Biomarkers of Total Dietary Fiber Intake: A Challenge to Meet

Dietary fiber is considered an important health-promoting food component; however, to better characterize associations between fiber consumption and health outcomes, it is important to identify reliable biomarkers of intake. Dietary fiber includes all nondigestible carbohydrates that are characterized by different chemical structures, yet have a common resistance to digestion and absorption in the intestine, where they act as substrates for the resident microbiota. Considering the structural complexity and diversity of fiber, its physico-chemical properties, the abundance of fiber-rich foods, and the complexity of the gut microbiota, the identification of biomarkers of fiber intake is quite challenging. To this aim, a narrative review was performed to define the critical issues for fiber biomarker identification and then to analyze and validate putative biomarkers of total fiber intake. On the basis of the common properties of the different types of fiber, we have identified the following as possible candidate biomarkers: fecal fiber components; stool weight; fecal microbiota composition; microbiota metabolites, such as fecal and plasma short-chain fatty acids and plasma odd-chain fatty acids; and breath hydrogen and methane. The scientific evidence to support their role as biomarkers of intake was evaluated by applying the validation criteria developed by the FoodBAll Consortium. We also explored whether a combination of validated biomarkers of food intake relative to the main dietary fiber sources could be considered to develop a score-like evaluation of fiber consumption in dietary patterns. What emerged from our analysis was that the complexity of the relationship between diet, fiber composition, and microbiota, as well as the limited and fragmented scientific evidence currently hamper the identification of robust biomarkers of fiber intake. Indeed, none of the potential biomarkers considered in our analysis completely fulfilled all of the validation criteria; however, microbiota composition and breath hydrogen appeared to be the most promising candidates.