FibreFiber · Dietary fibre
Carbohydrate the body cannot digest, feeding gut bacteria and supporting satiety.
Fibre is carbohydrate your own enzymes cannot digest. It passes into the large intestine intact, where some of it is fermented by gut bacteria and some simply adds bulk and holds water.
The old split into soluble and insoluble is being replaced by a more useful description based on what a fibre actually does: how viscous it is, how fermentable it is, and how much bulk it adds. Oats and beans are viscous and fermentable; wheat bran mostly adds bulk; inulin ferments readily and, in quantity, produces the wind people blame on the beans.
It is the closest thing nutrition has to a consensus win. A series of systematic reviews pooling 185 prospective studies and 58 trials found 15 to 30% lower all cause and cardiovascular mortality, and lower incidence of coronary heart disease, stroke, type 2 diabetes and colorectal cancer, comparing the highest fibre eaters with the lowest. Benefit was greatest between 25 and 29 g a day and the dose response curve suggested more may be better still.
It works through several routes at once: feeding the bacteria that produce short chain fatty acids, slowing glucose absorption, lowering cholesterol where the fibre is viscous, adding stool bulk, and increasing fullness for very few calories.
UK guidance sets 30 g a day for adults and national diet surveys show most people fall well short, which makes the gap itself the most valuable and cheapest improvement available in most diets.
Increase it gradually and with water. Going from 12 g to 30 g overnight produces bloating and discomfort that people then interpret as an intolerance, when the actual problem was the speed of the change.
In practice, the sources that do it are ordinary: pulses, wholegrains, fruit with the skin on, vegetables, nuts and seeds. Our macro calculator tracks the intake; the food reference gives the numbers per portion.