Fast Food and Full-Service Restaurant Consumption Show Different Associations with Systemic Inflammation Among U.S. Adults: Dietary Fiber as the Primary Mediator in NHANES 2015–2018

This study aimed to determine whether fast food and full-service restaurant consumption have different associations with systemic inflammation and to identify nutrient mediators. This cross-sectional analysis included 6357 adults from NHANES 2015-2018. Calories from fast food and full-service restaurants were calculated using 24-hour dietary recall food-source codes. Systemic inflammation was assessed using serum high-sensitivity C-reactive protein (hs-CRP). Baron-Kenny mediation tested dietary fiber, saturated fat, sodium, and total sugar as candidate mediators. Survey-weighted models adjusted for age, sex, race/ethnicity, education, income, BMI, energy intake, and smoking. Fast food consumption was significantly associated with higher hs-CRP (β = 0.014 per 10% caloric increase; P = 0.035), whereas full-service restaurant consumption was not (β = −0.005; P = 0.578). In a head-to-head model, fast food remained significant and full-service dining remained null. Fiber mediated 34.2% of the fast food-CRP pathway, and saturated fat mediated 12.6%. After adjustment for all nutrients, the direct fast-food effect became nonsignificant ( P = 0.213). The association was strongest among adults aged 20-39 years ( P = 0.016). The fast food-CRP association was mediated primarily by lower fiber density, supporting fiber-focused counseling across eating contexts. Findings emphasize nutrient composition rather than restaurant use alone as the central public-health target.