Fibre: how much, what kind, how to add it

Fibre is the most under-eaten nutrient in the modern diet, and the one with the strongest evidence for everything from bowel health to cardiovascular protection. The official UK target is 30 g a day. Most adults eat 19 g. Closing that gap is one of the highest-use things you can do for your gut.

What fibre actually does

• Bulks stool, speeds transit, prevents constipation.
• Feeds beneficial gut bacteria. They ferment it into short-chain fatty acids (butyrate, acetate, propionate) that strengthen the gut barrier and reduce inflammation.
• Slows post-meal blood sugar spikes by adding viscosity in the small intestine.
• Lowers LDL cholesterol by binding bile acids in the gut.
• Supports satiety. Fibre meals keep you fuller for longer than equivalent low-fibre meals.

Three types, you need all three

Soluble fibre

Dissolves in water, becomes gel-like in the gut. Slows digestion, lowers cholesterol, regulates blood sugar. Sources: oats, oat bran, chia seeds, flaxseed, psyllium, kiwi, apple flesh, citrus, beans, lentils.

Insoluble fibre

Doesn't dissolve. Adds bulk to stool, speeds transit. Sources: wheat bran, whole grains, brown rice, leafy greens, vegetable skins, seeds, nuts.

Fermentable fibre

Selectively feeds gut bacteria. Some are also FODMAPs, useful for most people but problematic for IBS. Sources: onion, garlic, leek, asparagus, Jerusalem artichoke, bananas, beans, chicory root, oats.

Daily targets by age

• Adults: 30 g (UK SACN), 25-38 g (US Institute of Medicine).
• Children 5-11: 20 g.
• Children 11-16: 25 g.
• Pregnancy: same as adult target plus extra water.
• Older adults (70+): often need slightly less (25 g) because of reduced energy intake and water absorption.

How to ramp up to 30 g (without the gas)

1. Week 1: add 5 g per day. One swap (e.g. white bread → wholemeal) plus a piece of fruit. You're now at ~24 g.
2. Week 2: add another 5 g. Add a serving of beans/lentils to one meal a day, or 1 tbsp chia in your breakfast. You're at ~29 g.
3. Week 3: get the variety up. Aim to eat 30 different plant foods over the week, including spices and herbs.
4. Week 4: settle. Track daily fibre for 5 days; you'll likely be at 30+ g without effort.
5. Drink at least 2 litres of water across the day. Fibre without water makes constipation worse, not better.
6. If gas is severe, slow the ramp. Adding 3 g a week instead of 5 is gentler and usually settles within 2-3 weeks of any new fibre level.

Easy 30 g day examples

Plant-forward

• Breakfast: porridge oats (50 g) with chia (1 tbsp), banana, mixed berries, about 11 g.
• Lunch: lentil soup with wholemeal bread, about 12 g.
• Snack: apple with skin, handful of almonds, about 5 g.
• Dinner: chickpea curry with brown rice and leafy greens, about 15 g.
• Total: ~43 g.

Mixed eater (no calorie counting)

• Breakfast: 2 slices wholemeal toast with avocado, eggs, about 8 g.
• Lunch: chicken salad with kale, beans, tomato, sunflower seeds, about 12 g.
• Snack: pear, dark chocolate (1 square 70%+), about 5 g.
• Dinner: salmon, roasted broccoli, sweet potato (skin on), about 10 g.
• Total: ~35 g.

When fibre causes problems

• IBS: high-FODMAP fibres (onion, garlic, beans, asparagus) often trigger symptoms. Switch to low-FODMAP soluble fibre (oats, chia, kiwi) until you've done the FODMAP elimination.
• Diverticulitis flare: low-fibre diet during acute flare, then ramp back up.
• After abdominal surgery: follow surgical team guidance.
• Severe constipation already: psyllium with water for 1-2 weeks before increasing variety.
• Iron deficiency on bran: bran can interfere with iron absorption. Take iron supplements 2 hours away from bran-heavy meals.