The current situation of obesity in the dog and cat I: Risk factors, associated diseases and pathophysiological background Recent surveys have reported a prevalence of obesity in dogs and cats in industrialized countries of about 40%. The risk factors for obesity in dogs are female gender, castration, middle age, certain breeds, feeding of treats and table scraps, a low level of exercise, and having an overweight owner who is not aware of the health requirements of his or her dog. In cats, male gender, castration, and over-humanization by the owner have been identified as risk factors for being overweight. A number of diseases have been associated with obesity, including orthopaedic disorders, skin disorders, heart disease, diseases of the urinary tract, diabetes mellitus and cancer. Obesity lowers exercise tolerance as well as reduces quality of life and longevity. But these changes are not only caused by the increase in body weight and an oversupply with nutrients, In humans and horses, the combination of the clinical symptoms of obesity, hypertension, insulin resistance and chronic low grade inflammation is termed the 'metabolic syndrome' Although investigations on this topic in cats and dogs are scarce, signs of subclinical inflammation have also been found in these species. Differences in the composition of the intestinal microbiota found in laboratory rodents and humans have initiated the discussion on the role of the microbiota in the development of obesity, since the intestinal microbiota influences the energy metabolism of the host via several pathways. In dogs, no obvious differences in the composition of the faecal microbiota between lean and obese animals have been found, and studies in cats are currently lacking.