Fiber

Fiber is a carbohydrate, but of a kind apart: the small intestine has no enzymes to cut it. It passes through whole and reaches the colon exactly as it was eaten. That is the definition EFSA uses, and it covers a whole family: cellulose, pectins, beta-glucans, resistant starch, lignin.

Everything else follows from that. What is not absorbed adds bulk and speeds up transit. What adds bulk fills the stomach without bringing calories. And what reaches the colon intact feeds the bacteria there, which turn it into the short-chain fatty acids the cells of the gut wall live on.

They also slow the absorption of the rest of the meal, which flattens the rise of sugar in the blood: that is why wholemeal and white bread do not have the same glycemic index, as Glycemic index and glycemic load explains.

EFSA settles on 25 g a day in adults, the figure above which bowel function is normal. It adds that beyond 25 g the benefits keep coming: less cardiovascular disease, less type 2 diabetes, weight better held.

ANSES aims higher, 30 g a day, because that is the level at which the drop in colorectal cancer risk becomes clear. France eats about 18 g: the gap is a good third.

Ranked raw, seeds and cocoa crush everything. One line per family says the important thing better: it is everywhere, in every plant aisle.

The families missing from this table are missing for a simple reason: no animal food contains fiber. The best-placed meat in the database is Chorizo, at 0.9 g, and what it shows comes from the spices in its recipe. Meat, fish, egg, cheese: zero.

Per-100 g figures mislead, because nobody eats 100 g of chia seeds. What counts is what an ordinary day adds up to, in real portions:

None of these lines is an effort: wholemeal bread rather than white, a plate of lentils, a vegetable, a fruit, a handful of hazelnuts. It is the swapping that makes the total, not a supplement.

It is also why fiber weighs on satiety: the same volume on the plate, for far fewer calories, see Losing weight.

Soluble fiber dissolves in water and forms a gel: the pectin of apples and citrus, the beta-glucans of oats and barley, the mucilages of chia and psyllium. These are the ones that slow digestion, calm the sugar rise and best feed the colon bacteria.

Insoluble fiber does not dissolve: the cellulose and lignin of wheat bran, vegetable skins, whole grains. It makes the bulk and pushes transit along.

The distinction is worth knowing, but it does not need managing: no food has only one or the other, and eating varied settles it without thinking about it.

Doubling your fiber overnight is paid for in bloating. The cause is not the fiber but the flora: the bacteria that can digest it are still few, and those that take it on produce a lot of gas. They need two or three weeks to settle in.

Hence the one rule that matters: go up in steps of a few grams, over a few weeks, and drink. The fiber that adds bulk does so by holding water; without water, it hardens the stool instead of softening it, and the cure becomes the problem.

25 g a day for transit, 30 g for everything else, and France sits at 18 g.

No animal food has any: fiber comes only from plants, and from every family of plants.

An ordinary day gets there with wholemeal bread, a plate of pulses, a vegetable, a fruit and a handful of nuts. No supplement needed.

Go up slowly, and drink: the early bloating comes from the flora, not from the fiber, and it passes in two or three weeks.