Although the adverse health effects of consuming ultra-processed foods (UPFs) are well-documented, limited evidence exists on their impact during lactation. This study examined the association between maternal UPF consumption, feeding practices, and malnutrition in breastfed infants. A cross-sectional analysis was conducted with 111 mother–child pairs up to 150 days postpartum. Infant feeding practices were assessed using WHO indicators, and malnutrition was evaluated by length-for-age and BMI-for-age. Maternal dietary intake was estimated using two 24 h recalls, and UPF consumption was classified by the Nova classification. Dyads were grouped based on the highest UPF consumption quartile, and adjusted binary logistic regression was applied. UPFs accounted for 26% of the maternal diet on average. While 71.2% of infants were exclusively breastfed, one-third were overweight, and 11.7% were stunted. High maternal UPF consumption (>32% of energy) was associated with increased odds of malnutrition by BMI-for-age (wasting or overweight) (OR 3.38; 95% CI: 1.29–8.83) and stunting (OR 3.89; 95% CI: 1.04–14.58). Feeding practices showed no association. These findings highlight that maternal UPF consumption is associated with malnutrition odds in breastfed infants in the population assessed, emphasizing the need for dietary guidance during lactation to improve infant health outcomes.

Background: Effective postpartum weight management is essential for maternal health, yet the impact of specific nutritional strategies during lactation remains poorly defined. Objectives: This systematic review evaluated the impact of nutritional interventions on weight management among lactating women. Methods: A systematic search was conducted in PubMed, Web of Science, Scopus, and Bireme/LILACS (December 2022; updated March 2026). We included clinical trials assessing nutritional interventions for weight management during the lactation period. Study quality was evaluated using the Risk of Bias Tool 2 and the Risk of Bias in Non-Randomized Studies of Interventions. Results: Seven clinical trials met the inclusion criteria, initiated during pregnancy ( n = 1) or lactation ( n = 6), with intervention durations ranging from 11 days to 12 months. Caloric restriction (deficit of 500 kcal/day) combined with macronutrient adjustment was associated with significant postpartum weight loss and improved body composition. Conversely, micronutrient supplementation alone demonstrated no effect on weight reduction. The evidence is constrained by a limited number of studies, heterogeneous intervention protocols, varying postpartum stages, and incomplete reporting, which restricts the generalizability of the findings. Conclusions: While evidence remains limited, caloric restriction for durations exceeding 1 month appears to promote weight loss in lactating women without self-reported adverse effects on breastfeeding performance. However, caution is important regarding the use of caloric restriction during lactation, due to the knowledge gap concerning its impacts on the mother-infant dyad and breast milk composition.