SlimFast is a meal replacement program that involves swapping out food for one of their branded shakes. While it's possible that the diet can help you lose weight, replacing whole foods with shakes may not always be the best idea. So, what are the side effects of drinking SlimFast, exactly?
SlimFast shakes come in five varieties: Original, Keto, Low-Carb, Advanced Energy and Advanced Nutrition, according to the company's website. The idea behind the diet is that you replace two meals per day with a shake (and eat SlimFast-branded snacks in between), thereby cutting your calories to promote weight loss.
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Some older research — like this December 2004 study in the International Journal of Obesity and Related Metabolic Disorders — found that SlimFast helped people lose weight. Another January 2009 study in Obesity came to the same conclusion: However, some of the researchers received grant support from SlimFast and served on the company's Advisory Board.
In other words, new and unbiased research is lacking when it comes to surveying whether SlimFast is actually effective.
What's more, there are certain drawbacks to the diet. Here are SlimFast side effects to be aware of, along with the dangers of SlimFast to know about if you're considering trying the program for weight loss.
Talk to your doctor before trying SlimFast or any weight-loss program that involves supplements, as they may not be safe or recommended for you, per the Mayo Clinic.
1. It Can Deprive You of Adequate Fuel
According to the 2020-2025 Dietary Guidelines for Americans, adults should aim to eat the following amount of calories per day (though your actual daily caloric intake may vary based on factors like your or activity level or health status):
- People assigned female at birth (AFAB): 1,600 to 2,000 calories
- People assigned male at birth (AMAB): 2,000 to 2,400 calories
If you're trying to lose weight, it's best to do it at the safe and sustainable pace of 1 to 2 pounds per week, which you can typically do by creating a daily calorie deficit of about 500 calories through eating less and moving more, according to the Mayo Clinic.
But this doesn't typically mean severely restricting your food intake. Per Harvard Health Publishing, if you're replacing an entire meal with a shake — as you do on the SlimFast diet — it should contain at least 400 calories.
The original SlimFast shake, however, only contains 180 calories a bottle, according to the company's website. If you use these shakes as a total meal replacement, then you may not be getting adequate fuel — this can slow your metabolic rate, which can leave you feeling fatigued and get in the way of weight-loss efforts.
And while severely cutting calories may initially lead to weight loss, this is usually the result of losing water weight and burning muscle rather than fat, neither of which is good for your body in the long term, according to the Mayo Clinic.
Calorie intake should not fall below 1,200 per day for people AFAB or 1,500 per day for people AMAB, except under the supervision of a doctor, per Harvard Health Publishing.
2. It Can Lead to Nutritional Deficits
On a similar note, another side effect of drinking SlimFast instead of eating meals is that not getting enough calories can deprive you of essential nutrients, according to Harvard Health Publishing.
Plus, it's always best to get your nutrition through whole foods rather than supplements like shakes (unless your doctor recommends it), according to the Mayo Clinic. That's because supplements don't provide you with the same nutritional benefits as foods like fruits, vegetables, whole grains and lean proteins.
Is SlimFast Good for You?
Perhaps you're wondering if SlimFast is healthy in the first place. Per Harvard Health Publishing, an 8-ounce meal replacement shake should ideally be low in sugar and contain 10 to 20 grams of protein, a maximum 6 grams of fat and a maximum 40 grams of carbohydrates (including sugar).
According to the SlimFast website, their original shake fits the bill for macros. However, it also contains 19 grams of sugar, meaning it might not be the best option.
And while SlimFast isn't necessarily bad for you every now and then, it's still always best to get your nutrients through whole foods rather than supplements, per the Mayo Clinic.
3. It May Lead to Weight Regain
Getting too little fuel isn't the only SlimFast danger to consider. The opposite can also be true: If you're wondering why you're gaining weight on SlimFast, it may be because meal replacement diets can sometimes result in eating too many calories.
For instance, this can be the case if you drink the shakes while also continuing to eat full meals, resulting in high calorie intake, according to Harvard Health Publishing. This is why doctors may prescribe drinking SlimFast to gain weight for people with underlying conditions that cause unintentional weight loss.
And while the SlimFast diet may help you lose weight initially, it doesn't appear to be more effective for short-term weight loss than similar programs, according to April 2016 research in the Annals of Internal Medicine.
In fact, the program can result in regaining those lost pounds (and, sometimes, additional weight), as restricting calories can lead to metabolism changes and a tendency to eat "off-limits" foods, both of which contribute to weight regain, according to the American Psychological Association.
This isn't a side effect per se, but it's important to note if you're considering the program for sustained weight loss.
What's more, the diet doesn't help you build long-term habits like eating nutritious foods and exercising regularly. Without developing these patterns, it can be hard to maintain weight loss, according to the Mayo Clinic.
Is SlimFast Bad for Your Liver?
There's no scientific evidence to suggest that SlimFast can cause damage to your liver.
4. It Can Cause Digestive Symptoms
If SlimFast shakes hurt your stomach, there's a reason for that: Sweeteners.
According to Harvard Health Publishing, it's best to stick to nutritional shakes where sugar is not one of the top three ingredients. The original SlimFast shake lists sugar as its third ingredient, per the company website. It also contains artificial sweeteners like fructose, high-fructose corn syrup and sucralose.
These sweeteners can lead to digestive distress, per the Mayo Clinic, including symptoms like:
Artificial sweeteners aren't the only reason SlimFast keto shakes (and other varieties) can cause stomach pain. If you're lactose intolerant, milk may be to blame.
Indeed, fat-free milk is the first ingredient listed on the original shake's nutrition label, per the SlimFast website. Other variations — like the keto shake — also contain dairy products like milk protein concentrate, according to the website.
People with lactose intolerance lack the enzyme lactase and are thus unable to digest milk sugar (lactose), according to the Mayo Clinic. The result? Unpleasant symptoms like:
- Stomach cramps
Does SlimFast Have Laxatives in It?
Per the company's website, there are no laxatives listed in the shakes' nutritional labels.
5. It Can Contribute to Headaches
Artificial sweeteners aren't only responsible for digestive symptoms: They may also be to blame if you experience SlimFast-related headaches.
For instance, sucralose in particular can lead to increased migraines, according to University of Michigan.
6. It Can Spike Your Blood Sugar
Another side effect of drinking SlimFast? Potential blood sugar spikes. Sugary foods — like these shakes — can increase your blood glucose levels, according to the American Medical Association.
Artificial sweeteners may also contribute to higher blood glucose (though more research is needed to confirm this link), per the Centers for Disease Control and Prevention.
In addition, certain food stabilizers like maltodextrin (which is used to help create SlimFast shakes' creamy texture) can raise your blood sugar levels significantly, according to a May 2019 study in Nutrients.
The takeaway: There are a number of ingredients in SlimFast shakes that can lead to blood sugar spikes. Per the Cleveland Clinic, high blood sugar levels can cause symptoms like:
- Increased thirst and/or hunger
- Blurred vision
- Frequent urination
Can SlimFast Cause Cancer?
There has been speculation about whether artificial sweeteners are linked to cancer. But according to the National Cancer Institute, there is no clear evidence to demonstrate a connection between sweeteners and cancer in humans.
- SlimFast: "How It Works"
- Harvard Health Publishing: "Supplemental Nutrition Drinks, Help or Hype?"
- University of Michigan: "Fake Sweeteners Make for Real Problems"
- Nutrients: "Predicting Glycemic Index and Glycemic Load from Macronutrients to Accelerate Development of Foods and Beverages with Lower Glucose Responses"
- Mayo Clinic: "Lactose Intolerance"
- Obesity: "One-Year Weight Losses in the Look AHEAD Study: Factors Associated with Success"
- International Journal of Obesity and Related Metabolic Disorders: "Feasibility of a partial meal replacement plan for weight loss in low-income patients"
- SlimFast: "SlimFast Original Shakes"
- U.S. Department of Agriculture and U.S. Department of Health and Human Services: "2020-2025 Dietary Guidelines for Americans"
- Mayo Clinic: "Why do doctors recommend a slow rate of weight loss? What's wrong with fast weight loss?"
- Harvard Health Publishing: "Calorie counting made easy"
- Mayo Clinic: "Supplements: Nutrition in a pill?"
- Annals of Internal Medicine: "Efficacy of commercial weight loss programs: an updated systematic review"
- American Psychological Association: "Why do dieters regain weight?"
- Mayo Clinic: "Artificial sweeteners and other sugar substitutes"
- American Medical Association: "Foods that spike a patient’s blood glucose are not what you think"
- Centers for Disease Control and Prevention: "10 Surprising Things That Can Spike Your Blood Sugar"
- Cleveland Clinic: "Hyperglycemia"
- National Cancer Institute: "Artificial Sweeteners and Cancer"
- SlimFast: "SlimFast Keto Meal Shakes to Go - Creamy Milk Chocolate - 4 Count"