The effect of rapid weight reduction with supplemented fasting was studied in a group of 46 individuals with moderate to severe obesity. The preparation used contained a mixture of protein, carbohydrate, and essential fatty acids providing 420 kcal daily. It was supplemented with a complement of electrolytes, minerals, and vitamins.Serum concentrations of electrolytes, urea nitrogen, creatinine, uric acid, glucose, cholesterol, and triglycerides were measured prior to the onset of the study and at two-week intervals for a six-week study period. In addition, blood pressure, heart rate, and body weight were recorded regularly. A mild and transient fall in serum bicarbonate concentration and a rise in uric acid level was observed.In contrast to other regimens, hypokalemia was not observed in the present study. In fact, serum K+ concentration rose slightly while serum Na+ concentration remained virtually unchanged. There was a transient rise in serum creatinine concentration followed by a fall to values below the baseline. Serum glucose, cholesterol and triglyceride levels, blood pressure, and heart rate decreased significantly. Body weight fell from 232.7 ± 58 lb at the onset of the study to 176.4 ± 47.9 lb at the conclusion of the study.The protocol was well tolerated and the side effects were mild and infrequent. In conclusion, the present protocol provides a safe and effective means for rapid weight reduction in individuals with moderate to marked obesity without producing severe electrolyte disturbances seen with other modalities.