Exposure to a High-Fat Diet Compromises Gut Health, Behavior, and HPA Axis Function, with Partial Reversal When Limited to Adolescence

High-fat diet (HFD) consumption contributes to obesity, yet its impact on females of (pre)reproductive age and the effects of dietary modification after adolescence remain underexplored. This study examined how continuous HFD exposure or an adolescent switch from HFD to a standard diet (SD) shapes the gut microbiome, behavior, neurochemistry, metabolism, and key components of the hypothalamic–pituitary–adrenal (HPA) axis in female rats. Because HPA-axis alterations can occur across generations after HFD exposure, we examined reproductive-tissue HPA-axis components as potential mechanisms of transmission. Females received SD, HFD, or HFD followed by SD after majority of adolescence (postnatal day 60). HFD exposure impaired HPA-axis regulation and switching to SD during adolescence did not prevent persistent dysfunction into adulthood. However, reproductive HPA-axis components remained unaltered. Diet also strongly influenced the microbiome: while HFD disrupted microbial composition in adolescence, switching to SD partially restored it by adulthood. Behavioral and metabolic effects, including increased adiposity and anxiety-like behavior, emerged only with prolonged HFD exposure. Brain neurotransmitter concentrations remained largely unaffected. Overall, dietary history across adolescence and early adulthood shaped long-term HPA-axis function, microbiome composition, and behavioral outcomes. The absence of reproductive HPA-axis alterations suggests it is not a major mediator of maternal HFD-induced intergenerational effects. Persistent HPA-axis dysfunction despite dietary switching indicates limited reversibility, whereas the microbiome showed the greatest adaptive capacity. In contrast, lasting behavioral and metabolic consequences of HFD require continued exposure to adulthood. • HFD had a direct impact on the HPA axis. Switching from HFD to a standard diet during adolescence did not prevent lasting HPA axis dysfunction into adulthood. • Although HFD disrupted the HPA axis, it did not affect the reproductive system, suggesting that HPA axis alterations are not the primary mediators of maternal diet-induced offspring impairments. • The diet directly influenced the microbiome, which exhibited clear adaptation after an adolescent switch to a standard diet, indicating partial reversibility of dietary effects. • In adulthood, HFD affected body weight, metabolic measures, and behavior, suggesting that prolonged exposure beyond adolescence is necessary to produce substantial alterations in these parameters.