SlimFast is a true OG when it comes to fad diets. Founded in 1976 by S. Daniel Abraham, the company was sold to Unilever in 2000 for $2.3 billion, then sold again in 2014 for an undisclosed sum (via Forbes). So, while it’s true that the SlimFast plan is controversial, there’s also no denying the fact that it’s been popular.
In fact, according to Kainos Capital, the company’s current owner, SlimFast has the number one brand recognition of any weight loss company — more than Weight Watchers, Jenny Craig, and even Atkins. SlimFast is practically synonymous with weight loss shakes, and it’s hard to imagine that anyone hasn’t seen the iconic cans or bottles on grocery store shelves.
In the 40-plus years that SlimFast has been in business, plenty of experts have weighed in on the plan itself, as have diet veterans themselves. Whether you’re thinking about trying the SlimFast plan yourself or you’re just curious about what it entails, and whether or not it’s actually healthy, here’s everything you need to know about SlimFast and the SlimFast plan.
The SlimFast plan calls for drinking two shakes and eating one meal per day
SlimFast sells meal replacement shakes, meal replacement bars, and snack bars. But what they’re really selling is the SlimFast plan, a diet they say will help you lose weight. According to the SlimFast website, the SlimFast diet requires that you drink two meal replacement shakes and eat one "sensible" meal per day. The traditional shakes contain a balance of protein, carbs, and fat, while the new Keto shakes are high fat and low carb.
You might be wondering what a sensible meal entails, exactly. Although the SlimFast website doesn’t spell it out, WebMD explained that your one meal on the SlimFast diet should contain around 500 calories. Presumably, a "sensible" meal is one that contains a combination of vegetables, lean protein, and high-fiber starchy carbs like whole grains and potatoes. However, the focus of the SlimFast diet seems to be more on how many calories you consume than what kind of food you eat, so it’s probably within the rules to eat any type of meal as long as it clocks in at right around 500 calories.
You can eat three 100-calorie snacks on the SlimFast plan
In addition to your meal shakes and your one meal, you’ll be able to eat three 100-calorie snacks each day on the SlimFast plan to tide you over in between (via SlimFast). What you choose to eat is up to you. Whether you’re hoping for something sweet or savory, there are plenty of low-calorie snacks to choose from. As Eat This, Not That explained, many brands sell pre-packaged 100-calorie versions of their products, like Health Warrior Chia Bars, Clif Bars, Krave Pork Sticks, Blue Diamond Almonds and even Oreos. Or, you could go the DIY route and put together a 100-calorie snack yourself.
If you’re in the mood for something sweet but still want a little protein, try a half-cup of cottage cheese or yogurt with a half-cup of chopped melon. If you’d rather something savory, go for two tablespoons of hummus served up with two-thirds cup of carrots. If you want something that tastes like breakfast, slice a hard boiled egg and lay it on a plain rice cake.
SlimFast shakes don’t contain as many calories as you might expect
The thing about the SlimFast meal replacement shakes is that they’re far lower in calories than your typical meal. Each one of the original SlimFast shakes contains between 180 and 190 calories, 5 grams of fat, 25 grams of carbs, and 10 grams of protein (via SlimFast).
The brand also have two additional lines of shakes with the same number of calories but different nutrients compared to the original shakes. The Advanced Nutrition line is higher in protein and fat and lower in carbs, with between 180 and 190 calories per shake, 9 grams of fat, 7 grams of carbs, and 20 grams of protein, whereas the Keto line is higher in fat and a little lower in protein than the Advanced Nutrition line, with 180 calories, 14 grams of fat, 7 grams of carbs, and 8 grams of protein.
No matter which line you choose, the bottom line is that the shakes are very low in calories. While all contain fat, protein, and fiber to help keep you full, it’s likely that this small amount of calories won’t be enough for many people.
The SlimFast plan provides approximately 1,200 calories per day for women
Add up two 180-calorie shakes, one 500-calorie meal, and three 100-calorie snacks, and the SlimFast diet comes in at around 1,200 calories per day for women (via Healthline). For reference, the United States Department of Agriculture’s 2015–2020 Dietary Guidelines for Americans recommends 2,000 calories per day for most adults, although obviously the actual number of calories you need in a day varies greatly depending on your age, weight, gender, lifestyle, genetics, sleep quality, stress levels, and more. Still, 1,200 calories is a very, very low calorie number for most people.
Although it may be possible to sustain yourself on this amount of calories, experts believe it is the bare minimum. "The big picture is to consume enough calories with a balance of nutrients and engage in physical activity for good health management to achieve one’s weight goals," dietitian Andrea Spivack told Everyday Health. "Consuming less than 1,200 calories per day may make it difficult to meet vitamin and mineral needs via food." That might be even more of a concern when you’re getting about 400 of those calories — one-third of your total intake — from processed shakes instead of vitamin and mineral-rich whole foods.
The SlimFast plan provides approximately 1,600 calories per day for men
If you read the fine print, the SlimFast plan is actually slightly different for men and women. Whereas women drink only a single shake for meal replacement, SlimFast recommends that men add another 200 calories to each meal replacement — either by drinking two pre-made shakes, eating two meal replacement bars, or adding two scoops of SlimFast powder to make a shake instead of just one (via SlimFast). That additional 400 calories per day means that men eat around 1,600 daily calories on the SlimFast diet: two 400-calorie meal replacements, one 500-calorie meal, and three 100-calorie snacks.
Sticking to 1,600 calories per day can be an effective way for some women to lose weight, but active women and men will likely need more calories to feel satisfied, registered dietitian nutritionist Anne Danahy explained in an article for Livestrong. As such, the 1,600 calorie plan may actually be more appropriate for women, and men may need to eat more to feel satisfied and prevent symptoms like ongoing hunger and fatigue.
Experts say the SlimFast plan will produce short-term weight loss
If you eat a very low calorie diet, which is essentially what the SlimFast plan is, you’ll almost definitely lose weight in the short term. According to the experts that review diets for the U.S. News & World Report annual diet rankings, the SlimFast diet is designed that way.
The SlimFast diet performed above average compared to other diets in the short-term weight loss category. "The diet will likely deliver fast results, thanks in large part to its 1,200-calorie-a-day regimen," the report stated. "Experts said it’s a moderately effective way to lose weight in the short term."
As the experts pointed out, the short-term weight loss isn’t thanks to the shakes themselves, but because of the low-calorie nature of the SlimFast plan. In other words, you won’t get any benefit from drinking SlimFast shakes or eating SlimFast bars if you’re not also drastically cutting back on the other foods you’re eating. As the diet stipulates, the SlimFast shakes are meant as meal replacements, not supplements to a normal diet.
The SlimFast plan is a "Band-Aid approach to weight loss"
According to the experts who review diets for the U.S. News & World Report annual diet rankings, the SlimFast plan will probably not create lasting, long-term weight loss. Compared with other popular diets, the SlimFast plan was ranked below average in the long-term weight loss category, and was rated as being "minimally effective" for long-term weight loss. As one expert on the panel pointed out, liquid diets that rely on meal replacement shakes just aren’t a great long-term solution. "It’s a Band-Aid approach to weight loss," the panelist said.
Why? Well, drinking two shakes per day in lieu of real meals gets boring pretty quickly. Plus, there are limited SlimFast flavors available. Even if that weren’t the case, though, do you really want to drink two meals a day forever? There’s a certain satisfaction that comes from sitting down to a meal and actually chewing it. There’s also satisfaction in varying your meals, which the SlimFast doesn’t really allow for because of the very limited nature of the shakes and bars. If you’re looking for long-term weight loss, SlimFast is likely not the best plan for you.
The SlimFast plan might make it difficult to socialize
It’s naive to think that food is just about fueling our bodies. Food can make us feel happy and comforted, and it’s a great way to bond with others. Unfortunately, the SlimFast diet makes this tough to do. One expert who reviewed the SlimFast diet for the U.S. News and World Report annual diet rankings stated, "This plan makes living a normal life outside of a can or bar difficult." Because SlimFast dieters must replace two meals every day with a shake or bar, and must stick to 500 calories or less for the third meal, there’s really not much room to enjoy food.
Of course, that could put a real damper on your social life. All of us take pleasure in eating with others, whether that means going to a restaurant with friends, grabbing lunch with a coworker, or hosting a dinner party. The SlimFast diet makes all of these things much harder. Sure, you could plan your one real meal per day to coincide with a social occasion, but you’re still stuck with that 500-calorie limit. Plus, what if you want to eat with others twice in one day?
Some consumers say SlimFast made them very bloated and gassy
The SlimFast plan may just cause some pretty uncomfortable side effects. One SlimFast diet veteran took to the Consumer Affairs website to explain one of downsides to the SlimFast plan: extreme gas and bloating. The dieter explained that they had medical issues "that can make eating difficult [and] exercising even 30 minutes a day difficult." This dieter thought that SlimFast was a good solution, but later found that this wasn’t the case. Although they liked a few of the flavors, the shakes created serious gastrointestinal discomfort.
"They definitely keep you full," the dieter wrote. "But that’s because if you don’t take twice the regular dose of an anti-gas [medication] before drinking them (and occasionally, even if you do) you’ll bloat up, usually within a half hour." She described feeling "sudden and uncomfortable fullness" after drinking a shake, her shirt getting tighter, and her stomach looking visibly distended. This dieter wasn’t the only one to experience such side effects. Other consumers also complained of extreme gas and stomach discomfort.
You could end up feeling dizzy on the SlimFast plan
"If you follow their plan, you start getting dizzy and lightheaded by the afternoon because your sugar level drops because all you had that day was their shakes for breakfast and lunch," one dieter explained in a Consumer Affairs review of the SlimFast plan.
Experts say that this is a common side effect of a weight-loss diet that’s too low in calories. In an article for SF Gate, registered dietitian Erin Coleman wrote, "Any time you drastically reduce your calorie intake to lose weight you may experience dizziness." Why? "Insufficient carbohydrates can cause dizziness because your brain — which controls your balance — prefers carbohydrates as its main source of energy."
While the original SlimFast shakes are higher in carbs than the Keto or Advanced Nutrition lines, they still have fewer carbohydrates than a typical meal (via SlimFast). If you experience dizziness, you should probably stop the diet and try something else. If you’re set on SlimFast, eat carb-rich snacks to prevent big blood sugar drops.
SlimFast is all about processed foods, which experts recommend limiting
There’s just no getting around the fact that SlimFast shakes, powders, and bars are processed foods. While SlimFast products do contain a balance of protein, carbs, and fat, plus added fiber, vitamins and minerals, they aren’t as nutritious as eating, say, a meal rich in fruits, vegetables, fiber-rich starches, lean proteins, and healthy fats.
It’s no secret that whole foods are good for you. In a 2019 randomized controlled trial published in Cell Metabolism, researchers found that adults who were given a diet full of processed foods ate more and gained more weight than adults who were given a diet that contained exactly the same number of calories but was made up of whole foods instead.
That’s bad news for SlimFast and other companies that rely on dieters buying processed meal replacement bars and shakes. The research suggests that calorie control isn’t the only factor in appetite and lasting weight loss, and that calories from processed food is less healthy and more likely to cause weight gain than the same number of calories from whole foods. Plus, wouldn’t you rather bite into a tasty sandwich or salad than drink an artificially flavored shake?
The SlimFast plan doesn’t teach you how to cook or eat healthy
One question you might be asking yourself about the SlimFast diet: What happens when I don’t want to keep replacing two of my daily meals with shakes anymore? That’s a valid question, and the truth is that SlimFast doesn’t have a very satisfying answer. In fact, they recommend that you continue to replace one of your daily meals with a shake even after you’ve reached your goals, presumably forever (via SlimFast).
Nevertheless, research says that this isn’t a very good strategy for long-term health. A huge 2017 cohort study published in the International Journal of Behavioral Nutrition and Physical Activity assessed the health and eating patterns of over 40,000 French adults for ten years. The researchers found that the ability to plan and cook healthy meals was strongly associated with healthy eating and healthy weight.
The takeaway? If you want to be healthy, don’t turn to meal replacement plans like the SlimFast diet. Instead, learn to plan and cook healthy meals, so that you have the tools you need to eat healthfully for life.
The SlimFast plan isn’t much different from other low-calorie diets
There’s nothing all that special about the SlimFast plan. Really, it’s just a gimmicky way to eat fewer calories by replacing two meals a day with processed, low-calorie shakes or snack bars. You could lose weight sans the shakes by replacing two meals per day with a 200-calorie bowl of cereal, salad, or slice of pizza.
"Sometimes people on meal replacement plans like this see immediate results because the diet is a shock to the body," registered dietitian Amy Goodson told Everyday Health. "You may be losing a bit of fat at the time, but it’s more likely water weight initially." She added that you can’t keep that rate of weight loss — eventually it will slow down or stop altogether.
As with all weight-loss diets, you just can’t expect to continue losing weight so rapidly, since all of your water weight will be gone and fat loss is a much slower process. And, as with all low-calorie diets, you can expect to lose one to two pounds per week.
The SlimFast diet isn’t very exciting or flavorful
After a few weeks on the SlimFast diet, you may find it tough not to get bored of eating the same meal replacement bars and drinking the same meal replacement snacks over and over again. "As with any meal replacement product, it could get tiresome," registered dietitian nutritionist Keri Gans said of the SlimFast plan when speaking with Everyday Health. While you can mix up your free meal and snack to add variety, there aren’t a lot of other choices you can make.
Dietitian Angela Lemond plainly stated in the same article that drinking shakes day in and day out lacks flavor and gets boring, and dietitian Dawn Blatner added that this boredom also comes with a pretty hefty price tag. "They are more expensive than produce," she said.
The bottom line: The SlimFast plan is restrictive and repetitive. Even if you shake up your one daily meal and three daily snacks, there’s not much variety in the shakes and bars that make up two of your daily meals.