Dietary fiber

Dietary fiber comprises non-digestible plant polysaccharides and oligosaccharides that resist hydrolysis by human intestinal enzymes. The two principal classes are soluble fiber (e.g., pectin, beta-glucan, inulin), which dissolves in water and forms a viscous gel, and insoluble fiber (e.g., cellulose, lignin), which adds stool bulk and accelerates colonic transit. In the colon, soluble fiber is fermented by resident microbes — including members of Bacteroidetes and Firmicutes — into short-chain fatty acids (SCFAs): acetate, propionate, and butyrate. Butyrate is the primary energy substrate for colonocytes and acts as a histone deacetylase inhibitor, dampening NF-κB-mediated inflammatory signalling; propionate travels to the liver to regulate gluconeogenesis; acetate circulates systemically and modulates appetite via GPR41 and GPR43. These mechanisms are directly relevant to aging: chronic low-grade systemic inflammation (inflamm-aging) is attenuated by higher SCFA production, and an age-related decline in SCFA-generating microbiota is well documented. Dose-response meta-analyses of prospective cohorts (Yao et al. 2023) show that each 10 g/day increment in fiber intake associates with roughly 10% lower all-cause mortality and 13% lower cardiovascular mortality; Reynolds et al. 2019 (Lancet) further established that intakes of 25–29 g/day confer the greatest reduction in non-communicable disease risk. These associations are observational, and randomised trial evidence on hard outcomes remains limited.