Why most diets fall short on fibre — and what that does to your gut

Fibre is one of those nutrients everyone knows they should get more of, and almost nobody actually tracks. Most adults get roughly half the fibre intake nutrition guidelines recommend — and the gap shows up less in digestion than people expect, and more in the makeup of the gut itself.

Your gut bacteria eat what you don't

Dietary fibre passes through the small intestine largely undigested by you — but not by the roughly 100 trillion bacteria living in your colon. Different fibres feed different bacterial populations. A diet built around only one or two fibre sources (say, just wheat bran, or just a fruit smoothie) only feeds a narrow slice of that ecosystem.

This matters because a more diverse, well-fed gut microbiome is linked to a broader range of everyday functions: how full you feel after eating, how your body handles blood sugar after a meal, and even mood and stamina, since gut bacteria influence far more of the body than digestion alone.

What a genuinely varied fibre intake looks like

Nutritionally, the fibre sources people usually reach for — resistant starch, beta-glucans from oats, inulin, psyllium husk — each behave differently in the colon. Resistant starch, for instance, specifically helps blunt the blood sugar spike that follows a carbohydrate-heavy meal. Beta-glucans from oats are one of the few fibres with a recognised role in maintaining normal cholesterol levels. Relying on just one source means missing what the others do.

The practical takeaway

You don't need to overhaul your diet to close this gap — a fibre blend that combines several sources in one serving (mixed into water, a smoothie, or a shake) is a straightforward way to feed a wider range of gut bacteria than most single-ingredient fibre supplements manage, without needing to plan every meal around it.