Obesity increases the likelihood of various diseases, particularly heart disease, certain types of cancer and osteorthritis. Obesity is most commonly caused by a combination of excessive dietry calories, lack of physical activity, and genetis susceptibility, although a few cases are caused solely by genes, endocrine disorders,medications or psychiatric illness. Evidence to support the view that some obese people eat little yet gain weight due to a slow metabolism is limited; on average obese people have a greater energy expenditure than their thin counterparts due to the energy required to maintain an increased body mass.