Obesity by definition is an increase in the amount of adipose tissue mass or fat mass. Adipose tissue is relatively ine from a metabolic rate. Basal metabolic rate depends on the amount of metabolically active tissue or the lean body mass. An obese individual has higher propoion of fat relative to lean body mass than a non- obese individual with the same weight, and hence a lower basal metabolic rate. Ref: Understanding Nutrition By Ellie Whitney, Eleanor Noss Whitney, Sharon Rady Rolfes, Page 246; Biochemistry By Debajyoti Das, Page 570.