Quantity and source of dietary protein influence metabolite production by gut microbiota and rectal mucosa gene expression: a randomized, parallel, double-blind trial in overweight humans.

Randomized, parallel, double-blind, three-arm trial in overweight adults aged 18 to 45 years at Avicenne Hospital in Bobigny, France. Among the active intervention arms, 12 participants were randomized to casein and 13 to soy protein; both groups were overweight, with mean BMI 28.27 kg/m2 and 27.36 kg/m2, respectively.

After a dietary normalization run-in, participants received an isocaloric protein powder providing 15% of habitual energy intake three times daily for 3 weeks, dissolved in water. The active regimens were milk protein isolate enriched in micellar casein (casein arm) or isolated soy protein (soy arm); maltodextrin served as the control comparator.

High-protein supplementation altered gut microbial metabolite production and rectal mucosa gene expression, but did not change fecal water cytotoxicity or mucosal inflammatory markers. Compared with maltodextrin, both casein and soy reduced fecal butyrate; casein increased 2-methylbutyrate, while soy increased valerate, phenylacetate, tyramine, and acetoin signals. Urinary isobutyrate, indoxyl sulfate, and phenylacetylglutamine increased with protein intake, and casein specifically increased p-cresyl sulfate. Casein and soy also differentially affected rectal mucosa genes involved in mucosal homeostasis, cell cycle, and cell death, indicating that protein source influences host-microbiota responses.

Small active-arm sample sizes and a short 3-week intervention limit clinical interpretation. The trial was conducted in a single center among overweight adults, so generalizability is limited. Outcomes were largely mechanistic and surrogate-based, and adverse events were not described in the extracted data.