Dietary fibre is the most evidence-backed thing you can eat more of

A 2019 meta-analysis in The Lancet, commissioned by the WHO, reviewed 185 prospective studies and 58 clinical trials and found that each 8g increase in daily dietary fibre intake was associated with a 5–27% reduction in all-cause mortality, stroke, coronary heart disease, type 2 diabetes, and colorectal cancer. The average adult in the UK consumes 19g per day against a recommended 30g. Legumes, whole grains, vegetables, and fruit are the most practical sources; supplements are less effective than whole foods.

Source: Reynolds et al., The Lancet (2019)