“Fiber vs Probiotics: Which Does What in the Gut?” was written by Beverly Garden, RDN, LDN & edited/reviewed by Aly Bouzek, MS, RDN.
Fiber, probiotics, prebiotics… it seems like “everyone’s” talking about them. We know they’re important for good health, but what do they really do? Do they all have the same job in our body?
It can be overwhelming and confusing to sort through all the research to know what’s what! We can help with that! This article will discuss fiber vs probiotics, and we’ll delve into the role each plays in the body and the health benefits of each.
A special focus will be placed on the health advantages of a healthy gut microbiome and how probiotics, fiber, and a type of fiber called prebiotics, work together to achieve this.
Finally, we’ll give practical ideas to ensure adequate intake of fiber and probiotics in the diet.
What is Fiber?
In general, fiber is a type of carbohydrate that the body cannot digest. Fiber is in plant-based foods such as fruits, vegetables, whole grains, and legumes.
Unlike other food components such as fats, proteins, or carbohydrates, fiber is unable to be broken down by the body. Instead, it passes through and out of the digestive system relatively intact.
There are two types of fiber: soluble and insoluble.
Soluble fiber dissolves in water to form a gel-like material.
It is in foods such as oats, peas, beans, apples, citrus fruits, carrots, barley, and chia seeds.
Insoluble fiber does not dissolve in water, and it promotes the movement of material through the digestive system and increases stool bulk.
(Great for treating/preventing constipation!).
Good sources of insoluble fiber include: whole wheat flour, wheat bran, quinoa, beans and legumes, nuts, fruits, and vegetables.
Benefits of Fiber
Consuming a diet with adequate dietary fiber can have many health benefits. Some of these benefits include:
- Regulating bowel movements
- Maintaining bowel health
- Preventing colon cancer
- Lowering LDL (“bad”) cholesterol levels
- Decreasing the risk of cardiovascular disease
- Regulating blood sugar levels
- Improving satiety and satisfaction with meals
- Increasing longevity
- Preventing diverticular disease
What are Probiotics?
Probiotics are live microorganism or “good bacteria” and/or yeasts that are found naturally in the body. Probiotics are also obtained via probiotic supplements and various fermented and cultured dairy products and other foods.
In the digestive system, probiotics help ward against too many “bad bacteria” in the gut.
They help maintain a healthy balance between harmful and helpful bacteria to promote a healthy gut microbiome. “Microbiome” is another word for the microorganisms in a particular environment.
Benefits of Probiotics and a Healthy Gut Microbiome
Many health benefits are associated with adequate probiotics and a healthy gut microbiome. For example, they:
- Play a role in immunity
- Help prevent infection
- Facilitate digestion and absorption of food/nutrients
- Synthesize vitamins in the body
- Help digest and absorb medications
- Prevent or improve diarrhea from antibiotic use or an infection
- Prevent and treat constipation, diarrhea and bloating
- Decrease inflammation
- Reduce the risk of colon cancer
- Reduce the risk of type 2 diabetes
There is also emerging research showing that a healthy gut microbiome helps improve symptoms of depression and anxiety. (1)
This connection between the health of the gastrointestinal tract, brain chemistry, and mental health is often referred to as the gut-brain connection.
Fiber vs Probiotics: Working Together in the Gut
Although both fiber and probiotics are important for gut health in their own right, the optimal gut health benefits occur when when we eat adequate amounts of both.
This is because fiber is the fuel for the probiotics to grow and multiply.
What About Prebiotics?
Prebiotics are a type of fiber that produce short-chain fatty acids when consumed by microorganisms (probiotics) in the gut.
In addition to several of the same benefits of a healthy gut microbiome noted above, additional benefits are linked to prebiotics and the associated production of short-chain fatty acids when “consumed” by probiotics. For example, they:
- Aid in calcium and phosphorus absorption
- Create neurotransmitters between brain and gut (causes changes in mood)
- Help reduce sugar and other food cravings
Fiber vs. Probiotics: Which is More Important?
As explained above, both fiber and probiotics work together for optimal gut health. However, if you can only focus on one, fiber may be slightly more important than probiotics. This is because good bacteria in your gut can only grow and multiply if they have the proper fuel.
That means that even with ideal probiotic intake, the sought after benefits will not be achieved if there is nothing for the healthy bacteria in the gut to feed on (they need fiber!).
The result could mean an imbalance of the gut microbiome, where the “bad” bacteria overtake the “good” bacteria.
Fiber vs Probiotics: How Do I Make Sure I Get Enough Fiber in My Diet?
The Dietary Guidelines for Americans, 2020-2025, recommends the following total fiber intake according to age and gender:
| Gender | Age | Fiber Recommendation |
| Men | 31–50 years old | 31 grams |
| Men | 50+ years old | 28 grams |
| Women | 31–50 years old | 25 grams |
| Women | 50+ years old | 22 grams |
Unfortunately, the typical American diet does not include the recommended amounts of fiber – only 3% of men and 9% of women consume enough fiber.
Tips on Increasing Fiber Intake
Consuming a diet rich in fruits, vegetables, whole grains, beans, and legumes will help ensure a diet rich in fiber. If necessary, fiber supplements are available for use. Some top dietary sources of fiber include:
- Lentils
- Split peas
- Chia seeds
- Avocado
- Chickpeas
- Pears
- Apples
- Almonds
- Blueberries
- Broccoli
- Brussels sprouts
- Whole wheat bread
- Whole wheat spaghetti
For more detailed information on the fiber content of food and ideas on incorporating more fiber in your diet, check out our Insoluble Fiber Foods Chart article and our chart on soluble fiber food sources.
Food and Supplement Sources of Prebiotic Fiber
Although all prebiotics are fiber, not all fiber is a prebiotic. Remember, prebiotics have unique benefits noted above due to the production of short-chain fatty acids when they are “consumed” by probiotics. Good food sources of prebiotics include:
- Apples
- Bananas (particularly green bananas)
- Chicory root
- Dandelion greens
- Garlic
- Jerusalem artichokes
- Onions
- Oats
For more information on prebiotic supplements, check out our article on Best Prebiotic Supplements According to a Dietitian.
Take it Slow and Hydrate!
When increasing fiber (including prebiotic fiber) in the diet, it is very important to do so gradually and to drink plenty of fluid! Increasing fiber too fast and/or not consuming enough fluids can result in diarrhea, constipation, and/or uncomfortable gas and bloating!
Sources of Probiotics
There are no specific recommendations for what is considered an optimal probiotic intake. Therefore, a general recommendation is to strive to include probiotic-containing foods in your diet on a regular basis.
Foods high in probiotics are fermented or cultured. Examples of such foods include:
- Buttermilk
- Certain cheeses such as cottage cheese, Swiss, provolone, cheddar
- Miso soup
- Sauerkraut
- Tempeh
- Yogurt
Numerous probiotic supplements are also available and may be beneficial.
They each contain different strains of probiotics. Please discuss with your healthcare provider if a probiotic supplement is right for you.
Fiber vs Probiotics Summary
Consuming a diet that’s adequate in fiber has numerous health benefits. Fiber also acts as the fuel for probiotics to flourish in the body. Adequate intake of both fiber and probiotics (also known as the “good bacteria”) is important for a healthy gut microbiome.
The benefits of a healthy gut microbiome are far-reaching.
These benefits range from the prevention of diseases such as colon cancer, supporting immune health, blood sugar regulation, synthesis of vitamins in the body, promoting bowel health, bowel regulation, and even an improvement in depression and anxiety!
In addition, when prebiotics (a type of fiber) are consumed by probiotics in the gut, short-chain fatty acids are formed. These provide unique health benefits as well.