Table of Contents


Slim-Fastis the trademarked brand name of both a line of diet products and a weight-management program known as the Slim-Fast Optima Diet. Slim-Fast Foods, the manufacturer of the diet products, was acquired by Unilever N.V., a company headquartered in the United Kingdom, in 2000. Slim-Fast diet shakes are perhaps the best-known products in the line, which also includes snack bars, meal bars, smoothies, cookies, and powders for reconstituting by mixing with skimmed milk. The Slim-Fast 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 2006 to account for over $1 billion in consumer purchases each year.


Although Slim-Fast as a specific product was introduced only in the early 1980s, 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 Dex-tri-Maltose, introduced a liquid diet formula called Metrecal. Metrecal was packaged in 8-oz cans, each containing 225 calories’ worth of product. The dieter


Slim-Fast®productCalories per servingProtein(g)Carbohydrates(g)Fat(g)Cholesterol((mg))Sodium(mg)Potassium(mg)Fiber(g)
Original shake22010402.5-352206005
Easy to digest shake1801024-265>5200500-6003
Low-carb shake180-190204-6915220-2605502-4
High-protein shake1901523-24510220550-6005
Optima shake180-1901023-255-65200550-6005
Original nutrition bar140-150519-205-6565-80115-1602
Low-carb nutrition bar1201-614-214.5-5>570-80n/a1-2
High-protein nutrition bar190-2001520-216-70–>5200270-3002

Amounts vary with product flavors

(Illustration by GGS Information Services/Thomson Gale.)

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 invalids 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 their 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.

Metrecal lost much of its market in the 1980s as a result of competition from Slim-Fast, which cost much less and was aggressively promoted in the mass media. In addition to lower price, the original Slim-Fast products tasted much better to most consumers than Metrecal, which had a noticeably chalky taste—so much so, in fact, that one team of researchers in Philadelphia used Metrecal to test its effects on the concentration of gastric acid in patients diagnosed with peptic ulcer. In addition to a more pleasing taste, the original Slim-Fast formula came in a wider variety of flavors and included breakfast and lunch meal bars as well as the canned shakes and a powdered formula that the dieter could mix with skimmed milk at home.

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 nonsoluble dietary fiber, which also increases the dieter’s feeling of satiety. The new line of Slim-Fast LMRs is called Slim-Fast Optima Hunger Control Shakes. In addition, the company has 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 2007, there are five separate lines of Slim-Fast diet products:

  • Original Slim-Fast formula: available in ready-to-drink shakes, smoothies, meal bars, and powder.
  • Slim-Fast Optima: available in shakes, meal bars, snack bars, and powder. Cookies, a new product, were added to this line in early 2007. Each serving of one of these products supplies about 8 grams or 14% of an average adult’s daily protein requirements. The products come in a range of vanilla, chocolate, peanut butter, coffee, caramel, and strawberry flavors, as well as various combinations of these.
  • Slim-Fast High Protein: available in shakes and meal bars. These products contain almost twice as much protein (15 grams per serving) as the Optima products.
  • Slim-Fast Easy-to-Digest: available only in shakes, this formula is lactose- and gluten-free, for people who cannot digest products containing wheat or milk.
  • Products for a Lower-Carb Diet: available as shakes or snack bars.


The Slim-Fast Optima diet plan is available in a 44-page booklet that can be downloaded from the Slim-Fast website. The booklet explains that the Slim-Fast plan is based on the dieter’s present weight level rather than a one-size-fits-all calorie level or rigid menu. The dieter is instructed to substitute Slim-Fast products for two meals per day, use them for a between-meals snack if desired, drink plenty of


Gluten— An elastic protein found in wheat and some other grains that gives cohesiveness to bread dough. Some people are allergic to gluten and cannot digest products containing wheat.

Lactose— 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 for one or more meals a day as part of a weight-loss regimen or source of nutrition for people who cannot eat solid foods.

Metrecal— The first product marketed as an LMR for weight reduction, introduced in 1960 by Mead Johnson.

Satiety— 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.

Smoothie— A blended beverage resembling a milkshake in texture but often made with nondairy ingredients. Slim-Fast and other diet product companies market prepackaged smoothies as well as shakes.

water, and add 30 minutes per day of physical exercise to their lifestyle. The dieter does not have to give up coffee, tea, or other low-calorie caffeinated beverages. The daily meal plans for the four specific weight levels (for adults) are as follows:

  • Up to 140 pounds: 1 Slim-Fast Meal On-the-Go; 1 Slim-Fast Meal Combination; 1 Sensible Meal; 3 fruits or vegetables
  • 141-170 pounds: 1 Slim-Fast Meal On-the-Go; 1 Slim-Fast Meal Combination; 1 Sensible Meal; 4 fruits or vegetables; 1 snack
  • 171-200 pounds: 1 Slim-Fast Meal On-the-Go; 1 Slim-Fast Meal Combination; 1 Sensible Meal; 4 fruits or vegetables; 3 snacks
  • Over 200 pounds: 1 Slim-Fast Meal On-the-Go; 1 Slim-Fast Meal Combination; 1 Sensible Meal; 5 fruits or vegetables; 4 snacks

Dieters can arrange these meals, snacks, and meal combinations in any daily pattern that works for them. The plan defines its various components as follows:

  • Meal-On-the-Go: A Slim-Fast meal replacement liquid shake or solid bar (180-220 calories)
  • Meal combination: A Slim-Fast meal replacement shake or bar combined with a serving of ‘a favorite healthy food’ (180-220 calories plus 200 calories; healthy food suggestions include a cup of lentil soup; half a roast beef sandwich; cottage cheese plus a glass of tomato juice)
  • Sensible meal: A nutritious meal of about 500 calories, accompanied by a large glass of water or other calorie-free beverage. The booklet contains a diagram of a plate divided into half with the top half divided in half again. The dieter is instructed to think of the bottom half of the plate as filled with vegetables and the two top segments as filled with lean protein and starch (preferably whole grains) respectively. Instead of counting calories or weighing and measuring, the person is advised to visualize portions as follows: 1 cup = the size of a softball; 1/2 cup = size of a light bulb or baseball; 3 ounces (meat or fish) = size of a deck of cards or the palm of the hand; 2 tablespoons = size of a ping-pong ball. Thus the sensible meal is about 1/4 protein food, 1/2 vegetables, and 1/4 whole grains.
  • Fruits and vegetables: 1 serving = 1 medium-size whole fruit or 1/2 cup sliced; 1 cup raw vegetables or 1/2 cup cooked.
  • Snacks: Slim-Fast snack bars (120 calories).

The Optima diet plan allows a daily calorie count of 1250-1400 calories for a 140-pound dieter and up to 1850 or 1900 for a dieter over 200 pounds.


The Slim-Fast Optima diet plan and the various Slim-Fast 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 Slim-Fast diet plan has several advantages:

  • It is intended to produce a safe, moderate weight loss of 1-2 pounds per week, which allows many people to use it without constant medical oversight or intervention. Some published studies indicate that this relative independence of medical monitoring is an attractive feature to many people.
  • The Optima diet plan is available in a downloadable booklet free of charge on the Internet; in addition, the various boxed Slim-Fast products have thumbnail summaries of the diet printed on the side or back panels. The Slim-Fast website contains much more general information about nutrition, weight reduction, and exercise than most product-related diet websites. It also contains a number of recipes for such popular foods as chili, barbecued chicken, steak with mushrooms, and meat loaf.
  • The diet plan urges physical exercise as an important part of a healthy lifestyle as well as a weight reduction program.
  • The Optima diet plan follows the U. S. Department of Agriculture’s (USDA) 2005 dietary guidelines.
  • It does not require calorie counting or careful measurement of portion size.
  • The plan’s recommendation of support groups and the use of a food diary are important psychological helps to many dieters. In addition, the company has weight loss advisors and professional dietitians available by telephone for dieters who need ‘help in setting realistic goals’ or specific dietary advice.
  • The diet plan’s overall flexibility in regard to meal replacements and ‘sensible meals’ makes it easier for dieters to tailor it to their own time schedules and meal preferences.
  • The fact that both the diet plan and the products have been evaluated in clinical trials is reassuring to many dieters.

The Slim-Fast products themselves are tastier and appear to satisfy hunger better since their reformulation in 2004. Some studies indicate that the solid bars, however, are more effective in controlling feelings of hunger than the LMRs. An additional advantage is the relatively low cost of Slim-Fast products compared to other prepackaged diet formulas. In fact, Slim-Fast owed its initial success in competing with Metrecal in the 1980s to its considerably lower price. One clinical study of Slim-Fast focused specifically on its effectiveness in helping low-income dieters lose weight, on the grounds that the incidence of obesityis high in this population. The study found that the subjects were significantly more successful in losing weight (7% of body weight on average) with the Slim-Fast plan (2 meal replacements per day plus one sensible meal) than they were when they simply attended a nutrition clinic. However, cost will still be a barrier for some. Typical prices for Slim-Fast products as of early 2007 are $10 for a can of Optima powder (14 servings); $5.79 for a box of 6 snack


  • Would you recommend the Slim-Fast diet plan for a person of my age, sex, lifestyle, and medical history?
  • Have you read any of the medical research about Slim-Fast?
  • Have any of your other patients tried Slim-Fast, and if so, did they lose weight? Did they like the products?
  • Do you know of anyone who has developed a new health problem or had a previous condition grow worse from using Slim-Fast?

bars; $4.29 for a box of 6 cookie bars; and $6.79 for a six-pack of Optima shakes.

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


In general, women who are pregnant or nursing; adolescents under 18 years of age; and anyone who needs to lose more than 30 pounds and/or has not been physically active should consult their physician before starting any weight reduction program. The Slim-Fast plan and the products themselves, however, are less likely to cause health problems than very low calorie diets (VLCDs) or fad diets. The Slim-Fast plan booklet specifically warns against eating less than 1200 calories per day.


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

Research and general acceptance

Published studies of Slim-Fast in the United Kingdom as well as the United States report that it enables dieters who follow the program to lose significant amounts of weight in a safe manner with minimal medical intervention or problematic side effects. One 2002 study of Slim-Fast products as part of weight reduction programs in four high-stress occupations (police, medical professionals, firefighters, and flight crew members) found that the products were effective in reducing weightand body mass index (BMI) even in overweight adults whose stressful jobs would encourage overeating. Four-fifths of the subjects completed the 12-week clinical study and were maintaining their weight loss at six-month follow-up, with the firefighters losing the most weight and the medical professionals the least.

The Slim-Fast plan and products were also shown in a study published in 2001 to be safe and effective for patients diagnosed with type 2 (adult-onset) diabetes. The study of the diabetic patients reported that the subjects showed improvements in blood sugar, insulin, hemoglobin A1c, and blood lipid levels as well as losing weight. The chief drawback reported, as with all weight reduction programs, is patient compliance. About 40% of the subjects in one clinical study of Slim-Fast 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.

Another problem with meal replacements is that they do not necessarily change eating habits so when they are stopped weight regain can occur.


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North American Association for the Study of Obesity (NAASO), The Obesity Society. 8630 Fenton Street, Suite 918, Silver Spring, MD 20910. Telephone: (301) 563-6526. Website:

Rudd Center for Food Policy and Obesity. 309 Edwards Street, Yale University, New Haven, CT 06520-8369. Telephone: (203) 432-6700. Website:

Slim-Fast Foods Company. Website: Contact by e-mail only, at Telephone line for nutritional advice: (800) 754-6327.

Rebecca J. Frey, Ph.D.