Objective The objective of this study is to assess changes in the dimensions of the food system and consumption associated with body weight variations during the first month's lockdown in Peruvian adults in Metropolitan Lima. Methods A cross-sectional study conducted during the first months of lockdowns in Peru. 694 adults completed a web-based survey about changes experienced in the process of acquiring food during lockdown, changes in their intake and self-perceived body weight. A multinomial logistic regression analysis was conducted to evaluate the factors associated with changes in body weight. Results Weight gain was perceived in 38% of the participants and 22.8% perceived weight loss. 39.2% did not perceive changes in their weight. Risk factors for body weight gain were increased alcohol consumption (OR=4.510, 95% CI 1.764 to 11.531) and decreased fruit consumption (OR=2.129, 95% CI 1.290 to 3.515), while decreasing cereal intake (OR=0.498, 95% CI 0.269 to 0.922) and choosing nutritious food as a driver for purchase (OR=0.512, 95% CI 0.320 to 0.821) were found to be protective against gaining weight. Decreasing food intake during the pandemic (OR=2.188, 95% CI 1.348 to 3.550) and having to miss important foods (OR=2.354, 95% CI 1.393 to 3.978), were associated with weight loss. Conclusions During confinement, weight gain was mostly associated with food consumption and personal food system factors. Meanwhile, weight loss was associated with external food system factors.