Studies have shown that obesity has become increasingly prevalent in both people and animals such as pets and laboratory animals.[4] There have been no linkages found between this obesity trend and diet and exercise. According to Professor Robert H. Lustig from the University of California, San Francisco, "[E]ven those at the lower end of the body mass index (BMI) curve are gaining weight. Whatever is happening is happening to everyone, suggesting an environmental trigger."[5] The theory of environmental obesogens proposes a different causal facet to obesity – that lifetime exposure to xenobiotic chemicals may change the body's metabolic system.