Host metabolism shapes the intestinal microbiota: a top-down paradigm of environmental selection pressure

Intestinal homeostasis is not a stochastic microbial assembly but a deterministic outcome orchestrated by host-mediated metabolic gating. Traditional research has prioritized the microbiota's impact on host physiology. However, the consistent expansion of facultative anaerobes, such as Enterobacteriaceae, observed in pathological states like intestinal inflammation, suggests that dysbiosis is fundamentally a consequence of impaired host regulation. Here, we propose a "top-down" paradigm of host metabolic regulation, framing the host as an "ecological engineer" that actively shapes the microbiome through metabolism. We detail three critical metabolic filters: (1) the maintenance of epithelial hypoxia via mitochondrial β-oxidation to suppress aerobic respiration; (2) the implementation of "nutritional immunity" to restrict glucose and inflammation-derived electron acceptors (nitrate and tetrathionate); and (3) the energy-dependent synthesis of the gel-forming mucin 2 (MUC2) mucus layer and antimicrobial peptides (AMPs). We argue that the breakdown of these filters leads to "niche opening," which acts as the fundamental driver of dysbiosis. Finally, we discuss therapeutic strategies aimed at restoring host bioenergetics-including Peroxisome proliferator-activated receptor gamma (PPAR-γ) agonists, melatonin, and ketogenic diets-to rebuild the host's ecological filtration capacity and fundamentally correct dysbiosis.