Obesity is a major problem worldwide, and it is related to abnormalities in glucose and lipid metabolism. Preclinical studies have shown that weight gain and insulin resistance may be prevented by oral administration of the probiotic Bifidobacterium animalis ssp. lactis 420. Furthermore, the prebiotic polydextrose has shown efficacy on satiety in clinical settings. The purpose of this study is to investigate the effects of these products, individually and combined, on change in body fat mass in a double-blind, randomized, placebo-controlled intervention trial. The study is conducted at four research clinics in southern Finland. The supplement is provided in a sachet, mixed into a fruit smoothie and ingested once per day for the duration of six months. One month from the end of the intervention participants will attend a follow-up visit. The study will enroll 232 participants, who will be randomized into blocks using a computerized procedure.