Fiber
The carbohydrate you can't digest—but your bacteria can. Fiber passes through your digestive system, feeding beneficial gut bacteria that convert it to powerful metabolites like butyrate. Most people get far less than the recommended 25-35g daily.
Types of Fiber
Soluble Fiber
- Dissolves in water, forms gel
- Slows digestion, steadies blood sugar
- Binds cholesterol and bile acids
- Fed to bacteria → SCFAs
- Sources: oats, beans, apples, citrus
Insoluble Fiber
- Doesn't dissolve, adds bulk
- Speeds transit time through gut
- Reduces constipation
- Less fermentable
- Sources: wheat bran, vegetables, whole grains
Special Fiber Types
Resistant Starch
Starch that resists digestion. Best butyrate producer. Found in cooled potatoes/rice, green bananas.
Inulin/FOS
Fructans from chicory, garlic, onions. Highly fermentable prebiotic. Can cause gas if introduced too fast.
Beta-Glucan
From oats and mushrooms. Immune-modulating. FDA approved for cholesterol claims.
Pectin
From apple peels, citrus. Gelling properties. Feeds beneficial bacteria.
Psyllium
Highly mucilaginous. Great for regularity. Less fermentable = less gas.
Arabinogalactan
From larch tree. Supports immune function. Well-tolerated prebiotic.
What Fiber Does
Feeds Microbiome
Bacteria ferment fiber into SCFAs (butyrate, propionate, acetate). This is their food.
Blood Sugar Control
Slows carbohydrate absorption. Reduces glucose spikes. Improves insulin sensitivity.
Cholesterol Binding
Soluble fiber binds bile acids, forcing liver to use cholesterol to make more.
Satiety
Fiber fills stomach, slows digestion, triggers fullness hormones. Helps with weight.
Regularity
Adds bulk and softness to stool. Both constipation and diarrhea can improve with fiber.
Toxin Binding
Fiber can bind some toxins and excess estrogens for elimination via stool.
Fiber Cautions
Start Slowly
Rapid increases cause bloating and gas. Increase by 5g/week. Let microbiome adjust.
Drink Water
Fiber absorbs water. Without adequate hydration, it can cause constipation.
SIBO/IBS Caution
Fermentable fibers can worsen symptoms in SIBO. Low-FODMAP may be needed first.
Diversity Matters
Different fibers feed different bacteria. Aim for variety, not just one type.