Scientists suggest that where body fat is stored, not total body mass index, is strongly associated with all-cause and cause-specific mortality.
Body mass index (BMI) measurement has been the gold standard for determining if a person has obesity. Healthcare providers calculate BMI by evaluating an individual's height and weight. However, this measurement does not determine the ratio of fat, muscle, bone, or other tissue in the body.
Moreover, research has found that while a BMI of 30 or over raises the risk of death by up to 108%, other BMI ranges do not increase mortality risks.
With obesity is on the rise in the United States, it's critical that scientists identify the best method for determining whether the excess weight a person carries could be detrimental to their health and lifespan.
In a new study published on September 20 in JAMA Network Open, scientists investigated BMI, waist-to-hip ratio (WHR), and fat mass index (FMI) — a measurement that includes body weight, height, and fat mass content — to determine which measurement had a consistent association with mortality.
They found that fat distribution measured by hip-to-waist ratio is strongly linked to death from all causes.
Researchers used mortality data from 387,672 participants in the UK Biobank and a validation group of 50,594 people to compare mortality rates to BMI, FMI, and WHR.
The analysis showed that associations between BMI and FMI and mortality were J-shaped, and links between WHR and all-cause mortality were linear.
In addition, genetically determined waist-to-hip ratio — the genetic predisposition to accumulate fat in the abdominal area — was more significantly associated with all-cause mortality than BMI. Moreover, this association was stronger in male participants.
The study authors say these findings support a possible causal relationship between WHR and mortality, meaning having a high waist-to-hip ratio could potentially lead to death from all causes.
Using WHR measurements in a clinical setting may be more effective than BMI or FMI measurements in determining whether a person is at risk of obesity-related health conditions. In addition, assessing an individual's hip-to-waist ratio could help monitor fat loss during weight loss interventions.
The scientists suggest that clinical recommendations should focus on how fat is distributed in a person's body rather than total body mass index.
How to measure waist-to-hip ratio
To measure WHR, hold a flexible tape measure around the middle of the abdomen near the belly button and note the measurement. Then, measure the circumference around the widest part of the hips. Finally, divide the waist measurement by the hip circumference.
According to WHO, waist-to-hip ratios of 1.0 or higher indicate a person is at higher risk of experiencing health problems. Moreover, a waist-to-hip ratio above 0.90 for males and above 0.85 for females indicates abdominal obesity.