Fibre is one of the most common shortfalls in the modern diet, and one of the clearest-cut: US national survey data (NHANES) suggests around 95% of people fall below the recommended intake.
Fibre isn't absorbed for energy the way most nutrients are; it does its work passing through your digestive system. It supports healthy digestion and regularity, feeds the beneficial bacteria in your gut, and a consistently good intake is associated with steadier blood sugar and better cholesterol levels over time. A low intake tends to show up as digestive sluggishness rather than anything dramatic.
It's easy to miss because refined and processed foods have most of their fiber stripped out, so a diet heavy on white bread, refined grains, and packaged food can fall short even when it feels substantial. Whole grains, legumes like beans and lentils, fruit, vegetables, nuts, and seeds are all strong sources.
Adults benefit from around 25 g a day for women and 38 g for men. There's no upper limit or toxicity ceiling here, though it's worth increasing intake gradually and drinking enough water, since a sudden large jump can cause temporary bloating.