Walnuts are not a classic “single-substrate” prebiotic like inulin, but they can support prebiotic-like effects through their food matrix. They provide fiber, polyphenols, and a characteristic fatty-acid profile that can shape the gut ecosystem indirectly. Polyphenols can be metabolized by gut microbes into bioactive compounds, potentially favoring beneficial taxa. Strictly speaking, a prebiotic claim typically requires evidence that a specific substrate is selectively utilized by beneficial microorganisms and confers a health benefit; for walnuts, effects are more multi-factorial. Practically, pairing walnuts with high-fiber foods (vegetables, legumes, whole grains) creates a stronger gut-supportive pattern. Because walnuts are energy-dense, portion control matters, and intake should be individualized for allergies, anticoagulant use, or dietary restrictions.