Dietary Fiber and the Human Gastrointestinal Microbiota as Predictors of Bone Health

Abstract

BackgroundIncreasingly, the gastrointestinal (GI) microbiota is emerging as a factor relevant to bone health. Proposed mechanisms of the inter‐relationships among dietary fiber, the GI microbiota, and bone health include increased availability of absorbable calcium because of lowered gut pH related to bacterial fermentation of fiber; short‐chain fatty acid (SCFA) signaling from bacterial metabolism of dietary fibers that regulates mineral absorption and cell proliferation, thereby increasing surface area for absorption; and suppression of systemic inflammation, a risk factor for osteoporosis, by the microbiota. However, these relationships are under‐investigated in human populations.ObjectiveWe aimed to determine the relationships among bone health, dietary fiber consumption, and the GI microbiota and their metabolites (i.e., SCFAs) in adults.MethodsCross‐sectional analyses were conducted on 25–46‐year‐old adults (n=64, 36 females). Dietary fiber, calcium, and vitamin D intake were assessed using a 7‐day diet record. Body composition and bone density were assessed using dual‐energy X‐ray absorptiometry (DXA). Fecal samples were utilized to assess bacterial composition, SCFA concentrations, and pH. Barcoded amplicon pools of bacterial sequences were generated using high‐throughput sequencing followed by analysis using QIIME 1.9.0. SCFAs were quantified on a dry matter basis using gas chromatography. Fecal pH was measured using a pH meter. Relationships between bacterial relative abundances, SCFA concentrations, and pH were first assessed using Pearson (for normally distributed variables) and Spearman (for non‐normally distributed variables) correlations, with partial correlation for normalized dietary fiber intake (fiber/kcal). Then, a regression model was conducted to assess the relationship between fiber and bone density after adjusting for covariates including age, sex, and BMI.ResultsBivariate analyses revealed that greater bone mineral density was correlated with higher intakes of fiber (r=0.24, p=0.03). Bone mineral density was inversely related to Blautia (r=−0.28, p=0.03) and Clostridiaceae (rho= −0.31, p=0.01). Greater bone mineral density tended to be related to higher fecal butyrate concentration (r=0.31, p=0.08) and lower fecal pH (r=−0.30, p=0.06). Similar to bone mineral density, bone mineral content was inversely related to Blautia (r=−0.26, p=0.04) and Clostridiaceae (rho=−0.27, p=0.03); and greater bone mineral content tended to be positively related to fecal butyrate concentration (r=.30, p=0.09) and negatively related to fecal pH (r=−0.30, p=0.06). Interestingly, Blautia and Clostridiaceae were not related to fecal pH or butyrate. The linear regression model revealed a positive relationship between greater bone mineral density and dietary fiber intake (p=0.02)ConclusionsIn summary, bone mineral density was associated with greater intakes of fiber, lower fecal pH, and higher fecal concentrations of butyrate, which supports proposed mechanisms. Additional study is necessary to determine the role of specific microbes in this relationship.Support or Funding InformationPartial funding for this study was provided by the Hass Avocado Board and the USDA National Institute of Food and Agriculture, Hatch project ILLU 538 384.