Practical Guidance: BMI, Waist Circumference, and Body Weight Categories
Based on the JAMA Network Open study, waist circumference (WC) alone or combined with BMI is the most practical and accurate tool for the general population — more accurate than the full Lancet obesity definition and considerably more accurate than BMI alone . Below is a practical reference combining both metrics, broken out by sex, since normal ranges differ meaningfully between men and women.
BMI Categories (Adults, All Ages 20+)
BMI is calculated as weight in kilograms divided by height in meters squared, and the standard adult categories apply uniformly across sexes and most age groups from 20 onward.cdc+1
The study's key finding, however, is that BMI alone catches barely half of true obesity cases (sensitivity 51.3%) when validated against actual body-fat scans, so it should never be treated as the final word on an individual's health status .
Waist Circumference Thresholds (Sex-Specific)
Waist circumference is measured with a tape directly on the skin, halfway between the lowest rib and the top of the hip bone, roughly at the level of the navel, after a normal exhale . This single measurement performed nearly as well as combining BMI and WC together (AUROC 0.815 vs 0.818) and is the simplest tool for home use .
Note that recommended cutoffs are lower for people of Asian descent, since diabetes and related metabolic conditions tend to develop at lower absolute body weights in this population — roughly 90 cm for men and 80 cm for women in several international guidelines.
Combined BMI-or-WC Approach: The Practical Standard
The study's core recommendation is to flag increased risk if a person meets either an elevated BMI or an elevated WC threshold — this combined rule delivered the best overall balance of sensitivity and specificity, with far fewer false positives than the full Lancet definition (about 5.5 million vs 15.5 million among US adults ages 20–59) . In practice, this means:
If BMI is in the healthy range (18.5–24.9) and WC is below the sex-specific threshold, risk is genuinely low.
If BMI is elevated or WC is elevated (even if the other looks normal), further evaluation is warranted — this is exactly the group BMI-alone tends to miss, particularly people with a "normal" BMI but high abdominal fat, who research shows carry worse long-term survival outcomes than some overweight or obese people without central fat accumulation.bmj+1
Age Considerations
The study population was limited to adults aged 20–59, so these specific accuracy figures don't directly extend to older adults or adolescents . That said, general clinical practice applies the same BMI categories from age 20 onward without age-based adjustment, while acknowledging that body composition shifts with age — older adults tend to carry relatively more fat and less muscle at the same BMI, which is one reason waist circumference becomes an even more informative supplementary measure later in life. For adults over roughly 65, some clinicians favor slightly higher BMI thresholds before flagging concern, since modest excess weight has been associated with better outcomes in certain frail or older populations, but this remains an individualized clinical judgment rather than a fixed rule.niddk.nih+1
Bottom Line for Everyday Use
For most people without access to a DEXA scan, checking both numbers — BMI from a standard height/weight calculation, and waist circumference with a tape measure — captures meaningfully more real risk than either number alone, while avoiding the excess false-positive labeling that comes with the more complex Lancet criteria requiring hip measurements . Waist circumference is arguably the single most valuable number to track over time, since it directly reflects visceral fat, the type most strongly linked to cardiometabolic disease, and it can meaningfully change month to month with lifestyle adjustments .
Mykola Iabluchanskyi together with Andriy Yabluchanskiy
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