This randomized controlled trial aims to evaluate the effects of structured lactation nutrition education combined with Dali ni Horbo supplementation on maternal dietary intake, breastfeeding practices, infant growth, and infant gut microbiota. Lactating mothers with healthy infants aged 0-1 month will be recruited from selected primary healthcare centers in Medan, Indonesia, and assigned to an intervention or control group. The intervention group will receive structured nutrition education and daily Dali ni Horbo, a traditional Batak fermented buffalo milk product, while the control group will receive standard care. Maternal energy and nutrient intake, nutritional knowledge and attitudes, exclusive breastfeeding practices, maternal and infant anthropometric measurements, intervention adherence, adverse effects, and infant fecal microbiota composition will be assessed before and after the intervention. This study is expected to determine whether integrating nutrition education with a nutrient-dense local food can improve maternal nutritional adequacy, support exclusive breastfeeding, promote optimal infant growth, and beneficially influence early-life gut microbiota.