The present invention relates to polyhydroxyalkanoate (PHA) containing monomer units of 3-hydroxybutyrate for use, by oral administration, in the prevention or treatment of an overweight or obesity condition, or of metabolic dysfunctions related to said condition. A marked effect was observed on the reduction of the body weight, both in the case of a standard diet and in the case of a diet rich in fats, a reduction that can be attributed to an increase in lipid oxidation to the detriment of the deposit of excess lipids, as confirmed by the decrease in the body weight and by analyses of the body composition. A positive effect on the metabolic dysfunctions related to an overweight or obesity condition has also been verified, in particular on dyslipidemias, such as the blood level of low-density lipoproteins (LDL cholesterol) and triglycerides, as well as an improvement in the inflammatory state.