Background
Optimal feeding practices during the first 2 years of life are vital for child survival and growth. Current nutrition programmes in low-income and middle-income countries focus primarily on long-term dietary improvement, overlooking the acute challenges that heatwaves present to daily feeding practices in already nutritionally vulnerable populations.

Methods
By using the Demographic and Health Surveys data, we analysed data from the youngest child aged 6–23 months (including both infants aged 6–11 months and young children aged 12–23 months) in 293 137 households across 36 low-income and middle-income countries from 2000 to 2019. Our data came from mothers’ 24-h dietary recall interviews about what foods they fed their youngest child on the previous day and night. Child feeding indicators included minimum dietary diversity (MDD), minimum meal frequency (MMF), and minimum acceptable diet (MAD), following WHO standards. Heatwave exposure was defined using location-specific temperature thresholds (92·5th, 95th, and 97·5th percentiles) with different duration criteria (≥2 or ≥3 consecutive days). Mixed-effects logistic regression models with distributed lag model frameworks were adopted to examine the cumulative effects of heatwaves over 14 days, adjusting for potential confounders.

Findings
Heatwaves significantly disrupted feeding practices among children aged 6–23 months, with the strongest effects observed on the risk of not achieving MDD (odds ratio [OR] for not meeting MDD: 6·19 [95% CI 5·46–7·16] for 3-day heatwaves at 95th percentile threshold). More severe heatwaves additionally compromised the likelihood of achieving adequate meal frequency (OR for not meeting MMF: 2·78 [2·36–3·37] at 97·5th percentile), ultimately affecting children’s ability to receive a minimum acceptable diet (OR for not meeting MAD: 4·66 [3·60–6·62]). These effects persisted up to 2 weeks post-exposure and showed strong negative impacts on consumption of nutrient-rich foods (OR 5·82 [4·44–7·65] for vegetables and vitamin A-rich fruits). Heightened vulnerability to inadequate feeding practices was observed in rural areas, low-income households, families with multiple young children, and those lacking cooling infrastructure (refrigerator or air conditioning), with ORs consistently higher than in their counterpart groups.

Interpretation
Our findings reveal that heatwaves rapidly disrupt child feeding practices, and these disruptions continue for up to 2 weeks. This evidence calls for urgent integration of short-term heat adaptation strategies within existing long-term nutrition programmes, particularly for access to nutrient-rich foods. Improving access to basic cooling facilities, especially in vulnerable communities, is vital for safeguarding child nutrition as global temperatures rise.