Chrononutrition-based hypoenergetic diets with varying macronutrient and protein distribution in shift workers with prediabetes or type 2 diabetes: protocol for a randomised controlled trial

Shift work is associated with an increased risk of metabolic disorders, including type 2 diabetes, largely due to circadian misalignment, irregular meal patterns, and suboptimal diet quality. Chrononutrition, which focuses on aligning nutrient intake with circadian rhythms, has emerged as a promising strategy to improve metabolic health. Protein intake plays a key role in glucose homeostasis, and high-protein hypocaloric diets have shown benefits in people with type 2 diabetes. However, the effects of higher evening protein intake in shift workers remain unclear. This trial aims to analyze the effect of three hypocaloric diets differing in macronutrient composition and distribution throughout the day on glycemic control, body composition, and other secondary outcomes in shift workers with overweight or obesity and prediabetes or type 2 diabetes. This is a 12-week, three-arm, parallel-group, single-blind randomized controlled trial including 126 shift workers. Participants are randomized equally to: (A) a high-protein diet with protein-enriched dinner (50-60% of daily protein); (B) a high-protein diet with protein-restricted dinner (10-20% of daily protein); or (C) a normoproteic control diet with usual protein distribution. Primary outcomes include changes in glycemic control and DXA-derived body composition. Secondary outcomes are lipid profile, sleep quality, and quality of life. Other clinical and lifestyle parameters are evaluated to monitor changes throughout the intervention. Assessments are performed at baseline, week 6, and week 12. All analyses will follow the intention-to-treat principle. This study will provide new evidence on how evening protein intake may influence metabolic health in populations exposed to circadian disruption.