BACKGROUND: Establishing optimal maternal nutritional habits during pregnancy, breastfeeding, and early life is crucial for the health and welfare of both the mother and the child. However, research is lacking to understand the associated mechanisms linking maternal diet to health outcomes. The objective of this study was to assess the potential influence of two distinct diets, consumed during gestation and lactation, on the microbiota composition, immunity and lipid metabolism of Lewis dams. METHODS: Diet 1 (D1, Mediterranean diet-like) was characterized by a high fibre content, vegetable protein, and fish oil; whereas Diet 2 (D2, slightly Western diet-like) was enriched with animal protein and lard. Fecal samples were collected weekly throughout the nutritional intervention. Blood, tissue samples (adipose tissue, intestine, mammary gland, spleen and liver) and cecal content were collected from the mother at the end of lactation (day 21) to examine the effects on the epithelial barrier, lipid metabolism, microbiota composition and metabolites, as well as the mucosal immunity. FINDINGS: According to our findings, consumption of the D1 diet had a beneficial impact on the mothers compared to the D2 diet. D1 increased the intestinal surface area and enhanced the mucosal immunity, as evidenced by a rise in fecal immunoglobulin (Ig) A and Ig-coated bacteria levels, along with an increase in total IgG in the mesenteric lymph nodes, as well as elevated levels of T helper (Th)1-associated IgG2c isotype. Furthermore, D1 influenced the adiposity and exerted an anti-obesogenic effect on brown adipose tissue by up-regulating the expression of the genes Ucp-1, Cidea, Prdm16 and Gpr43. D1 also influenced the cecal microbiota composition, impacting its functions such as the production of short-chain fatty acids in the caecum. D1 reduced microbiota diversity by increasing beneficial taxa, such as Ruminococcaceae family and Turicibacter genus, among others. These genera showed correlations with the analysed immune and lipid metabolism parameters suggesting that microbiome modulation serves as a link between the observed systemic effects and the dietary intervention. INTERPRETATION: The study highlights that a diet rich in fibre, vegetable protein, and fish oil, consumed during gestation and lactation, enhances maternal health by improving intestinal function, mucosal immunity, and exerting anti-obesogenic effects on lipid metabolism, likely mediated by modifications in the cecal microbiota composition and function. FUNDING: The study was supported by LaMarató-TV3 (DIM-2-ELI, ref. 2018-27/30-31).