The aim of present study was to compare the effects of very low calorie diets - protein rich and dietary fiber rich food based - on gallstones formation during rapid weight loss. 68 patients were involved into the study. The body weight index in all cases exceeding normal value and equaled to 35±4,7 kg/m2. For weight correction purposes during 5 weeks the patients in first group were kept on a 520-800 kcal diet of "Margi" food products, prepared according our technology, and in the second group on a protein rich diet of the same calorie content. The body weight and changes in the gall-bladder wall and content were assessed by sonography before starting the diet, after three weeks from the commencement of the diet and upon its completion. The measurement of the body weight after completion of the 5 week diet revealed decrease by 10.9±1,5kg in the first group and by 11,2±1,1kg in the second group. Sonography disclosed growth in the amount of biliary sludge in 3 cases in the first group and in 9 cases in the second group. The statistical analyses of results indicate successful and nearly equal reduction of body weight by means of dietary fiber rich and protein rich diet, but high fiber consumption showed statistically significant benefits for prevention of biliary slug accumulation. The study showed that, in the respect to weight loss, diets based on fiber rich and protein rich food are equal, but fiber rich diet has considerable privilege in prevention of gallstone disease. Our findings support the presence of known association between increased dietary fiber consumption and reduction of gallstone formation. Obesity and rapid weight loss are risk factors for development of gallstones. Taking in an account the beneficial effect of dietary fiber, the food rich with this nutrient, particularly low-calorie fiber rich food "Margi", can be recommended for rapid weight loss in obese patients.