Background: Water-only fasting (WF) is a practice used to improve and maintain health. Objective: The aim of the study was to show whether WF performed for 8 days may be a threat to the health and/or life of people undergoing this practice. Methods: Twelve middle-aged men participated in the study. During the 8-day WF, the subjects ate no food except for drinking mineral water. Before and after WF, all subjects had a series of tests performed, beginning with the level of perceived stress and somatic measurements. The concentrations of creatinine, sodium (Naþ), potassium (Kþ), total calcium (Ca), magnesium (Mgþþ), urea (U), uric acid (UA) and total protein were determined in this urine and in the serum. For these substances, the values of clearance, renal filtration and fractional excretion were calculated. The osmotic clearance and free water clearance as well as the amount of daily urinary excretion of creatinine, Naþ, Kþ, Ca, Mgþþ, U and UA were also calculated. Moreover, the concentration of glucose in the serum and the concentration of b-hydroxybutyrate in the plasma were determined. In urine, specific gravity, pH and osmolality were also measured. Results: After 8 days of WF, the study showed a significant reduction in the level of perceived stress, weight loss, changes in body composition, dehydration, increased ketogenesis, hyperuricemia, decreased serum glucose concentration, and hyponatremia. These changes were accompanied by Naþ, Kþ and protein sparing, decreased serum Ca and Mgþþ concentrations, and reduced daily volume of more acidic urine with elevated specific gravity. Conclusions: After 8 days of WF, all subjects were found to remain safe and feel the sense of well-being. However, the appearance of the above-mentioned adverse metabolic effects, despite partially effective renal compensations, suggests that the further continuation of fasting intervention by the subjects would be detrimental to their body.