BMI versus waist to hip ratio

DEFINITION
Obesity is a state of excess adipose tissue mass. It is often viewed as equivalent to an increased body weight. This is not true because muscular individuals may have increased body weight but are not obese.

Weight follows a continuous distribution pattern in human population. The point at which mortality and morbidity becomes statistically significant is the cut-off to call a patient obese.

METHODS
Various methods have been used to measure obesity. These include:
1) Anthropometry (skin-fold thickness)
2) Densitometry (underwater weighing)
3) CT/MRI
4) Electrical impedance.

Still the most common techniques used in clinical practice are:
1) BMI measurement
2) Waist-to-hip ratio measurement.

BMI
It is not an accurate measure of obesity but since it is simple to calculate, it is the most frequently measured parameter.

At similar BMI, women usually have more fat than men. When the BMI > 25 Kg/m2, morbidity starts to increase and if associated with risk factors then therapeutic interventions should be considered.

WAIST-TO-HIP RATIO (WHR)
Intra-abdominal or abdominal subcutaneous tissue fat has more significant implications on morbidity than fat present in the buttocks and lower extremities. A WHR > 0.9 for women and > 1.0 for men is considered abnormal. (0.85 and 0.90 respectively according to WHO)

Insulin resitance, hypertension, dyslipidemia and diabetes mellitus are more strongly associated with intra-abdominal / abdominal subcutaneous tissue fat than to overall adiposity.

The probable explanation of this issue is that intra-abdominal adipocytes may be lipolytically more active than those from other depots. There is also release of free fatty acids into the portal circulation and this has adverse metabolic effects, especially on the liver.

A recent article published in the Annals of Internal Medicine concluded that
" normal-weight U.S. adults with central obesity have the worst long-term survival compared with participants with normal fat distribution, regardless of BMI category, even after adjustment for potential mediators. "

First published on: 30 November 2015

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