Because BMI doesn't reflect;

-age related differences
-sex related differences
-race related differences

BMI doesn’t indicate body fat distribution
BMI isn’t distinctive for determining risks of obesity related diseases
BMI is deceptive for children, pregnant or athletic patients

Age affects patients’ metabolic conditions. By the patient gets older fat deposit increases on abdominal area. Even if the body mass index is same – compared to younger patients – elder patients have higher level of blood sugar and blood lipids.

Male and female body compositions are different even though the body mass index is same. For example, a female patient has %45 of fat rate with BMI of 35 and male patient has fat rate of %35 with same BMI of 35. Thus, the metabolic features of male and female patients differ despite the same body mass index.

Asians have higher level of blood sugar and blood lipids comparing to Europeans with same body mass index.

The risk of metabolic syndrome is low if the over-eating related fat deposit is distributed to entire body. However, the risk is insulin resistance, type2 diabetes and cardiac diseases is higher if the fat deposit is in abdominal area (apple type obesity). So, body mass index doesn’t reflect the risks of different fat distribution in the body.

Due to all reasons stated above, body mass index is not an ideal criteria to determine range of obesity or risks of obesity related diseases.

However, it is the most commonly used and suggested obesity criteria by World Health Organization (WHO) for the last 50 years. This might be because there aren’t more valid, confidential and practical criteria other than body mass index.

It can be more suitable approach to take body mass index into consideration, along with body fat mass and waist circumference to determine the necessity for obesity surgery and range of obesity.