Autism prevalence and the limits of diagnostic expansion: a perspective on diagnostic validity, adult assessment, and phenotypic stratification.

Depression is a major public health concern in the United States, with evidence of increasing prevalence over time and persistent differences across population subgroups. Depression is also socially patterned, including by education and other socio-economic indicators, which may overlap with dietary behaviours, food access, and other health-related exposures. Although depression is multifactorial and cannot be explained by diet alone, diet is a modifiable exposure that can influence gut microbial composition and metabolism. Growing evidence suggests that major depressive disorder is associated with alterations in gut microbiota composition, supporting interest in the gut–brain axis as one potential pathway through which diet-related exposures may influence depressive symptoms. Prebiotics are selectively utilised dietary substrates that confer a health benefit through the gut microbiota. Because the definition and scope of prebiotics continue to evolve, the present study focused on International Scientific Association for Probiotics and Prebiotics (ISAPP)-recognised prebiotics relevant to the US diet, primarily galactooligosaccharides (GOS) and inulin-type fructans, including inulin, fructooligosaccharides (FOS), and oligofructose. These compounds are present in foods commonly consumed in the United States, including cereals and grains, fruits, vegetables, pulses, and legumes. However, intake depends not only on whether these foods are consumed, but also on portion size, food preparation, mixed-dish ingredients, and frequency of consumption. Estimated effective doses of prebiotics have been reported to range from 5 g/day to 15 g/day for adults, although this range should be interpreted as a physiological benchmark rather than a diagnostic or dietary cutoff. This benchmark is useful for evaluating whether estimated habitual intake approaches quantities that may be biologically relevant. Through their effects on gut microbial metabolism, prebiotics may plausibly influence depressive symptoms through pathways involving short-chain fatty acid production, immune-inflammatory signalling, gut barrier function, and neuroendocrine regulation. These mechanisms provide a rationale for studying prebiotic intake in relation to depressive symptoms, but they do not establish that higher prebiotic intake reduces depression risk in free-living populations.