PURPOSE: Different studies have pointed towards a positive effect of religious fasting on human health. Orthodox fasting (OF) regime could be characterized as a periodical vegetarian diet, demonstrating several common characteristics with the typical Mediterranean diet. The present systematic review aimed to synthesize available results regarding the potential impact of OF on human health.METHODS: Key biomedical databases were searched to identify studies examining the effects of OF on humans. Following implementation of specific criteria, ten studies were included in the analysis and their results were systematically reported and critically appraised in this review.RESULTS: According to the available limited results, OF periods are characterized by a restriction in total energy and fat intake, an increase in carbohydrate and fiber consumption, while in terms of protein intake, results are contradictive. The overall effect of OF on lipids profile seems to be optimal, with the reduction of total cholesterol and LDL-C levels, being a consistent finding across studies (up to 17.8 and 31.4%, respectively). However, the effect on HDL-C is still unclear. Conclusions regarding the impact on body weight and glucose homeostasis cannot be drawn, given that relevant data are limited with conflicting results. Any potential negative aspects of OF, mainly attributed to reduced dietary intake of vitamin D and B12 and minerals (mainly calcium), require further investigation.CONCLUSIONS: Given the limitations of available evidence, more studies are required before reaching definite conclusions about the effects of OF on human health.