Dietitians Review of The SlimFast Diet: Does it Work for Weight Loss?

ishonest diet score: 2.29 out of 5

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

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

While many tout success on the diet thanks to its simple and convenient plan, you may wonder if 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 if it’s worth trying.

  • 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.

What is SlimFast?

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

Originally, the diet consisted of two low calorie meal replacement shakes and 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 (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.

The SlimFast diet has been around since 1977. It’s currently based on following a 1-2-3 Plan, which involves eating 1 whole food meal, 2 meal replacements, and 3 snacks each day.

How does SlimFast work?

The SlimFast diet is a partial meal replacement plan that involves eating 1 “sensible” meal, 2 meal replacements, and 3 snacks.

You make one meal on your own, although there are guidelines for what foods to include, and it’s recommended to keep 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, resulting in weight loss.

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

The SlimFast Diet also claims to keep your metabolism burning by eating frequent meals and snacks.

According to SlimFast, the SlimFast diet provides approximately 1,200 calories per day for women and 1,600 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 now offers a convenient free app 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. You can select preapproved recipes or add your own. The app also has a built-in calculator to help you know if you’re meeting your calorie, macronutrient, and water goals for the day.

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

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

The SlimFast diet requires you to purchase SlimFast meal replacements and snacks to achieve a calorie deficit. You can use the SlimFast Together app to track your progress, meal plan, and even order groceries.

How to get started with SlimFast

SlimFast offers four plans: Original, Favorite Foods, Low Carb, or 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 of 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 your plan is selected, you can then start a meal plan for the week or receive premade meal plans, which will include the ability to make an automated grocery list.

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

To get started, you’ll need to download the SlimFast Together app, where you can select your diet and meal plan. Alternatively, you can follow the SlimFast Original plan.

What can you eat on SlimFast?

While there are no foods that are banned on the SlimFast diet, you must restrict your calories.

To allow for flexibility, the diet permits one “sensible” meal per day, which can include any foods of your choosing.

Alcohol is allowed, though it’s suggested to keep your consumption to a minimum.

If you choose the SlimFast Keto plan, the program will recommend that you still follow the 1-2-3 Plan but choose from SlimFast’s line of keto products.

The Keto plan also recommends low carb vegetables, protein, and higher fat at each meal. Also, the app provides several keto-friendly recipes to make following the diet easier.

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, which comes in a variety of flavors.

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

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

Finally, the plan encourages followers to have three SlimFast snacks each day. However, you’re allowed to have your own snacks as long as they’re no more than 100 calories.

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.

Keto sample meal plan

For men, it’s recommended to add 200 calories to each SlimFast meal replacement daily. Alternatively, the program suggests doubling your meal replacements.

Though no foods are off-limits, you’re only allowed one homemade meal per day, which should be fewer than 600 calories and be high in protein.

How much does SlimFast cost?

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

Here are prices for SlimFast ready-to-drink meal replacement shakes (4):

  • $9.99 for a pack of 4 shakes
  • $22.99 for a pack of 12 shakes
  • $91.96 for a 4-pack of 12 shakes (48 bottles total)

Purchasing a 4-pack of 12 shakes is the cheapest at around $1.92 per bottle.

Meal replacement bars (excluding the Keto Bomb Bars) cost (5):

  • $9.99 for a pack of 5 bars
  • $19.98 for a 2-pack of 5 bars
  • $39.96 for a 4-pack of 5 bars

The 4-pack of 5 bars is the cheapest option at around $2.00 a bar.

Overall, the cheapest meal replacement options are the SlimFast shake mixes, which cost (6):

  • $9.99 for a 14-serving (12.83-ounce) container
  • $12.99 for a 22-serving (20.18 ounce) container

Depending on the size you buy, each meal would cost $0.59–$0.72.

You can also purchase 14-day starter kits that cost $69.99 for the Favorite Foods and Low-Carb 14-Day kits. 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 around $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.

The SlimFast diet varies in price depending on the products you buy and in what quantity. At a minimum, it will cost $5.00–$6.42 per day for their products.

Is SlimFast effective for weight loss?

If followed properly, the SlimFast diet can be an effective way to lose weight.

It works by reducing your calorie intake to create a calorie deficit, which is when you expend more calories than you consume.

If followed correctly, you would only consume around 1,200 calories per day on the program. Considering 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 could be between 800–1,400 calories per day (8).

There have been a number of older studies on SlimFast, which suggest SlimFast products may support meaningful weight loss. 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 four 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 lost an average of 10.5 pounds (4.8 kg), or 4.9% of their body weight after 6 months (12).

Additionally, those who stuck with the SlimFast Diet for 12 months lost an average of 10.6 pounds (4.8 kg), while those in the control group gained 1.3 pounds (0.6 kg) (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. This suggests meal replacement programs such as SlimFast may be difficult to follow long-term (12).

One review from 2015 reported a high risk of bias for many SlimFast studies, as several of these studies were funded by SlimFast (13).

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

Therefore, it appears that using SlimFast products may help you lose weight in the short term, though little data exists to support its long-term use.

More recently, a 2019 review found that meal replacement products may be effective in 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 behavioral therapy, behavioral therapy combined with SlimFast meal replacements, or a comprehensive nutrition intervention that focused on both internal and environmental influences on food choice (16).

As the SlimFast diet doesn’t teach sustainable habits, such as learning to prepare and cook healthy meals or reading nutrition labels, there’s a high chance that you’ll regain weight once you discontinue the program.

Though you may lose some weight on the program, most research suggests SlimFast isn’t sustainable in the long term. As a result, there’s an increased chance that you’ll regain weight once you discontinue the program.

Easy to follow

Unlike other diets, SlimFast provides the majority of your meals — which require little or no preparation — making it 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.


Instead of having to prepare multiple meals a day, you only need to prepare 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.

Some benefits of the SlimFast Diet include its ease of use, convenience, and promotion of other healthy lifestyle habits.

Potential downsides

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.

Food is very processed

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

However, most of the 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 also want to pay close attention to each ingredient list.

Also keep in mind that as the products may have been manufactured in facilities that handle common allergens, prepackaged 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 a day, the diet is still very restrictive.

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

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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.

If you decide to discontinue the diet, you’re left back where you started, which can result in a return to old habits (17).

Meals are low in calories

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

If a person were to consume two meal replacements and three 100-calorie snacks per day, this equals 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 a day.

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

There are many downsides to the SlimFast diet: It’s restrictive, low in calories, highly processed, and focuses on a one-size-fits-all approach.

Alternatives to SlimFast

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.
  • Daily Harvest. Daily Harvest mostly offers convenient breakfasts and snack options such as smoothies, oat bowls, and harvest bowls made from whole, unprocessed ingredients.
  • HelloFresh. HelloFresh 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.
  • Purple Carrot. Purple Carrot is a vegan meal delivery service that provides breakfast, lunch, and dinner options made from completely plant-based ingredients.
  • The Good Kitchen. The Good Kitchen promises delicious and nutritious meals made from the highest quality ingredients. It works closely with farms to ensure ethical and sustainable practices while also supporting the local food communities.
  • Freshly. Freshly is ideal for people looking for single-serving meals that can be quickly reheated in the microwave. Meals require zero prep time, so Freshly is designed for busy adults in need of a quick and easy meal.

Should you try SlimFast?

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, 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.

The SlimFast diet is highly processed, expensive, keeps you reliant on their products, and isn’t sustainable. Due to this, it’s probably best to avoid it.

The bottom line

The SlimFast diet remains a popular weight loss program, despite its many flaws.

By following SlimFast’s 1-2-3 Plan, you may achieve short-term weight loss as 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.