective in patients with type 2 diabetes as well as in obese patients who are not diabetic, provided that the meal replacements are combined with one-on-one therapy. Other researchers reported that patients using meal replacement diets combined with behavioral change under medical supervision can also be successful not only in losing weight but also in keeping it off.

See also Atkins diet ; Fad diets ; High-protein diet ; Liquid diets ; Nutrisystem ; Obesity ; Sugar ; Vitamins .



Forman, Evan M., and Meghan L. Butryn. Effective Weight Loss: An Acceptance-Based Behavioral Approach: Clinician Guide. New York: Oxford University Press, 2016.


Carney, D., et al. “Successful Medical Weight Loss in a Community Setting.” Journal of Obesity and Weight Loss Therapy 5 (February 2015): pii: 248.

Coleman, C. D., et al. “Comparative Effectiveness of a Portion-Controlled Meal Replacement Program for Weight Loss in Adults with and without Diabetes/High Blood Sugar.” Nutrition and Diabetes 7 (July 10, 2017): e284.

Kahathuduwa, C. N., et al. “Effects of 3-Week Total Meal Replacement vs. Typical Food-Based Diet on Human Brain Functional Magnetic Resonance Imaging Food-Cue Reactivity and Functional Connectivity in People with Obesity.” Appetite 120 (January 1, 2018): 431–41.

McEvedy, S. M., et al. “Ineffectiveness of Commercial Weight-Loss Programs for Achieving Modest but Meaningful Weight Loss: Systematic Review and Meta-Analysis.” Journal of Health Psychology 22 (October 2017): 1614–27.

WEBSITES . “How It Works.” (accessed January 14, 2018). “Tasty Products.” (accessed January 14, 2018).

SlimFast YouTube Channel. (accessed January 14, 2018).

WebMD. “Diet and Weight Management: Slimfast.” (accessed January 14, 2018).


SlimFast Foods Company, 800 Sylvan Ave., Englewood Cliffs, NJ, 07632, (201) 567-8000, (800) 754-6327, , .

Rebecca J. Frey, PhD

  This information is not a tool for self-diagnosis or a substitute for professional care.