The principle characteristic of obesity is excess adipose tissue. Currently, body mas index (BMI) is an accepted and widely used measure for obesity.
BMI= weight (kg)/Height (m2)
According to World Health Organization (WHO) BMI classification;

BMI (kg/m2) Thinness (stage) Reference Overweight Obesity (class)
40 I
35 II
30 <30 III
25 <25 ≥25 I
18.5 ≥18.5
17 I
16 II
10 III

Above table shows obesity classes and thinness stages based on BMI distribution. BMI lower than 18.5 is considered as a stage thinness while BMI ≥30 is considered as obesity

Limitations of BMI

There are certain limitations of BMI as a measure of obesity. The body weight parameter does not tell us the percentage of body fat mass and lean mass. The other limitation is the lack of information regarding body fat distribution. Fat distribution in the body could be around the trunk and upper body called the android. Android fat distribution is seen as more common in males. Body fat distribution is not in the center but rather on the bottom, hips, and thighs making a pear shape called a gynoid. Gynoid fat distribution is more common in females. Android fat distribution contributes to obesity-related clinical complications. Finally, regarding ethnic differences, at any given BMI Asian females have higher body fat by 5% compared to Caucasians.

Clinical complications of obesity

Medical complications associated with obesity are:

Pulmonary disease

Obstructive sleep apnea
Abnormal function
Hypoventilation syndrome

Nonalcoholic fatty liver disease