Obesity: BMI calculation is not enough

Researchers found that the widely used BMI measure is insensitive for defining obesity. They recommend measuring body fat percentage.

What is BMI?

The Body Mass Index (BMI) is a parameter commonly used to assess the level of adiposity in the general population. The World Health Organisation defines overweight and obesity as an abnormal or excessive accumulation of fat that increases the risk of cardiometabolic diseases and certain cancers.

The association between BMI and all-cause mortality for BMIs above 25 kg/m2 (overweight) and above 30 kg/m2 (obese) is well documented. BMI has been widely used and accepted as a simple method to classify cardiometabolic risk based on weight.

BMI and fat mass index

Fat mass is the total amount of fat tissue in the body, which serves as an energy reserve. The results of a study conducted by researchers at the Tel Aviv University's School of Public Health - Faculty of Medicine lead to the conclusion that the highest amount of fat mass, measured as a percentage of body fat (BF%), is a much more reliable indicator of health and in particular of cardiometabolic risk than BMI. The study authors recommend that this measure should become the gold standard for assessing obesity.

The obesity paradox with a normal weight: a cross-sectional study

"Israel has alarming levels of childhood obesity and more than 60 per cent of the country's adults are defined as overweight," said the study's lead author, Prof. Yftach Gepner, in a TAU statement. "The prevailing index in this regard is the BMI, based on weight and height measurements, which is considered a standard indicator of an individual's overall health.

However, despite the intuitive link between excess weight and obesity, the actual measure of obesity is the amount of body fat mass, with normal maximum values set at 25 per cent for males and 35 per cent for females." He adds that the result of the disparity between the two indices, BMI and adiposity, is identified as 'the obesity paradox with normal weight' or, more simply, the obesity paradox.

The researchers also point out that the use of BMI to identify excess fat at the individual level has reasonable specificity but poor sensitivity, with about half of individuals with excessive body fat percentage (BF%) misclassified as non-obese. Their study was designed to identify the prevalence of metabolically obese normal weight (NWO) individuals in an Israeli population sample and to investigate the relationship between metabolically obese, metabolically healthy normal weight lean (NWL) individuals, and cardiometabolic risk.

Gepner's team recruited cross-sectional study participants in an Israeli nutrition clinic between 2015 and 2021. Candidates had to be at least 20 years old, regardless of BMI value. The researchers recorded weight, abdominal circumference (AbC) and BMI values. They used DEXA (dual-energy X-ray absorptiometry) to measure body composition. The researchers consulted medical records to detect serum markers of glycaemic control, lipids, liver enzymes and blood counts.

For their analysis, Gepner et al. applied multivariate linear regressions to assess associations between cardiometabolic markers in NWL vs. NWO participants, stratified by sex and adjusted for age and BMI. Using non-linear regression, they also analysed the relationship between BMI and BF%. Excess adiposity was defined as ≥25% for men and ≥35% for women.

Fat mass percentage: a new gold standard for defining obesity

The final cohort numbered 3001 participants, of whom approximately half were men and half women (51.9% and 48.1%, respectively). Body weight was within the normal range (BMI 18.5-24.9 kg/m2) for 967 (32.2%) of the participants, a group consisting of 326 men (mean age 36.1 years) and 641 women (mean age 34.7 years). The distribution of body fat in this group ranged from 4% to 49%.

Within the NWO group, 26% of the men and 38% of the women had a BF% above the specified limit value for excess adiposity. The researchers noted a discrepancy between this group and those in the overweight BMI category (BMI 25-29.9 kg/m2), where 69.6% of the men and 88.8% of the women were above the limit. In this case, we also show the reverse, i.e. that approximately 30% of the men and 10% of the women in the overweight BMI category could be classified as having a normal BF%.

Comparing clinical parameters indicating the risk of cardiometabolic diseases between NWL and NWO participants, the study showed that NWO men had elevated levels of:

Among NWO women, however, the only statistically significant difference observed was that triglycerides were higher among those with NWO (BF% ≥35) than among those with normal BMI/low BF% (101.4 vs. 84 mg/dL; P=.03).

Gepner et al. found no significant association between age groups and NWO in all age strata, but found similar associations between age groups and cardiometabolic risk for each sex. Among NWO subjects, the team found a higher AbC in 60% of women (≥88 cm) versus only 4% of men (≥102 cm).

Gepner says that the NWO condition appears to be much more common in Israel than his team thought. The finding worries him, as those within the normal range of the BMI index usually go 'under the radar', being overlooked for treatment or counselling on lifestyle and behaviour change, thus increasing the risk of cardiometabolic diseases.

According to the authors, body fat percentage is a much more reliable indicator of an individual's overall health than BMI. Therefore, they recommend that all clinics be equipped with suitable devices to measure body fat percentage and gradually turn it into the gold standard both in Israel and worldwide to prevent disease and premature mortality.

  1. Lahav Y, Kfir A, Gepner Y. The paradox of obesity with normal weight; a cross-sectional study. Front Nutr. 2023 Jun 9;10:1173488. doi: 10.3389/fnut.2023.1173488. PMID: 37360304; PMCID: PMC10287971.