BMI stands for body mass index and is one of the most widely tools for measurement of the degree of obesity of an individual. It is the ratio of weight and height of an individual.
The average BMI of an adult is between 20-25 kg/m2 which is considered normal. A BMI of less than 20 kg/m2 is considered underweight and that more than 25 kg/m2 is overweight. BMI more than 30 kg/m2 comes in the category of obesity which again sub categorised into mild , moderate and severe. Mild obesity is between 30-35 kg/m2 , moderate is 35-40 kg/m2 and severe/morbid obesity is more than 40 kg/m2.
Although increasing body weight in adults is accompanied by corresponding increase in BMI , the increased BMI might not accurately depict the changing body composition and fat distribution. For example, a person undergoing weight training may be gaining more muscle mass in relation to body fat but the increased BMI would not reflect this parameter. Similarly , certain medical conditions like liver failure, kidney disease and hypothyroidism lead to excess fluid accumulation in the body which can lead to increased BMI measurement. The increased BMI in these conditions do not necessarily imply increase in body fat percentage or obesity. Also , the BMI is not an accurate predictor of obesity in young children and adolescents.
For these special conditions, a full body scan depicting the detailed body composition can accurately determine the degree of obesity and the need for intervention. This can be performed in the clinician’s office using a variety of commercially available machines.
A whole body MRI scan remains the most accurate tool for determining the exact body composition and fat distribution but is not widely used because of lack of accessibility and financial constraints.