BACKGROUND/OBJECTIVES: Greek Orthodox fasting (OF), which involves 180-200 days of fasting per year, is dictated by the Christian Orthodox religion. For the first time, this cross-sectional study examines the characteristics and the effects of OF on anthropometry, cardiometabolic markers and calcium homeostasis in Athonian monks (AMs).SUBJECTS/METHODS: Daily intakes of energy, macro- and micronutrients of a day during a weekend of Nativity Fast, defined as non-restrictive day (NRD), and a weekday during Great Lent, labeled as restrictive day (RD) were recorded.RESULTS: The daily energy intake of 70 AM (age=38.8±9.7 years) was low during both RD and NRD (1265.9±84.5 vs 1660±81 kcal, respectively, P<0.001). Paired samples t-test showed statistically significant difference between daily intakes in RD and NRD: carbohydrates (159.6±21.8 vs 294.3±23.4 g, P<0.0001) and saturated fat (12.7±0.0 vs 16.4±0.0 g, P<0.0001) were lower, whereas protein (89.2±1.3 vs 72.35±1.3 g, P<0.001) was higher during RD. A subsample of 50 monks (age=38.7±10.6 years) formed a study cohort for cardiometabolic and calcium homeostasis assessment. Body weight (74.3±12.9 kg) and body mass index (BMI; 23.8±4.1 kg/m) were independent of level of physical activity. Optimal profiles for lipid and glucose parameters (total cholesterol: 183.4±41.7 mg/dl, LDL: 120.6±37.6 mg/dl, triglycerides: 72.2±31.3 mg/dl, HDL: 48.5±14.2 mg/dl and homeostasis model assessment of insulin resistance (HOMA-IR) 1.02±0.40) were found. Profound hypovitaminosis D (8.8±6.2 ng/ml), high parathyroid hormone (PTH): 115.5±48.0 pg/ml with normal serum calcium levels (8.9±3.2 mg/dl) was observed.CONCLUSIONS: Unaffected by variation in lifestyle factors, the results of this unique study offers clear evidence for the health benefits of the strict Athonian OF through optimal lipid and glucose homeostasis.