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Can Slim Fast Cause Diarrhea? – 5 Reasons Why It Does!

Slim Fast is a clinically-proven diet plan designed to make you lose weight and keep it off. But while its regular, high-protein, and low-sugar shake varieties have been effective for weight loss, some people may have difficulties tolerating the drink. Does Slim Fast cause diarrhea? Stay on for an interesting read!

Slim Fast: Background Info

Source: slimfast.com

The Slim Fast diet is a weight loss plan that the Thompson Medical Company began in 1977 which introduced a 1,200-calorie-per-day meal replacement product. The original diet involved 2 meal replacement shakes daily for breakfast and lunch augmented by a healthy dinner.

Later on, the company extended its product line to cover delicious bars, smoothies, and snacks as well. In 2000, its founder, Daniel Abraham, sold the company to Unilever for $2.3 billion.

Slim Fast products are intended for people who want to lose weight quickly and easily. They are effective, easy to follow, and offer the following health benefits:

  • Effective for weight loss and weight management
  • Increases the feeling of fullness which curbs the craving to eat more
  • Improves insulin, glucose, and lipid levels
  • Supports heart health and reduces the risk for heart disease
  • Decreases calorie intake without decreasing nutrient intake
  • Modifies to follow a gluten-free or high-fiber diet

The Slim fast diet plan may work for short-term weight loss but it may be difficult to sustain in the long run as it can be restrictive and quite expensive. Moreover, ingesting their products may result in certain digestive issues.

Does Slim Fast Cause Diarrhea?

A significant number of people have reported experiencing diarrhea right after starting with the Slim Fast diet plan. This potentially ‘explosive’ digestive reaction can be attributed to some of the ingredients in the Slim Fast products, such as:

  1. Slim Fast contains milk.

Source: slimfast.com

Slim Fast shakes with flavors such as Strawberries & Cream, Cappuccino Delight, and Creamy Milk Chocolate all contain milk. Milk, along with other dairy products such as cream and cheese has lactose, a type of sugar that causes digestive issues in some people right after ingestion.

Over 1 in 3 Americans suffer from lactose intolerance which means that they are unable to fully digest lactose from the foods and drinks with dairy that they consume. The partially digested food passes through the colon a lot quicker than normal, which causes diarrhea.

  1. Slim Fast contains sugar substitutes.

Digestive residues of the sugar substitutes used for calorie control may gather in the colon causing it to secrete more water to dilute the waste which leads to diarrhea. Some of these sweeteners are:

  • Sucralose
  • Acesulfame potassium
  • High Fructose Corn Syrup
  • Maltodextrin
  1. Slim Fast contains food additives.

Source: slimfast.com

Most meal replacements, especially dairy-free ones, are full of food additives. Food additives are not bad in small quantities but they tend to add up when you drink a protein shake daily. Food additives can be hard to digest and they can cause diarrhea when consumed in higher quantities. Here are some of the food additives in Slim Fast:

  • Carrageenan
  • Soy Lecithin
  • Guar Gum
  • Gellan Gum

In general, you will be able to identify which of the ingredients are food additives because they are difficult to pronounce.

  1. Slim fast contains vitamins and minerals that may cause diarrhea.

Some vitamins and minerals when taken over the recommended daily amount tend to contribute to the development of diarrhea, and here are some of them:

  • Vitamin C

Vitamin C is usually absorbed in the upper part of the intestine. However, when there’s too much, the remaining unabsorbed Vitamin C proceeds into the lower intestine and causes diarrhea.

  • Zinc

Zinc is a trace mineral that strengthens the immune system. Diarrhea is a sign of zinc deficiency but too much zinc may also cause stomach cramps, nausea, and vomiting. In one study, diarrhea was one of the side effects experienced by 40% of the participants who took a zinc supplement to recover from a cold.

  • Magnesium

Overloading magnesium from foods is not much cause for concern among healthy individuals. However, high doses of magnesium from medications, fortified foods, or supplements are another matter as these can cause you to display mild symptoms of overdose such as diarrhea, abdominal cramping, and nausea.

According to a senior research scientist with the National Institutes of Health’s Office of Dietary Supplements, Johanna Dwyer, RD, “It’s pretty hard to overdo it from food alone.” So the culprits may well be the fortified foods or drinks that you ingest or the supplements that you take. In addition, Dwyer advises that it’s better to stick to a supplement that is no more than the daily value.

  1. Some Slim Fast products contain caffeine.

Source: slimfast.com

All Slim Fast products in chocolate flavors naturally contain some caffeine, but they would be no more than what is found in a bar of chocolate. Slim Fast also has a product line called Advanced Energy and all products belonging to this line contain 100 mg of caffeine which is equivalent to 1 cup of coffee.

Have you experienced an urge to go to the toilet right after drinking coffee? This is because the caffeine in coffee has laxative properties which speed up digestion. Sometimes when your stomach contents reach your small intestine too quickly, it results in gas, bloating, and diarrhea.

Other Side Effects of Slim Fast

Just like every good thing, Slim Fast may have some side effects although they are not serious enough to be considered a health risk. Some side effects of Slim Fast that may accompany diarrhea include:

  • Abdominal pain
  • Gas
  • Oily spotting
  • Fatty stools
  • Abdominal cramps
  • Soft stools

However, people who are taking medication for certain health conditions may experience other side effects due to drug interactions. This is why they should consult their doctors first before using Slim Fast or other weight loss products.

Foods Compatible With Slim Fast

Although no foods are prohibited in the diet plan, the following are compatible with Slim Fast products because they allow you to stay within the caloric limit and even help with diarrhea:

  • Lean proteins, such as lean beef, chicken, turkey, fish, tofu, and legumes

When you have diarrhea, you should avoid fried and greasy foods that are high in fat content. It is better to bake or broil lean meats such as beef, chicken, or turkey since strong spices and seasonings can aggravate your system.

  • Whole grains, such as oatmeal, brown rice, quinoa, buckwheat, and millet

Oatmeal and brown rice both contain soluble fiber which slows down digestion and adds bulk to the stool which helps with diarrhea. However, the bran and germ retained in brown rice can irritate the gut and bring on digestive issues so it is better to replace brown rice with white rice when you have diarrhea because it is well-tolerated by the stomach.

  • Fruits, such as apricots, bananas, apples, kiwi, and strawberries

Fruits like apples, bananas, apricots, and strawberries are low in calories but high in soluble fibers called ‘pectin’ which is good for treating diarrhea. Pectin helps absorb liquid in the intestines while preventing constipation. In addition, bananas and apples are easy to digest and have a bland taste that does not aggravate the gut. This is why bananas and apples are part of the BRAT diet which are known to help with diarrhea, among other things.

  • Vegetables, such as asparagus, cucumber, tomatoes, and celery

Vegetables are full of nutrients but they can be difficult to digest when eaten raw. The best way to cook vegetables when you have diarrhea is to boil or steam them. Cucumber and tomatoes have high water content so they can help combat dehydration, while the fiber in celery helps prevent diarrhea by absorbing water and solidifying stool with roughage.


There is no fixed answer as to whether a certain type of food can cause diarrhea or not because every person has a different response, so what may be good for one may not be good for another. The best option to tell what foods can trigger diarrhea for you is to try the elimination diet.

The Slim Fast weight loss plan involves calorie restriction through meal replacement and encourages other healthy lifestyle practices such as exercise, sleep, and stress management. The program certainly has its pros and cons, but does Slim Fast cause diarrhea? Yes, it does, and for so many reasons!

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Coreen Tan-Tambasen

A Mass Communications graduate as well as a registered nurse in the Philippines, Coreen has been a freelance writer since 2015. Most of her writing projects are in the health and medical niche, and she is committed to sharing interesting articles about digestion and gut health that can be applied to everyday life.

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