Association between the diet gut microbiota index and survival in adult cancer survivors: Findings from the National Health and Nutrition Examination Survey (2007-2018).

The gut microbiota (GM) is increasingly recognized as a key factor influencing cancer progression, treatment response, and long-term survival. Dietary patterns are major modulators of GM composition and may thus impact clinical outcomes in adult cancer survivors. However, the association between GM-related dietary indices and mortality in this population remains unclear. We analyzed data from 2711 adult cancer survivors (≥20 years, defined by self-reported physician diagnosis) in the National Health and Nutrition Examination Survey 2007-2018. The dietary index for gut microbiota (DI-GM) was calculated from two 24-hour dietary recalls (in person and telephone). Due to the study design, the timing of dietary data collection relative to cancer treatment status was not available. Cox proportional hazards models with appropriate weighting assessed the association between DI-GM (continuous and quartiles) and all-cause mortality, adjusting for key sociodemographic and clinical covariates. Mediation analysis assessed the role of systemic inflammation. Compared to the lowest quartile (Q1), the highest quartile (Q4) showed a 29% lower risk of all-cause mortality (hazard ratio = 0.71, 95% confidence interval: 0.57-0.88, P = .002). While a trend of decreasing mortality risk was observed across all quartiles, only Q4 reached statistical significance. Dose-response analysis indicated a nonlinear inverse relationship, with the protective effect plateauing at a DI-GM score of 6.78. Mediation analysis suggested that systemic inflammation, particularly assessed by the systemic inflammation response index, statistically but modestly mediated this relationship (average causal mediation effect = 0.994, 95% confidence interval: 0.988-0.998), confirming a partial mediation. Cancer-specific mortality was not assessed due to data limitations. A higher DI-GM score, reflecting a GM-friendly dietary pattern, was independently associated with lower all-cause mortality among US adult cancer survivors. These findings support the potential role of GM-friendly dietary patterns as a modifiable factor associated with improved long-term survival in US adult cancer survivors.