The Science Behind Dietary Fibre and Why Most People Don't Get Enough

What Fibre Actually Does in the Body

Fibre's reputation as a digestive aid barely scratches the surface of what it does. Once consumed, the two main categories — soluble and insoluble — take distinct paths through the digestive system, each contributing differently to health.

Soluble fibre dissolves in water and forms a thick gel in the gut. This gel slows the emptying of the stomach, which helps moderate the rate at which glucose enters the bloodstream. It also binds to cholesterol particles, supporting their removal from the body. Oats, apples, barley, and legumes are particularly rich in soluble fibre.

Insoluble fibre does not dissolve. Instead, it adds bulk and softness to stool, speeding its passage through the colon. Whole wheat, bran, and most vegetables are good sources. Regular, comfortable bowel movements — often associated with insoluble fibre — are a basic marker of digestive health.

Beyond these two roles, fibre feeds the gut microbiome — the trillions of bacteria that live in the large intestine. When these bacteria ferment certain fibres (particularly a subtype called prebiotic fibre), they produce short-chain fatty acids that may reduce intestinal inflammation and support immune function. Research into the gut microbiome is still evolving, but the connection between fibre intake and microbial diversity is well-established.

“Dietary fibre is arguably the most underappreciated nutrient in the modern diet. Its effects extend well beyond digestion — influencing metabolic health, immune function, and the composition of the gut microbiome.”

— Dr. Walter Willett, Professor of Epidemiology and Nutrition, Harvard T.H. Chan School of Public Health

Why the Fibre Gap Is So Wide

National nutrition data consistently shows that fibre intake among US adults averages well below recommended levels. Understanding why requires looking at how the modern food supply is structured.

Processed and ultra-processed foods — which make up a large share of the average American's diet — are typically made from refined grains and added sugars, with most of the naturally occurring fibre removed during manufacturing. White bread, white rice, pastries, and many packaged snack foods are filling in terms of calories but low in fibre.

Busy lifestyles and the convenience of fast food or ready-made meals compound the issue. Fibre-rich foods like legumes and whole grains often require more preparation time than their refined counterparts. Cost and food access also play a role — fresh produce and whole grain products can be less accessible in some communities.

Reading food labels is one practical step toward improving fibre awareness. Our guide to reading a nutrition label can help you identify fibre content and make more informed comparisons at the grocery store.

Practical Ways to Close the Fibre Gap

Increasing fibre intake doesn't require a dramatic dietary overhaul. Small, consistent substitutions tend to be more sustainable than wholesale changes.

• Swap refined grains for whole grains: Choose whole wheat bread, brown rice, or oats instead of white bread or instant rice.
• Add legumes regularly: Lentils, chickpeas, and black beans are versatile and inexpensive. Adding them to soups, salads, or grain bowls significantly boosts fibre with minimal effort.
• Keep the skin on produce: Much of the fibre in apples, pears, and potatoes sits in or just under the skin.
• Reach for whole fruit over juice: Juicing removes most of the fibre. A whole orange provides fibre; a glass of orange juice largely does not.
• Snack on nuts and seeds: Almonds, chia seeds, and flaxseeds offer a useful fibre boost alongside healthy fats.

One caution: increase fibre gradually. Adding large amounts too quickly can cause bloating and gas as the gut microbiome adjusts. Drinking adequate water is equally important — fibre works best when it has fluid to absorb and move through the digestive tract.

For a broader framework on how fibre fits into your overall eating pattern, see our article on building a balanced plate.

This article is for general informational purposes only and does not constitute medical or dietary advice. Consult a qualified healthcare professional or registered dietitian for guidance specific to your health needs.