Obesity is a condition associated with an increased risk of metabolic disorders, and in particular of type-2 diabetes (T2D). The treatment and prevention of obesity and associated metabolic disorders present great medical challenges. A major therapeutic goal in T2D is to control blood glucose levels, which can be achieved by pharmacological and nonpharmacological measures. The latter include increased physical activity and reduction of body fat mass by limiting dietary caloric content. Low-calorie diets (LCDs) involve a reduction in daily caloric intake by 25% to 30%. LCDs should be individualized depending on the patient's energy requirements, the severity of the obesity, and any accompanying diseases and treatments. Intermittent fasting (IF) involves caloric restriction for one or several days a week, or every day as the prolongation of the overnight fast. The results of recent clinical trials have shown that LCDs and intermittent fasting in patients with obesity (including those with coexisting T2D) can lead to a reduction in body fat mass and metabolic parameter improvements. These beneficial effects arise not only from the loss of body mass, but also from the activation of metabolic pathways specific to fasting conditions. However, the paucity of large-scale randomized controlled trials makes it difficult to prescribe LCDs or IF as reliable, routine methods for successful and stable weight loss.