Earlier during the prehistoric period, food supply was very irregular, so our body has developed the ability to store excess food for the immediate requirement of survival. It is stored in the form of fat cells, which are residing in the adipose tissue. These adipose tissues are widely distributed in the different parts of the body. These adipose tissues are adapted to store excess energy as triglyceride and, releases stored energy as free fatty acids for the use at other site, as and when required. This function (storing and releasing energy) is regulated by the endocrine and neural pathways. This helps humans to survive starvation for long time. However in the presence of abundance of nutritional food and sedentary life style increased the fat storing capacity of adipose tissues in our system, and that affected health adversely.
Obesity is a state where excess storage of fat in the adipose tissue in the body takes place which results in increased body weight. High masculine people however has very low fat stored in their body. They will also have high BMI, but these individuals are not considered obese. There are other method of quantifying obesity including anthropometry, densitometry, CT or MRI, and electrical impedance.
Fats are usually stored subcutaneously in the intra abdominal and abdominal region, this can be determined by measuring the waist-to-hip ratio. Women with hip ratio >0.9 and men with >1.0 is considered abnormal. When compared with men and women with similar BMI, women have more body fat than men. Based on many epidemiological studies, BMI 25 kg/m2 is fixed as maximum and BMI above 30 kg/m2 is considered threshold for obesity.