What Is the SlimFast Diet?

slimfast

Verywell / Debbie Burkhoff 

At Verywell, we believe there is no one-size-fits-all approach to a healthy lifestyle. Successful eating plans need to be individualized and take the whole person into consideration. Prior to starting a new diet plan, consult with your healthcare provider or a registered dietitian, especially if you have an underlying health condition.

The SlimFast diet is a weight loss plan centered on meal-replacement shakes and snack bars formulated by the company. The shakes come in regular, low-sugar, and high-protein varieties, and the snacks are 100 calories each. The company claims that weight loss results are possible within one week.

The SlimFast meal plan and products have changed throughout the years since the company was first founded in 1977. The original line of diet shakes came in chocolate, vanilla, and strawberry flavors, and the diet plan included a shake for breakfast and lunch followed by a "sensible dinner."

During the 1990s, SlimFast expanded its line to include meal replacement bars. As low-carb diets became popular in the mid-2000s, the company launched low-carb and low-sugar alternatives with the SlimFast Optima product. In 2009, SlimFast updated its program to the 3-2-1 plan which included three snacks, two meal replacements (shakes or bars), and one sensible meal.

Many people have had success with short-term weight reduction using SlimFast (and similar diets like Herbalife) because it's an easy-to-follow low-calorie plan. According to the company, the SlimFast plan has been tested in 50 clinical studies, and evidence shows it can be an effective short-term program. However, it is not a sustainable plan for weight maintenance.

The 2021 U.S. News and World Report Best Diets ranks the SlimFast diet number 26 in Best Diets Overall and gives it an overall score of 2.8/5. Its ranking is based on its success rate for short-term weight loss, but it received lower marks based on its nutrition content and mixed results for long-term weight loss.

Quick weight loss is likely on the SlimFast diet and proponents claim you can stay on the program as long as you like. However, nutrition experts do not recommend SlimFast as a long-term eating plan. As with most meal replacement diets that restrict calories, regardless of how long you stay on them, you are likely to regain any weight once you resume normal eating patterns. Staying on the diet for the long term excludes many healthy foods and important nutrients from the majority of meals.

What Experts Say

"Diets that promote eating bars and shakes instead of real foods aren't sustainable and often aren't healthy. You might lose weight, but replacing your meals with highly processed meal replacements won't deliver long-lasting, satiating results."

Kelly Plowe, MS, RD

What Can You Eat?

If you decide to go on the SlimFast plan, you will eat six times a day: Three meals and three snacks. Two meals are replaced with a SlimFast shake and one meal is portion-controlled. Each snack is 100 calories.

Your non-SlimFast meal can be eaten out or cooked at home, and though no foods are restricted, you are encouraged to eat healthfully. The meal must also be very low in calories, restricted to just 500 calories for women and 800 calories for men. You may choose to have alcohol in moderation, but SlimFast suggests limiting consumption since alcohol provides only calories and no nutrients.

What You Need to Know

You can purchase SlimFast branded snacks or make your own 100-calorie snacks at home. The 100-calorie snack packs are part of SlimFast's Advanced Nutrition program. There are a variety of both salty and sweet snacks to choose from.

According to SlimFast, women on the diet consume approximately 1,200 calories per day on the plan and men consume 1,600 calories per day. The company claims this allows for a reasonable rate of weight loss of about 1 to 2 pounds per week. However, some people may find that they're not getting enough daily calories, and replacing meals with shakes is not a very satiating way to eat.

If you follow this diet plan, make sure your one sensible meal a day includes foods with fiber and protein.

What to Eat
  • SlimFast shakes, snacks, and meal bars

  • One "sensible" meal a day

  • Alcohol

  • Coffee

What Not to Eat
  • No foods are explicitly banned on the SlimFast diet, but calories are restricted

The main focus of the SlimFast diet is the shakes, which come in a few different formulas and flavors. The newer Advanced Nutrition Shakes and Smoothies provide more protein and less carbohydrate than the original shakes.

Keep in mind that SlimFast products contain artificial ingredients. But many people opt for them out of convenience, since making a nutritious smoothie away from home is usually not an option.

For context, here are a few nutrition profiles for individual servings of SlimFast shakes. The "other ingredients" include additives such as fructose, carrageenan, and maltodextrin.

Original SlimFast Chocolate Royale Shake Mix

  • 110 calories
  • 4 grams fat
  • 18 grams carbohydrate
  • 4 grams fiber
  • 10 grams sugar
  • 2 grams protein

Ingredients: Sugar, sunflower creamer, alkalized cocoa powder, and other ingredients.

Original SlimFast Rich Chocolate Royale Shake (Ready-to-Drink)

  • 180 calories
  • 5 grams fat
  • 26 grams carbohydrate
  • 5 grams fiber
  • 19 grams sugar
  • 10 grams protein

Ingredients: fat-free milk, water, sugar, cocoa, canola oil, and other ingredients.

Advanced Nutrition SlimFast Creamy Chocolate Smoothie Mix

(Before mixing with 8 ounces of skim or fat-free milk)

  • 100 calories
  • 3 grams fat
  • 7 grams carbohydrate
  • 5 grams fiber
  • 1 gram sugar
  • 12 grams protein

Ingredients: Whey protein isolate, soy protein isolate, cocoa, sunflower oil, and other ingredients.

When you compare the ingredients and nutrition for SlimFast products with a smoothie you make at home with fresh fruit, fiber-rich nuts and seeds, and other nutritious ingredients, you may find that DIY smoothies are a healthier choice. It would be unusual to add artificial sweeteners and thickening agents to typical smoothie recipes.

Pros and Cons

Pros
  • Accessible

  • Less expensive than other commercial plans

  • Easy to follow

  • Portion-controlled

Cons
  • Heavily processed food

  • Does not teach healthy eating skills, like portion control, calorie counting, or low-calorie cooking

  • No personalized guidance

  • Poor taste

Pros

Products Are Accessible

SlimFast products are available at many supermarkets, grocery stores, and pharmacies around the United States. They are also available online.

Costs Less than Some Other Plans

Other diet plans like Jenny Craig, Nutrisystem, and Weight Watchers require you to commit to a long-term program to get a lower price. But SlimFast does not require a commitment. You can try it for a day or a month without having to cancel a subscription meal service or plan.

Easy-to-Follow Instructions

No foods are off-limits for the sensible meal and the products are portion-controlled, which makes it easy to follow.

Provides Built-In Calorie Counting

There is no need to download an app, keep a journal, or log foods. Almost all of the math is done for you—except for the one daily meal, which you will have to keep track of.

Encourages Exercise

SlimFast is a proponent of exercise but no formal program is provided. Some research has shown that exercise combined with meal replacement programs like SlimFast can lead to weight loss. However, these studies typically included guided support or education programs as part of the trial.

Cons

Contains Processed Foods

The program is built primarily on processed foods. Most health experts recommend that you decrease the amount of processed food that you consume and increase your intake of fresh or frozen fruits and vegetables, lean meats, seafood, and whole grains for optimal health.

Limits Healthy Cooking

You may not learn healthy cooking methods on the SlimFast diet plan—and cooking is not required on the program. And since you are able to consume a hearty 500 to 800 calories at your main meal, it could be tempting to just eat out or buy ready-to-eat processed food.

Disregards Portion Control

If you reach your goal weight on the SlimFast diet, you're going to have to eventually wean yourself off the program and maintain your weight loss. Because the diet products are already portion-controlled, you may not have the skills you need to make it through this transition without weight regain.

No Long-Term Maintenance Plan

Once you reach your goal weight, you are advised to slowly decrease your consumption of SlimFast and increase your intake of regular foods. But beyond that, there is no structured plan that helps to transition you off the diet.

No Personalized Support 

While there is no personalized nutritional guidance, the program has a Facebook page and provides recipes on the website. These resources may not be enough for some people.

Lacks Good Flavor

Not everyone likes the taste of SlimFast. Those interested in trying this diet might benefit from tasting a few different products before buying anything in bulk.

Is the SlimFast Diet a Healthy Choice for You?

The U.S. Department of Agriculture's 2020–2025 Dietary Guidelines for Americans recommends consuming a variety of nutrient-dense foods and beverages for a balanced diet. These include:

  • Vegetables and dark, leafy greens (kale, spinach, broccoli, Swiss chard, green beans)
  • Beans and legumes (all beans, lentils, peas)
  • Dairy (reduced-fat milk, cheese, yogurt)
  • Fruits (apples, berries, melon)
  • Grains (quinoa, brown rice, oats)
  • Lean meats (chicken breast, fish, turkey breast)
  • Nuts and seeds (walnuts, almonds, sunflower seeds)
  • Oils (olive oil, avocado oil) 

The USDA also advises limiting foods and beverages with higher amounts of added sugars, saturated fat, and sodium. While the SlimFast Diet does not restrict any foods, it does restrict all of the recommended foods for two out of three meals each day. Additionally, some SlimFast snacks may be high in sodium and a few shakes contain added sugars.

While the SlimFast diet plan has helped many people successfully lose weight in the short-term, it is not a long-term weight loss solution or a sustainable eating plan. The diet does not teach important skills, like healthy meal planning and preparing, needed for sustained weight loss.

The typical recommendation for calorie intake is approximately 1,500 calories per day for weight loss, but this number varies based on age, sex, weight, and activity level. The SlimFast Diet allows for 1,200 calories a day for women and 1,600 calories a day for men, which may not be enough calories for some people. Use this calculator to determine the right number of calories for you.

As a meal-replacement diet, SlimFast does not meet the USDA's definition of a healthy meal plan since it replaces real, whole foods with shakes containing artificial ingredients and added sugars. SlimFast snacks—though low in calories—are highly processed foods.

Health Benefits

The SlimFast diet restricts daily calories and will likely lead to short-term weight loss. However, research shows that meal replacement diets like SlimFast do little to promote long-term weight management.

Fortunately, the program does encourage exercise, which is essential to both weight loss and weight maintenance. Still, it might be hard to stick to the diet long enough to meet your weight loss goals. Replacing meals with shakes can be an unsatisfying experience compared to eating whole foods.

Health Risks

Though there are no common health risks or side effects associated with the SlimFast diet, the products are made from processed foods. Studies continue to show a link between processed foods and chronic diseases, which is why it's not recommended to consume these products for the long term.

Additionally, some SlimFast shakes are high in sugar, including added sugar. If you ate a lot of sweets before the diet, you may be tempted to return to these foods. Research shows that consuming foods high in sugar and saturated fat can increase your risk of heart disease.

A Word From Verywell

Restricting daily calories to create a calorie deficit can certainly lead to weight loss in the short term, but restrictive diets, in general, are not sustainable eating plans. A healthy diet that includes a variety of nutrient-dense whole foods combined with regular exercise can be one of the most effective weight loss strategies. If you're not sure what program or diet is best for you, ask your doctor or consult with a nutritionist or registered dietician for advice.

Remember, following a long-term or short-term diet may not be necessary for you and many diets out there simply don’t work, especially long-term. While we do not endorse fad diet trends or unsustainable weight loss methods, we present the facts so you can make an informed decision that works best for your nutritional needs, genetic blueprint, and budget, and goals.

If your goal is weight loss, remember that losing weight isn’t necessarily the same as being your healthiest self, and there are many other ways to pursue health. Exercise, sleep, and other lifestyle factors also play a major role in your overall health. The best diet is always the one that is balanced and fits your lifestyle.

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