A Herbivore-Carnivore (HC) index, built from bacterial groups that distinguish plant-eating from meat-eating mammals across 858 samples from 109 species, placed humans between the two poles. Hunter-gatherers and nonhuman apes sat on the herbivore side; urbanized populations drifted carnivore-ward. Applied to more than 72,000 participants worldwide, higher HC values tracked with plant-based, fiber-rich diets, greater microbial diversity, stronger genetic capacity to break down plant carbohydrates, better clinical measures, and fewer self-reported diseases.
The cross-species grounding is clever: it offers an interpretable axis for microbiome status rather than another opaque diversity score. But the associations are cross-sectional and rest partly on self-reported disease, so they cannot show that a herbivore-leaning microbiome improves health, or that fiber intake moves the index and drives the benefit. Diet, income, medication use and baseline health all shape both the microbiota and outcomes, and the abstract does not say how well these were adjusted for. Cohort heterogeneity across countries and sequencing methods adds noise. The practical takeaway is modest and consistent with existing evidence: eating more plants and fiber is the lever most likely to shift gut ecology in a favorable direction. The HC index is not yet a clinical test. This is a preprint that has not been peer reviewed, so effect sizes and conclusions may change. Editorially, it is incremental but useful, offering a conceptual framework more than a new intervention.