Association of macronutrient quality at dinner versus breakfast with body composition in adults living in Tehran: results from a cross-sectional study

Background We aimed to examine the relationship between the quality of macronutrients consumed at dinner versus breakfast and their effects on body composition among Iranian adults. Methods A cross-sectional study was conducted among 450 Iranian adults (aged 20–59). Anthropometric and body composition measures, including BMI, body fat percentage, fat-free mass, muscle mass, and waist-to-hip ratio, were assessed. Dietary data were collected using three 24-hour recalls, categorizing macronutrient quality (carbohydrates, fats, and proteins). ANCOVA was used to compare anthropometric and body composition measures across macronutrient tertiles for dinner versus breakfast. Non-linear associations were analyzed using restricted cubic splines. Results According to our findings, higher-quality carbohydrate intake at dinner versus breakfast showed a significant inverted U-shaped association with BMI. Conversely, saturated fat intake at dinner versus breakfast increased BMI in a U-shaped relationship, while linearly reducing muscle mass and fat-free mass. Unsaturated fat intake at dinner compared to breakfast showed a U-shaped association with BMI. Moreover, animal protein intake at dinner versus breakfast slightly increased muscle mass and fat-free mass in a reverse U-shaped pattern. However, higher plant protein intake at dinner versus breakfast reduced BMI linearly and decreased fat-free mass and muscle mass. Conclusions Higher intake of high-quality carbohydrates, unsaturated fats, and plant-based proteins at dinner versus breakfast may improve body composition, while increased consumption of saturated fats and animal proteins at dinner versus breakfast may worsen central adiposity and BMI.