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.
What is the SlimFast Diet?
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 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.
How the SlimFast Diet Works
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. 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).
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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.
Rules of the SlimFast Diet
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. 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 Products
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.
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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.
Cost of the SlimFast Diet
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.
Does SlimFast Work for Weight Loss?
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. 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).
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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. 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.
Benefits of the SlimFast Diet
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.
Downsides of the SlimFast Diet
While you may see some benefits to the SlimFast diet, there are many downsides to consider.
- One-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 Slimfast of today is different than you may remember: Gone are the days of shakes alone. The original plan promises you can drop pounds with tasty snacks, meal replacements, and one sensible meal a day. With the Slimfast plan, you eat two Slimfast meal replacements, three 100-calorie snacks (Slimfast snack bars, fruits, veggies, or even nuts), and one 500-calorie meal that you provide. With this plan you can eat 6 times per day. The program says you can lose a safe 1 to 2 pounds per week with this strategy.
SlimFast Advanced Nutrition
They also offer products called Slimfast Advanced Nutrition that provide high-protein meal-replacement shakes or smoothies with less sugar than many other brands. They have more protein than the original Slimfast products. The advanced plan also offers 100-calorie snacks and meal replacement bars. Meal replacements include a variety of flavors in their original and advanced protein meal bars, premade protein meal shakes, and powdered protein shake mixes (to be mixed with skim milk). In 2020, the company reformulated its drinks. For 100-calorie snack options, choose from a variety of flavors from their original Slimfast snack bars or from the Slimfast Advanced Nutrition snacks.
For the meal you provide, Slimfast recommends filling half of your plate with vegetables (such as green beans, carrots, eggplant, spinach, or broccoli); a quarter with lean protein (beef, poultry, pork, tofu, or fish); and the remaining quarter with starch (whole wheat pasta or bread, brown rice, potatoes, or corn). Nothing is totally off-limits with Slimfast, including alcohol. You have more flexibility with snacks and with the meal you provide, which includes lean protein, vegetables, and a starch.
Practical Considerations
- Cooking and shopping: Snack bars, meal replacement bars, and shakes are grab-and-go. You only plan and prepare one meal a day, which cuts down on trips to the grocery store.
- Packaged foods or meals: Required.
- In-person meetings: None.
- Exercise: Daily exercise, including walking and strength training, is encouraged. The program doesn’t offer a specific workout plan.
- Vegetarians and vegans: Vegetarians can have 4 to 6 ounces of a meat alternative, such as quinoa, tofu, lentils, or black beans, for their daily meal. Reduced- or low-fat dairy (yogurt, milk, cheese) is also OK. Vegans have fewer options, since most meal replacements are made with dairy or eggs, which aren't part of a vegan diet.
- Gluten-free: The Slimfast meal-replacement shakes and smoothies are labeled gluten-free. Other products are not labeled with regard to gluten, so check the ingredients list for possible sources of gluten, and contact the company if in doubt.
- Lactose-intolerant: If you have problems digesting lactose, you can combine Slimfast shake mixes with lactose-free milk or low-fat soy milk. Slimfast meal bars have 2 grams of lactose or less per serving. Slimfast Advanced Nutrition meal-replacement shakes are lactose-free.
- Cost: Slimfast products are sold in grocery stores and drugstores. Packages range from $9.99 to $89.99.
- Support: You can submit questions to Slimfast's online dietitian.
Is SlimFast Good for Certain Conditions?
Conditions such as diabetes, high cholesterol, and high blood pressure are driven by extra weight, so cutting calories must be part of their treatment. Anyone looking to lose weight on a calorie-restricted diet should work with a nutritionist or their doctor. Since the fastest weight loss happens during the first few weeks of dieting, your doctor may need to adjust any drugs you take, such as insulin or blood pressure medications.
Weight loss can make your blood sugar levels drop. Dizziness, fatigue, and other symptoms of low blood sugar may mean you need to check your blood sugar levels more often.