The Ethiopian Orthodox Christian (EOC) fasts, although adopted for religious purposes, combines aspects of energy restriction, time-restricted feeding, and a vegan dietary pattern, all of which have been independently associated with weight loss and healthier body composition. However, combined effect of these practices as part of EOC fast remains unknown. This longitudinal study design evaluated the effect of EOC fasting on body weight and body composition. Information on socio-demographic characteristics, physical activity level, and fasting regimen followed was captured through an interviewer-administered questionnaire. Weight and body composition measurements were taken before and at the end of major fasting seasons. Body composition parameters were measured by bioelectrical impedance (BIA), Tanita® BC-418®, Japan). Significant changes in body weight and body composition were observed for both fasts. Significant decreases in body weight (14/44 day fast: − 0.45;P= 0.004/− 0.65;P= 0.004), FFM (− 0.82;P= 0.002/− 0.41;P< 0.0001), and trunk fat mass (− 0.68;P< 0.0001/− 0.82;P< 0.0001) were observed after adjusting for covariates including age, sex, and physical activity. The EOC fasting regimen leads to significant reductions of body weight and compositions. Longer fasting duration led to much higher effects in body weight and body composition and may be non-pharmacological strategy in prevention or treatment of chronic diseases.