Habitual and supplemented prebiotic diets and their links to inflammatory serum markers and hypothalamic microstructure in young, overweight adults: a pre-registered study.

BackgroundPrebiotic dietary fiber and related metabolites have been suggested to attenuate low-grade systemic and central inflammation through improving gut-brain axis signaling. We here aimed to test whether habitual or short-term high-dose fiber intake is linked to inflammatory markers in blood and to indicators of central hypothalamic inflammation. MethodsIn total, 59 adults (19 women, aged 28.3 years {+/-} 6.6 SD, mean body mass index, BMI, 27.3 {+/-} 1.5 SD) were included into analyses. Participants completed a food frequency questionnaire, underwent diffusion-weighted magnetic resonance imaging (MRI) at 3 Tesla for provision of mean diffusivity (MD) as a marker of brain tissue inflammation and donated fasting blood. Measurements took place at up to 4 timepoints, i.e. before and after 14 days of supplementary fiber and placebo intake, respectively. High-sensitive C-reactive protein (CRP), tumor-necrosis factor alpha (TNF-) and interleukin-6 (IL6) were assessed in serum. The study was preregistered at https://osf.io/uzbav. ResultsHabitual and interventional high-fiber diet was not significantly associated with neither inflammatory markers (|{beta}intervention|> 0.1, p > 0.32) nor with hypothalamic MD (|{beta}intervention| = 1.8, p = 0.07) according to linear mixed effects modeling. Male sex and higher body fat mass related to higher CRP. Further, higher BMI was borderline related to lower hypothalamic MD. ConclusionsIn this sample of overweight adults, dietary fiber intake was not related to inflammatory blood markers or hypothalamic microstructure. Instead, sex and body composition were of higher importance for prediction of interindividual differences in markers of (neuro)inflammation. Significance StatementPrebiotic dietary fiber has been discussed to lower systemic and central inflammation. While previous studies investigated the effects of fiber on inflammatory blood markers, the knowledge of the effect of fiber on neuroinflammation is limited. Thus, in this pre-registered randomized controlled trial analysis we examined the relationship between dietary fiber intake and inflammatory markers in blood and hypothalamus. 3T MRI and blood markers were assessed before and after high-fiber intake and placebo in 59 adults. In our overweight study sample of 19-42 years old adults, fiber intake had no significant impact on inflammatory markers. The current null findings can inform future nutrition neuroimaging trials and add to the discussion about how diet may affect brain structure and function.