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 about 20 g. Closing that gap is one of the highest-use things you can do for your gut.

• 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.

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: helpful for most people, though hard work for IBS-sensitive guts. Sources: onion, garlic, leek, asparagus, Jerusalem artichoke, bananas, beans, chicory root, oats.

• 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: the UK keeps the 30 g target for everyone over 16, though some guidance (e.g. US IOM) sets slightly lower intakes for over-50s (~21-30 g) reflecting lower energy needs.

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. In the American Gut study, people eating more than 30 different plant foods a week had more diverse gut bacteria than those eating fewer than 10. No guideline sets a target, but it is a reasonable thing to aim at, and herbs and spices count.
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.

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.

See a doctor before increasing fibre if you have blood in your stool, unexplained weight loss, a change in bowel habit that has not settled, iron-deficiency anaemia, or abdominal pain that wakes you at night. A change in bowel habit that has not settled is worth checking at any age. Fibre is not the answer to any of those, and loading up on it can delay finding out what is actually going on.

• IBS: high-FODMAP fibres (onion, garlic, beans, asparagus) often set sensitive guts off. Low-FODMAP soluble fibre (oats, chia, kiwi) is the friendlier route until you've worked out your own triggers.
• A diverticulitis flare-up changes the rules temporarily: follow the plan your own care team gives you.
• After abdominal surgery: follow surgical team guidance.
• Already badly constipated: gentle soluble fibre such as psyllium, with plenty of water, tends to be the kinder start before adding variety.
• Low on iron: bran can get in the way of iron absorption, so it is worth keeping iron supplements and bran-heavy meals a couple of hours apart. A pharmacist can advise.