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.

What Is the SlimFast Diet?

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 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.

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

The 7-Day Diet Plan

On the SlimFast plan, you can eat one calorie-controlled meal daily (500 to 800 calories) and three 100-calorie snacks. Besides the calorie restriction, these meals and snacks can be anything you like, so a week on the plan will vary considerably based on your choices and preferences.

  • Day 1: SlimFast shake; 1/4 cup raisins; SlimFast shake; SlimFast snack; palak chicken with brown rice; SlimFast snack
  • Day 2: SlimFast shake; SlimFast snack; SlimFast shake; 3 cups air-popped popcorn; baked pasta with lemon ricotta; SlimFast snack
  • Day 3: SlimFast shake; SlimFast snack; chicken verde quinoa casserole with 1/2 avocado; SlimFast snack; SlimFast shake; 1 cup unsweetened applesauce
  • Day 4: SlimFast shake; SlimFast snack; SlimFast shake; 4 whole-wheat crackers with fat-free cheese; habanero cheese grits with pan-blackened fish; SlimFast snack
  • Day 5: SlimFast shake; SlimFast snack; SlimFast shake; 1 cup mixed berries; Spanish-style shrimp paella; SlimFast snack
  • Day 6: SlimFast shake; SlimFast snack; beef, brown rice, and mushroom soup, fruit salad; 1 hard-boiled egg; SlimFast shake; SlimFast snack
  • Day 7: Buckwheat groats, turkey sausage; SlimFast snack; SlimFast shake; 1 ounce cream cheese with celery and bell peppers; SlimFast shake; SlimFast snack

What You Can Eat

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.

SlimFast Shakes

The SlimFast Diet is centered around shakes. There are several options for what to mix in with the shake powder. Mixing the powder with water is one option, but according to SlimFast, adding unsweetened nut milk can add up to 8 grams of protein to the shakes while adding as few as 25 calories. Some options include almond milk, cashew milk, macadamia milk, and coconut milk. The nutrition information below is for the shake mix only.

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.

"Sensible" Meals

The SlimFast diet allows for one "sensible" meal per day. You may consume any foods you like, as long as you stay within the caloric limits.

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

What You Cannot Eat

No foods are explicitly banned on the SlimFast diet, but calories are restricted. You may choose to have alcohol in moderation, but SlimFast suggests limiting consumption since alcohol provides only calories and no nutrients.

How to Prepare the SlimFast Diet & Tips

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 nutrient-dense foods. The meal must also be very low in calories, limited to just 500 calories for women and 800 calories for men.

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.

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 of the SlimFast Diet

While health experts do not recommend the SlimFast plan, it does have a few practical advantages.

  • 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 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, although 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.
  • May induce weight loss: 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.

Cons of the SlimFast Diet

There are some significant drawbacks to the SlimFast program, particularly if you follow it for an extended period.

  • Contains processed foods: The program is built primarily on processed foods. Most health experts recommend decreasing the amount of processed food you consume and increasing your intake of fresh or frozen fruits and vegetables, lean meats, seafood, and whole grains for optimal health. Studies continue to show a link between processed foods and chronic diseases, which is why consuming these products for the long term is not recommended.
  • 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 can consume a hearty 500 to 800 calories at your main meal, eating out or buying ready-to-eat processed food could be tempting.
  • 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, no structured plan helps to transition you off the diet.
  • No personalized support: While no personalized nutritional guidance exists, 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.
  • High in added sugar: SlimFast shakes are high in sugar, including added sugar. If you ate a lot of sweets before the diet, you might 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.

Is the SlimFast Diet a Healthy Choice for You?

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 most meals.

The U.S. Department of Agriculture's 2020–2025 Dietary Guidelines for Americans recommends consuming various 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 restricts all of the recommended foods for two out of three daily meals. Some SlimFast snacks may be high in sodium, and a few shakes contain added sugars.

While the SlimFast diet plan has helped many people 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 needed for sustained weight loss, like meal planning and preparation.

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 for some people. Use this calculator to determine the correct 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 whole foods with shakes containing artificial ingredients and added sugars. SlimFast snacks—though low in calories—are highly processed foods.

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 unsure what program or diet is best for you, ask your doctor or consult 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 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, 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 significantly affect your overall health. The best diet is always the one that is balanced and fits your lifestyle.

14 Sources
Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. U.S. News & World Report. Best Diets 2021: SlimFast.

  2. SlimFast & World Pantry. Shop SlimFast.

  3. SlimFast. Can I use water when using the smoothie mix?

  4. SlimFast. FAQ: Can I drink alcohol on the SlimFast plan?.

  5. SlimFast. How it works.

  6. Slim Fast. FAQ. What is the average weight loss on the SlimFast Plan?

  7. Brindal E, Hendrie GA, Taylor P, Freyne J, Noakes M. Cohort analysis of a 24-week randomized controlled trial to assess the efficacy of a novel, partial meal replacement program targeting weight loss and risk factor reduction in overweight/obese adults. Nutrients. 2016;8(5). doi:10.3390/nu8050265

  8. Lowe MR, Butryn ML, Thomas JG, Coletta M. Meal replacements, reduced energy density eating, and weight loss maintenance in primary care patients: a randomized controlled trialObesity (Silver Spring). 2014;22(1):94-100. doi:10.1002/oby.20582

  9. Hall KD, Ayuketah A, Brychta R, et al. Ultra-processed diets cause excess calorie intake and weight gain: An inpatient randomized controlled trial of ad libitum food intake. Cell Metab. 2019;30(1):67-77. doi:10.1016/j.cmet.2019.05.008

  10. Gramza-Michałowska A. The effects of ultra-processed food consumption-is there any action needed?Nutrients. 2020;12(9). doi:10.3390/nu12092556

  11. DiNicolantonio JJ, Lucan SC, O’Keefe JH. The evidence for saturated fat and for sugar related to coronary heart diseaseProg Cardiovasc Dis. 2016;58(5):464-472. doi:10.1016/j.pcad.2015.11.006

  12. U.S. Department of Health and Human Services and U.S. Department of Agriculture. 2020–2025 Dietary Guidelines for Americans, ninth edition.

  13. Berkowitz RI, Wadden TA, Gehrman CA, et al. Meal replacements in the treatment of adolescent obesity: a randomized controlled trial. Obesity. 2011;19(6):1193-9. doi:10.1038/oby.2010.288

  14. Ducrot P, Méjean C, Aroumougame V, et al. Meal planning is associated with food variety, diet quality and body weight status in a large sample of French adults. Int J Behav Nutr Phys Act. 2017;14(1):12. doi:10.1186/s12966-017-0461-7

By Malia Frey
 Malia Frey is a weight loss expert, certified health coach, weight management specialist, personal trainer​, and fitness nutrition specialist.