🌾 Dietary Fibre: Soluble vs Insoluble, Role in Microbiota Nutrition
Two types of dietary fibre and their different roles in health
Dietary fibre is plant carbohydrates that are not digested in the small intestine. It is divided into soluble and insoluble. Both types matter: soluble feeds the microbiota and lowers cholesterol, insoluble supports bowel motility.
What is fibre
Fibre consists of plant polysaccharides (cellulose, pectin, inulin, hemicelluloses, etc.) that are not broken down by human enzymes. It passes through the GI tract and affects digestion, microbiota, blood sugar and cholesterol.
- Not digested but fermented by microbiota.
- Supports regular bowel movements.
- Lowers blood sugar and cholesterol.
- Gives a feeling of fullness.
Soluble fibre
Soluble fibre dissolves in water, forming a gel. It slows digestion, reduces sugar and cholesterol absorption, and is fermented by bacteria in the colon, feeding the microbiota.
- Forms a gel in the GI tract.
- Lowers blood sugar and cholesterol.
- Serves as food for the microbiota (prebiotic).
- Sources: oats, legumes, fruit, barley.
Insoluble fibre
Insoluble fibre does not dissolve in water. It increases stool volume and speeds up its passage, supporting motility and preventing constipation.
- Increases stool volume.
- Speeds up bowel transit.
- Prevents constipation.
- Sources: bran, whole grains, vegetables.
Role in microbiota nutrition
Soluble fibre is the main food source for beneficial gut bacteria. Fermentation produces short-chain fatty acids (butyrate, acetate, propionate) that feed intestinal cells and support immunity.
- Fermented fibre feeds the microbiota.
- Short-chain fatty acids are produced.
- Butyrate is an energy source for intestinal cells.
- Fibre diversity = microbiota diversity.
Food: sources
| Food | Fibre | Type | Comment |
|---|---|---|---|
| Oat flakes | ~10 g/100 g | Soluble | Beta-glucans |
| Wheat bran | ~40 g/100 g | Insoluble | Top in fibre |
| Lentils | ~8 g/100 g | Mixed | Legumes |
| Apple | ~2 g/100 g | Soluble | Pectin |
| Broccoli | ~3 g/100 g | Insoluble | Vegetables |
| Chia seeds | ~34 g/100 g | Soluble | Concentrated |
How much to eat for the norm
- Norm 25–30 g/day: ~200 g oats + an apple + a portion of lentils + vegetables.
- Alternative: 2 tbsp of bran (~10 g) + vegetables + fruit.
- Chia seeds: 1–2 tbsp (~5–10 g of fibre).
- Introduce fibre gradually, increasing water.
Norms and dosages
| Indicator | Value | Comment |
|---|---|---|
| Adult norm | 25–30 g/day | Men up to 38 g |
| Women's norm | 25 g/day | Recommendation |
| Soluble | ~half of norm | Feeds microbiota |
| Insoluble | ~half of norm | Motility |
Practical tips
- Start the day with oatmeal — a portion gives ~4 g of soluble fibre.
- Add chia or flax seeds to yoghurt and smoothies.
- Replace white bread with whole-grain.
- Include legumes 2–3 times a week.
- Eat vegetables with every meal.
- Drink 1.5–2 l of water a day when increasing fibre.
What is fibre?
Fibre is plant carbohydrates not digested in the small intestine; important for digestion and microbiota.
What is soluble fibre?
Forms a gel, lowers sugar and cholesterol, feeds the microbiota.
Why is insoluble fibre needed?
Increases stool volume, speeds up motility and prevents constipation.
How much fibre a day?
Adults 25–30 g a day, men up to 38 g.
How does fibre feed the microbiota?
It is fermented by bacteria, producing short-chain fatty acids that feed intestinal cells.
Which foods have the most fibre?
Wheat bran (~40 g/100 g), chia seeds (~34 g), legumes, oatmeal.
Can you eat too much fibre?
A sharp increase may cause bloating; increase gradually and drink water.
Does fibre lower cholesterol?
Soluble fibre (oats, legumes) lowers cholesterol by binding it in the gut.
Which fibre is better?
Both types matter; a varied plant-based diet covers both needs.
Does fibre help with constipation?
Yes, insoluble fibre increases stool volume and speeds up motility.