Health 

The Vital Connection Between Fiber and Gut Health

March 09, 2021 6 min read

Woman holding a bunch of kale with fiber for gut health

Health experts have preached the importance of fiber for decades, and with good reason: Fiber and gut health go hand-in-hand.

When it comes to gastrointestinal wellness, fiber’s claim to fame has generally been its ability to encourage regular bowel movements. As it turns out, the benefits of fiber for gut health go well beyond preventing constipation. 

Before diving into what else fiber can do for your microbiome, let’s take a quick look at what exactly fiber is. 

What is fiber? 

Fiber is the indigestible component of plant foods. Dietary fiber is found in varying quantities in these foods, from fruits and vegetables to whole grains and legumes. 

High-fiber foods have been shown to help lower the risk of developing diabetes, heart disease, and certain kinds of cancer. Fiber is also linked to weight loss, a host of digestive benefits, and even longevity

But despite these wide-ranging benefits, the average intake of dietary fiber in the United States is concerningly low. 

While experts recommend 25g of fiber per day for women and 38g for men (21g and 30g respectively for those over the age of 50), the average intake of fiber continues to hover around 15g per day

Given that the Standard American Diet is generally dense in fast food and refined, processed carbohydrates, it’s not surprising that fiber intake is so scant. However, experts maintain that we should all take the importance of dietary fiber more seriously.

Fiber and digestion 

Fiber is generally described as the portion of plant food that resists digestion. A nutrient that we can’t digest but that we rely on for gut health might seem counterintuitive at first, so let’s take a quick look at the digestive process. 

When we eat something, the body uses digestive enzymes to break down the various components of the food (like fats and proteins) into nutrients we can use for energy and metabolic functions. 

Fiber, on the other hand, dodges breakdown by dietary enzymes. Instead, it either passes through the stomach, small intestine, and colon before exiting the body in the stool, or your gut bacteria break it down. 

Soluble and insoluble fiber  

There are two general categories of fiber: soluble (meaning it dissolves in water) and insoluble (cannot be dissolved in water). 

Insoluble fibers pass through the entire digestive system almost fully intact and contribute to stool bulk, while soluble fibers are partially fermented by gut bacteria. 

Health benefits of soluble fibers include lowering blood cholesterol and balancing blood sugar levels, making them helpful for the prevention and management of type 2 diabetes, prediabetes, heart disease, and metabolic syndrome. 

Good sources of soluble fiber include: 

  • Beans, lentils, peas 
  • Oats, psyllium 
  • Carrots, brussels sprouts, broccoli 
  • Sweet potatoes, turnips 
  • Apples, citrus fruits 
  • Flaxseeds, sunflower seeds, chia seeds 
White radishes rich in insoluble fiber

The primary benefits of insoluble fibers are preventing or treating constipation, promoting regularity of bowel movements, and increasing stool bulk. 

Good sources of insoluble fiber include: 

  • Wheat bran, barley 
  • Almonds, pine nuts 
  • Cauliflower, green beans, zucchini, celery
  • Beans, peas 
  • Prunes, skins of fruits including apples 
  • Spinach, swiss chard 

Soluble and insoluble fibers both offer benefits, and an ideal diet should provide both types. 

Fiber and gut health: the microbiome connection 

Dietary fiber appears to profoundly impact the health of the gut microbiome, along with overall well-being and metabolic function. 

The benefits of insoluble fiber for stool bulk and bowel movement frequency are relatively straightforward: Because we can’t digest these fibers, they can pass through the stomach, small intestine, and colon with relative ease and contribute to stool quality. 

But what about the benefits of soluble fibers, which are capable of being fermented by our gut bacteria? 

In order to understand the connection, it’s important to know a few things about the gut microbiome. 

The gut microbiome is home to hundreds of species of bacteria and other microorganisms, including fungi and viruses. We know that a healthy gut microbiome houses a diverse array of beneficial bacterial species, helping to push out the pathogenic or harmful kinds. 

Research into what makes a healthy gut microbiome has exploded over the last few decades. In that time, scientists have discovered all kinds of factors that contribute to the makeup of the microbiome, and how it can rapidly change when habits shift.  

One of the greatest determinants of gut composition is diet. What we eat is reflected in the collection of bacteria housed in our gut, and dietary fiber has an essential role to play. 

Benefits of prebiotic fibers 

Many soluble fibers can be further categorized as prebiotics. Prebiotic fibers are not just fibers fermented by gut bacteria. Rather, they are fibers that are fermented and broken down specifically by beneficialgut bacteria. In other words, prebiotics are food for friendly gut microbes, helping them flourish and improve the health of their host (you). 

Prebiotic fibers help nourish healthy bacteria so that they can grow, thrive, and multiply. 

When friendly gut bacteria ferment prebiotic fibers, they produce Short Chain Fatty Acids (SFCAs). Research has shown that SCFAs produced by gut bacteria can reduce inflammation, enhance metabolism, and improve insulin sensitivity

Therefore, feeding healthy bacteria with prebiotic fiber is a great way to support a healthy gut microbiome. Probiotic supplements (live beneficial bacteria) may also be helpful alongside a high-fiber diet, particularly when they contain prebiotics. 

Fiber and the gut lining 

Bacteria in the gut need to eat to survive, just like we do. And we know that prebiotics help feed the good guys. 

But when fiber is in short supply, what happens? Some bacteria will simply die of starvation, but others are more resourceful. For certain microbes determined to survive, the mucosal lining along the gut wall becomes a supplementary food source. 

With sufficient dietary fiber, we don’t just keep this from happening—we also feed the bacteria that make up the mucosal lining, helping maintain and strengthen it. 

The mucosal lining is the innermost portion of the gastrointestinal wall. When the mucus lining is thick and healthy, it helps protect the rest of the gastrointestinal wall, reducing the likelihood of inflammation, intestinal permeability (Leaky Gut Syndrome), and gastrointestinal illnesses.

This demonstrates the need for adequate fiber intake, both for the health and diversity of the gut microbiome and the strength of the gut lining. 

Three assortments of veggie avocado toasts rich in fiber for gut health

Fiber, the gut microbiome, and weight loss 

High-fiber diets have been known to help with healthy weight loss for quite some time. There are several factors that may contribute to this connection. 

By feeding beneficial gut bacteria, fiber can help change the makeup of the gut microbiome for the better. Research has shown that increasing fiber intake can actually alter the microbial profile from one associated with obesity to one associated with a healthier weight and BMI.  

The SCFAs produced as a result of the breakdown of prebiotic fibers by gut microbes have been linked to reduced inflammation and a decreased risk of obesity

High-fiber foods are also known to increase feelings of fullness, providing the energy and satiety we need with fewer calories. The anti-inflammatory, blood sugar-regulating, and cholesterol-lowering properties of fiber may also contribute to healthy weight loss. 

Adding a high-quality probiotic supplement containing bacterial strains linked to healthy weight loss may help enhance these benefits.* 

Tips for increasing fiber in your diet 

  • Start slow and work your way up. If you’re not used to a high-fiber diet, you may experience digestive symptoms like bloating or gas when introducing too much too fast. 
  • Go for a combination of soluble and insoluble fibers. The benefits of the two are complementary, and a healthy balance of both is ideal. 
  • Make sure to drink enough water (2-3 liters per day). This is always important, but especially alongside a high-fiber diet. 
  • Choose whole foods over fiber supplements. Fresh, whole, high-fiber foods are packed with vitamins, minerals, and other nutrients that offer all kinds of additional health benefits. 
  • Eat more legumes. Legumes like lentils, beans, and peas are excellent sources of fiber and plant protein.  
  • Try to include at least one high-fiber food with every meal, even if it’s just a matter of eating an apple or carrot sticks on the side.
  • Avoid refined grains and processed foods, which are stripped of a great deal of their fiber and other nutrients. 
  • If you get most of your fruits and vegetables from juices, keep in mind that the juicing process removes insoluble fiber. 
  • Mix it up! Try exploring your local farmer’s market for fresh, seasonal fruits and vegetables that you don’t typically eat. 
  • Make sure to eat sufficient fiber every day. Just a day or two without it can alter the gut microbiome. 

Key takeaways

Fiber might be most famous for its role in preventing and treating constipation, but it turns out the connection between fiber and gut health goes much deeper than this. The fiber found in plant foods, including fruits, vegetables, whole grains, and legumes, is a nutrient that has long been celebrated for its myriad health benefits. 

Research has linked high-fiber diets with enhanced digestion, weight loss, increased longevity, and reduced risk of developing heart disease, cancer, and type 2 diabetes. Fiber can also help reduce inflammation, boost metabolism, support healthy weight loss, and protect the gut lining. 

While insoluble fibers help support bowel movement frequency and quality, soluble fibers are fermented by gut bacteria. Many soluble fibers have prebiotic properties, meaning they feed beneficial gut microbes, supporting a balanced and healthy gut microbiome. 

Make it a goal to incorporate healthy fiber into your diet daily, ideally combining a variety of both soluble and insoluble fibers for optimal benefits. 

Ellie Ellias - Contributing Writer, Physician’s Choice