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The SlimFast plan is a popular weight loss program that has been around for decades.

It consists of meal replacement shakes and food products designed to promote weight loss.

While many people tout success on the diet thanks to its simple and convenient plan, you may wonder whether SlimFast really works and whether it’s sustainable.

This article takes a closer look at the SlimFast diet, including how to follow it, its potential benefits and downsides, and whether it’s worth trying.

Diet review scorecard
  • Overall score: 2.29
  • Weight loss: 2.5
  • Healthy eating: 2
  • Sustainability: 2
  • Whole body health: 2
  • Nutrition quality: 1.75
  • Evidence-based: 3.5

BOTTOM LINE: The SlimFast diet can help you lose weight if you are willing to trade most of your daily foods for ready-made snacks and shakes. It provides an easy short-term solution but may not be the best long-term investment.

Pros

  • may be effective for short-term weight loss
  • convenient
  • easy to follow
  • SlimFast Together App encourages other healthy lifestyle practices, including sleep, stress management, and exercise

Cons

  • expensive
  • one-size-fits-all approach
  • may feel restrictive
  • relies on highly processed products
  • makes unsubstantiated health claims about some products
  • is not designed to support sustainable, long-term weight loss

The SlimFast diet is a weight loss plan that has been around since 1977.

Originally, the diet consisted of two low calorie meal replacement shakes per day, along with a “sensible” dinner.

Since then, the company has expanded its product line to include shakes, smoothies, bars, and snacks. It also now offers products in Original, Advanced Nutrition, Advanced Energy, Diabetic Weight Loss, and Keto formulations.

The revamped SlimFast diet is based on a 1-2-3 Plan, which includes 1 “sensible” meal, 2 meal replacements, and 3 snacks per day. Under the 1-2-3 Plan, you can select from four versions: Original, Favorite Foods, Keto, and Low-Carb (1).

The company states that you can lose weight in as little as 1 week, but many skeptics believe the diet is unsustainable and expensive.

Who is SlimFast for?

SlimFast products are designed for people looking to lose weight quickly and conveniently.

The products are low in calories and easy to take on the go, so they can be a good option for busy people who are looking to lose weight.

SlimFast products are also commonly purchased by people who want a straightforward diet plan that involves minimal grocery shopping and food preparation.

However, keep in mind that these products aren’t cheap and may not be accessible for all budgets.

The SlimFast diet is a partial meal replacement plan that involves eating 1 “sensible” meal, 2 meal replacements, and 3 snacks per day. This is known as the “SlimFast 1-2-3 Plan” or the “SlimFast Plan.”

You can choose to follow one of four versions of this plan:

  • Original. This plan uses popular SlimFast products that have been around for more than 40 years. However, it appears that the company is slowly phasing out this plan in favor of the Favorite Foods, Keto, and Low-Carb plans.
  • Favorite foods. This plan does not have specific macronutrient goals and allows for one “SlimFast approved” meal per day as long as you stay within your calorie limits. This is considered the company’s most flexible and easy-to-follow plan.
  • Keto. Designed for people following a keto diet, this plan focuses on a high fat diet by combining SlimFast keto products and one “sensible” keto meal. This plan is the hardest to follow because of the strict macronutrient targets needed to maintain ketosis.
  • Low-Carb. Unlike the Keto Plan, the Low-Carb plan focuses less on consuming a high fat diet and more on eating fewer carbohydrates.

Regardless of which plan you choose, you’ll be instructed to make one meal per day on your own, although there are guidelines for which foods to include and the company recommends keeping the meal under 600 calories.

The company’s website and app offer numerous recipes that fit this requirement.

The rest of your meals will be replaced by a SlimFast shake, smoothie, or bar. You’re also allowed three 100-calorie snacks per day from the SlimFast snack product line.

The idea behind the program is that by replacing your regular meals and snacks with low calorie choices, you can reduce your daily calorie intake, which should result in weight loss.

Additionally, some SlimFast meal replacement products are high in protein, which may help reduce your appetite (2).

The SlimFast diet also claims to keep your metabolism burning because you consume frequent meals and snacks.

According to SlimFast, the SlimFast diet provides approximately 1,200–1,300 calories per day for women and 1,600–1,700 calories per day for men (3).

Additionally, the program recommends pairing the diet with at least 30 minutes of exercise per day to further promote a calorie deficit needed for weight loss.

By following the SlimFast diet, you’re expected to lose 1–2 pounds (0.45–0.91 kg) per week (3).

SlimFast Together app

SlimFast Together is a relatively new app the company offers to help you stay on track and achieve your weight loss goals.

When you first open the app, you’ll complete a 5-minute quiz to identify your goals and preferences.

You’ll then be able to use the app for meal planning by selecting preapproved recipes or adding your own. The app also has a built-in calculator to help you know whether you’re meeting your calorie, macronutrient, and water goals for the day.

Perhaps the most convenient aspect is the grocery ordering feature that automatically creates a shopping list from prepared menu items and allows you to place a grocery delivery or pickup order from a local retailer.

The app also includes a progress tracking feature that allows you to enter information about your current weight, measurements, diet, water intake, exercise, sleep, and stress. You can even add progress photos.

SlimFast Together is currently free to download on both Apple and Android devices.

SlimFast offers four plans: Original, Favorite Foods, Low-Carb, and Keto.

To sign up for the Favorite Foods, Low-Carb, or Keto plan, you’ll be required to first download the SlimFast Together app.

If you’re unsure which plan to choose, you can complete a quick quiz on the app, which will provide a personal plan recommendation based on your goals, preferences, and lifestyle.

Once you’ve selected a plan, you can choose a pre-designed meal plan or create a customized meal plan for the week. The app will then curate a shopping list, which you can take with you to the store or use to order groceries online.

If you prefer not to download the app, you can sign up for the SlimFast Original plan on the company’s website. With this plan, you’ll receive a quick-start guide and a 7-day meal planner to help you stay on track.

While no foods are banned on the SlimFast diet, you must restrict your calories. If you’re following the Keto or Low-Carb plan, you will also need to watch your macronutrient intake.

Alcohol is allowed, though the diet suggests keeping your consumption to a minimum.

To allow for flexibility, the diet permits one “sensible” meal per day, which can include any foods of your choosing from a list of “SlimFast approved” meals. These will vary depending on which plan you’re following.

Additional guidelines vary depending on the plan you choose. For example, the Keto plan requires you to eat a high fat diet and use only SlimFast keto products.

In contrast, the Low-Carb plan focuses mostly on the carb count of your meals and allows you to use a combination of SlimFast Advanced Nutrition and keto products.

The Favorite Foods plan is the most flexible option, allowing you to select any of SlimFast’s products. This plan also allows you to choose any “sensible” meal, regardless of macronutrient content, as long as it’s SlimFast-approved.

Regardless of which plan you choose, you must also have two SlimFast shakes, smoothies, or bars as a replacement for two meals each day.

SlimFast shakes and smoothies come as powdered mixes or in ready-to-drink bottles. Alternatively, you can have a meal bar. The bars come in a variety of flavors.

The meal replacement shakes and mixes provide roughly 180 calories and 14–20 grams of protein, though the amounts of carbohydrates, fiber, and added sugar depend on the product line (4).

Likewise, the meal replacement bars provide around 180 calories and 7–15 grams of protein, depending on the product line (5).

The plan also encourages followers to have three snacks each day. These can consist of SlimFast products or foods of your own choosing, as long as the snack has no more than 100 calories and fits into the macronutrient goals of your plan type.

Finally, the SlimFast Together app provides a meal planning tool and several recipes for your “sensible” meal to make the diet easier to follow.

Sample meal plan

The SlimFast website and SlimFast Together app provide a number of sample meal plans. Below are examples of what a day of eating could look like on each plan.

Favorite Foods sample meal plan

  • Breakfast: SlimFast Meal Shake
  • Snack: banana
  • Lunch: SlimFast Meal Bar
  • Snack: 3 cups (24 grams) air-popped popcorn
  • Dinner: Chicken Tikka Masala
  • Snack: SlimFast Double Chocolate Mint Crunch Mini Snack Bar

Low Carb sample meal plan

  • Breakfast: grilled veggie omelet
  • Snack: 1/2 cup (143 grams) Greek yogurt
  • Lunch: SlimFast Meal Shake
  • Snack: 1 slice of cheese
  • Dinner: SlimFast Diabetic Weight Loss Meal Bar
  • Snack: SlimFast Diabetic Weight Loss Snacks

Keto sample meal plan

  • Breakfast: SlimFast Keto Meal Shake
  • Snack: SlimFast Keto Fat Bomb Real Cheddar Cheese Crisp
  • Lunch: SlimFast Keto Fat Bomb Meal Bar
  • Snack: baked zucchini fritters
  • Dinner: baked salmon with lemon butter and steamed asparagus
  • Snack: SlimFast Keto Fat Bomb Dark Chocolate Almond Snack Square

For men, the program recommends adding 200 calories to each SlimFast meal replacement or doubling your meal replacements.

The cost of the SlimFast diet depends on which products you use, how often you use them, and whether you buy them in bulk.

Here are the current prices of popular SlimFast products (4, 5, 6):

  • Ready-to-drink meal replacement shakes: $9.99 for a pack of 4 shakes or $91.96 for a 4-pack of 12 shakes (48 bottles total)
  • Meal replacement bars (excluding the Keto Bomb Bars): $9.99 for a pack of 5 bars or $39.96 for a 4-pack of 5 bars (20 bars total)
  • SlimFast shake mixes: $9.99 for a 14-serving (12.83-ounce) container or $12.99 for a 22-serving (20.18-ounce) container

Overall, the cheapest meal replacement options are the SlimFast shake mixes, which cost $0.59–$0.72 per meal, depending on the size you buy (6).

You can also purchase 14-day starter kits. The Favorite Foods and Low-Carb 14-Day kits cost $69.99, and the Keto 14-Day Starter Kit is more expensive at $89.99 (7).

These kits include 2 meal replacements and 3 snacks per day and would have a daily cost of $5.00–$6.42 (excluding taxes) (7).

For a month, this equals around $140 at minimum. Keep in mind that this doesn’t include the cost of your “sensible” meal (7).

You can potentially lower the cost by preparing some of your own snacks instead of purchasing the premade snack options.

Learn more at SlimFast.

The SlimFast diet can promote weight loss by reducing your calorie intake to create a calorie deficit, meaning that you expend more calories than you consume.

If following the program correctly, you would consume only around 1,200 calories per day. Considering that the average, moderately active female needs around 2,000 calories per day, this means you’ll have at least an 800-calorie deficit (8).

Furthermore, as SlimFast recommends around 1,600 calories per day for men, the calorie deficit for men could be 800–1,400 calories per day (8).

Some older studies on SlimFast products suggest they may support meaningful weight loss. However, most studies on SlimFast are older, which may be due to the growing trend toward programs that promote sustainable weight loss (9, 10, 11).

For example, a 2006 study evaluated the effectiveness of 4 popular diet plans, including SlimFast, over the course of 6 months in 293 people with a BMI between 27 and 40 (12).

The study found that those in the SlimFast group had lost an average of 10.5 pounds (4.8 kg), or 4.9% of their body weight, after 6 months (12).

However, 29% of participants dropped out of the study because they couldn’t tolerate the diet, and only 9 of the original 58 participants were able to maintain the diet for 12 months at follow-up.

This suggests meal replacement programs such as SlimFast may be difficult to follow long-term (12).

Further, two reviews of meal replacement companies and popular weight loss programs found that SlimFast may support short-term weight loss but is not significantly more effective for weight loss than competitor programs (13, 14).

More recently, a 2019 review found that meal replacement products may be effective for achieving a calorie deficit but should be paired with other behavioral strategies, such as counseling, developing food skills, and enhancing self-efficacy (15).

Similarly, a 2018 study compared the effects of behavioral therapy, behavioral therapy combined with SlimFast meal replacements, and a comprehensive nutrition intervention that focused on both internal and environmental influences on food choice (16).

By the end of the study, the comprehensive nutrition intervention had led to the most weight loss. The authors suggested this was due to greater cognitive restraint, self-control skills, and self-efficacy, which are important factors for sustainable weight loss (16).

Though the SlimFast Together app provides some basic meal planning and progress tracking tools, it does not provide one-on-one counseling.

It also relies heavily on SlimFast products instead of teaching sustainable habits such as learning to plan and prepare all meals and reading nutrition labels.

Therefore, it appears that using SlimFast products may help you lose weight in the short term, but there’s a high chance that you’ll regain weight once you discontinue the program.

There are some benefits to the SlimFast diet.

Easy to follow

Unlike many other diets, SlimFast provides the majority of your meals, and the meals require little or no preparation, making the plan easy to follow.

Furthermore, as SlimFast meal replacements are pre-portioned, you don’t have to worry as much about figuring out portion sizes or overeating.

The program also provides easy-to-follow recipes, and the SlimFast app helps you stay organized.

Convenience

Instead of having to prepare multiple meals a day, you need to prepare only one. To make things even easier, SlimFast offers simple preapproved recipes that can help you stay on track.

Additionally, SlimFast products are convenient for people who are on the go and don’t have time to prepare a meal.

Promotes other healthy habits

The new SlimFast Together app includes useful tools to track your healthy lifestyle habits, such as sleep, stress, food and water intake, and physical activity.

Though SlimFast doesn’t provide personalized recommendations, this new addition can help users focus on a well-rounded healthy lifestyle.

While you may see some benefits to the SlimFast diet, there are many downsides to consider.

Ones-size-fits-all approach

A major downside to the SlimFast diet is its one-size-fits-all approach.

The diet assumes that everyone requires the same number of calories per day, without taking into consideration your height, weight, activity level, age, and other factors that can affect how many calories and nutrients you need in a day.

All four available plans focus on a calorie goal of 1,200–1,300 calories per day for women and 1,600–1,700 calories per day for men.

Highly processed foods

The healthiest diets consist of mostly whole, minimally processed foods high in fiber, protein, healthy fats, and beneficial plant compounds such as antioxidants.

However, most SlimFast products are highly processed, containing a long list of added ingredients such as added sugars, protein powders, and artificial sweeteners, colors, and flavors.

Lack of variety for different dietary needs

The majority of SlimFast products use milk as the main ingredient, and there are no alternative options for people who follow a plant-based diet or wish to avoid dairy.

If you have food allergies, you’ll want to pay close attention to each ingredient list.

Also keep in mind that, because they may have been manufactured in facilities that handle common allergens, packaged SlimFast products likely aren’t safe if you have a severe food allergy or intolerance.

Can feel restrictive

Though the diet allows you to have one meal of your choice each day, it is still very restrictive.

Even if you can eat whatever you want for one meal, you’ll still need to watch your portion sizes and calories. For many, this doesn’t leave much room for foods they enjoy.

Furthermore, if you choose to have your “sensible” meal in the morning, you’re only allowed to have meal replacements the rest of the day. This can be very limiting and not flexible enough for day-to-day life.

Doesn’t focus on behavior change

The SlimFast diet mostly focuses on calories rather than teaching its users to adopt healthy, sustainable habits such as home cooking, mindful eating, and reading nutrition labels.

Though the SlimFast app provides resources such as progress tracking, blog posts, and recipe ideas, the diet still relies on using products to replace two or more meals and snacks per day.

Therefore, it’s unlikely that you will develop sustainable habits, and you may be likely to return to your old habits once you chose to discontinue the diet (17).

Low calorie meals

Most SlimFast meal replacement shakes and bars contain around 180 calories each, which is not enough to replace a meal.

If a person were to consume two meal replacements and three 100-calorie snacks per day, that would equal around 660 calories from SlimFast products. This leaves around 540 calories for the “sensible” meal, which is 45% of your daily calories dedicated to one meal per day.

Ultimately, these meals may not provide enough calories and may leave you feeling hungry throughout the day.

SlimFast has been around since 1977 and has built itself a reputation as a popular weight loss brand. The brand has undergone multiple acquisitions in its lifetime.

Currently, SlimFast is headquartered in Palm Beach Gardens, Florida, and is owned by Glanbia.

The company currently holds a B+ rating from the Better Business Bureau and has not received any FDA or FTC warning letters. The company also provides contact information on its website, though it doesn’t include a return/exchange policy.

While the brand has historically been subject to lawsuits, it hasn’t been named in any lawsuits recently. However, the company’s current and previous owners (Glanbia, Unilever, Nutrisystem) have been involved in lawsuits unrelated to the SlimFast brand or products.

SlimFast as a company passed Healthline’s extensive vetting process, as it doesn’t have a known history of unethical business practices.

However, several of SlimFast’s products failed vetting because of sensationalized marketing language, including unsubstantiated claims that certain products, including SlimFast Advanced Energy meal replacement shakes, can “curb cravings” and “halt hunger.”

SlimFast products generally receive an average of 4 out of 5 stars on third-party retailer sites. This suggests that customers who use SlimFast products usually enjoy them.

SlimFast is far from the only diet program on the market. Here’s a quick look at how SlimFast compares to some of its main competitors:

BasicsMenu optionsCostProsCons
SlimFastrelies on meal replacement drinks and bars to achieve a calorie deficit• meal replacement drinks and bars
• 1 homemade meal per day
• 3 snacks per day, can be packaged or homemade
• keto and low carb plans available
• $69.99 for a 14-day starter kit
• varies by product type and amount
• convenient
• easy to follow
• app includes meal planning tools
• app provides more well-rounded lifestyle tips and guidance
• may support short-term weight loss
• expensive
• relies on highly processed products
• very low in calories and nutrients
• not supportive of long-term weight loss
Noomuses a mobile app to track food intake• green, yellow, and red light system for staying within calorie budget
• all foods are allowed
$199 per year• promotes behavior change
• does not restrict large groups of foods
• virtual coaching
• shown to be effective for short-term weight loss
• expensive
• focuses on weight loss over whole body health
• requires a smartphone or tablet
• low calorie diet may not be suitable for everyone
Optaviaencourages a low calorie and reduced carb diet via homemade meals, premade products, and personalized coaching• more than 60 Optavia products to choose from
• 1 low carb homemade meal per day
• 1 optional low carb snack per day
$2.96–$3.60 per serving for packaged items• easy to follow
• personal coaching
• good variety of premade food options
• expensive
• very low in calories
• not supportive of long term weight loss
• not suitable for all dietary needs
• uses highly processed premade products
• uses coaches who aren’t qualified to give nutritional and medical advice
Atkinsencourages a low carb diet patternlimit high carb food items and prioritize minimally processed, protein-rich foods• varies depending on foods purchased
• encourages expensive higher quality meats and dairy products
• may support long-term weight loss
• does not rely on premade products
• encourages whole foods
• may not provide enough carbs and fiber for some
• can be high in saturated fat
• may be difficult to follow

The best diets are those that are affordable, enjoyable, and sustainable (18).

While the SlimFast diet may help you lose weight, the products are expensive and highly processed and don’t provide enough calories to replace a meal.

What’s more, the SlimFast diet doesn’t provide support or teach you skills to help manage your weight when you discontinue the program. As such, you’re likely going to be reliant on SlimFast products or regain some of the weight you lost once you discontinue the program.

Instead, you’re better off focusing on positive behavior changes that lead to gradual, sustainable weight loss, such as getting regular exercise and eating whole, minimally processed foods that are high in protein, fiber, and healthy fats.

If you’re looking to lose weight or want to explore personalized nutrition recommendations, it’s best to work one-on-one with a registered dietitian.

Heads-up

Trying to “do it right” when it comes to nutrition may feel tempting, but it can backfire. If you are preoccupied with food or your weight, feel guilt surrounding your food choices, or routinely engage in restrictive diets, consider reaching out for support. These behaviors may indicate a disordered relationship with food or an eating disorder.

Disordered eating and eating disorders can affect anyone, regardless of gender identity, race, age, socioeconomic status, or other identities.

They can be caused by any combination of biological, social, cultural, and environmental factors — not just by exposure to diet culture.

Feel empowered to talk with a qualified healthcare professional, such as a registered dietitian, if you’re struggling. You can also chat, call, or text anonymously with trained volunteers at the National Eating Disorders Association helpline for free or explore the organization’s free and low cost resources.

If you want to manage your weight in a more sustainable way, there are many meal planning and delivery companies that provide nutritious prepared or partially prepared meals that are still convenient but can help with healthier eating and portion control, including:

  • PlateJoy. PlateJoy is a meal planning app that provides personalized, custom meal plans designed by registered dietitians. You can design your meal plan around your unique needs, dietary requirements, and lifestyle goals.
  • Sunbasket. Sunbasket provides weekly meal kit boxes with recipe cards and premeasured ingredients to help you make nutritious and delicious recipes. Plus, the service caters to a variety of dietary needs and preferences.
  • Daily Harvest. Daily Harvest mostly offers convenient breakfasts and snack options such as smoothies, oat bowls, and harvest bowls made from whole, unprocessed ingredients.
  • Factor. Factor is ideal for people looking for single-serving meals that can be quickly reheated in the microwave. Meals require zero prep time — Factor is designed for busy adults in need of a quick and easy meal.

Is SlimFast a good way to lose weight?

Though you may lose weight quickly using SlimFast products, you’ll likely regain the weight after you stop the program.

This is because the company fails to teach sustainable habits, such as meal planning, grocery shopping, and preparing nutritious meals. Therefore, you may be reliant on the products and approved recipes to sustain weight loss.

The program is also expensive and very low in calories, which can make it difficult to stick with.

How much weight can you lose in a week with SlimFast?

How much weight you will lose with SlimFast depends on the size of your calorie deficit, which depends on the numbers of calories you consume and expend through physical activity.

One SlimFast study found that participants lost an average of 10.5 pounds (4.8 kg) in 6 months, which equates to around 2.5 pounds (1.1 kg) per week. However, participants had a hard time sticking to the diet long-term, which may make it difficult to sustain this weight loss (12).

Most experts agree that losing 1–2 pounds (0.45–0.9 kg) per week is sustainable and healthy for most people as long as they do not feel deprived and can eat a variety of foods.

Can SlimFast make you gain weight?

SlimFast products alone won’t cause you to gain weight. However, if consuming the products causes you to consistently consume more calories than you expend in a day, you could end up gaining weight over time.

The SlimFast diet remains a popular weight loss program, despite its many flaws. It’s commonly known as the SlimFast 1-2-3 plan or simply the SlimFast Plan.

By following one of SlimFast’s 1-2-3 plans (Original, Favorite Foods, Keto, or Low-Carb), you may achieve short-term weight loss because you’re restricting your calorie intake. However, most research suggests that diets like SlimFast are not sustainable.

Additionally, the SlimFast diet relies on highly processed meal replacement products and fails to teach its users how to maintain their weight loss once they stop the program.

All in all, it’s probably best to skip the SlimFast diet.