Although weight loss is routinely recommended for obese person, the observational literature is controversial in respect of prolongation of life longevity after body weight loss. The positive long-term outcome has been found only in intentional weight loss in obese people with obesity-related comorbidities. The main task in obesity treatment strategy is "primum non nocere". The authors analysed the short-term effect and acceptability of low calorie diet (5 MJ) on health state of 150 obese people (BMI x = 35.9 kg/m2). The acceptability of this diet was 60% only, when the rest of patients has broken this regime mostly after second appointment with their physician. The successful patients achieved 10% weight loss from initial body weight during in average 5 months (95% CI for BMI reduction = (-4.55; -3.01), as well as reduction of percentage of body fat (95% CI = -7.7; -3). During this period has been observed in consistency with experimental evidence of other authors a decrease of plasma values of triacylglycerol, LDL cholesterol in hyperlipidemic patients, but not significant increase of HDL cholesterol. During this period obese persons have taken up more physical activities and felt themselves more psychologically comfortable.