Body Roundness Index (BRI)

Body Roundness Index (BRI)

The Body Roundness Index is a geometric anthropometric index that estimates body fat distribution — particularly visceral/abdominal fat — using only height and waist circumference. It was introduced as an alternative to BMI, aiming to better capture body shape rather than just overall size (Thomas et al., 2013).

Development and Concept

BRI was introduced by Thomas et al. in 2013 (Obesity). The underlying idea models the human body as an ellipse: height forms the major axis, and waist circumference (converted to a diameter) forms the minor axis. The index is essentially a reformulation of the waist-to-height ratio, expressed through the geometric concept of eccentricity — how much the body's cross-sectional shape deviates from a perfect circle.

The Formula

BRI = 364.2 - 365.5 × {1 − [(WC/2π)/(0.5 × H)]²}^0.5
WC: waist circumference, m
H: height, m

Both waist circumference and height must be in the same units (typically cm). The constants 364.2 and 365.5 were chosen by the original authors solely to rescale the eccentricity value into a more clinically accessible range, rather than carrying independent physical meaning (Thomas et al., 2013). This has drawn some mathematical criticism — one commentary noted the formula simplifies to a function of the simple waist-to-height ratio, and that the specific scaling constants are essentially arbitrary rather than derived (Cook, 2024).

How to Measure the Inputs

• Waist circumference: measured at the narrowest point of the torso, typically at or just above the navel, using a flexible tape measured parallel to the floor, without sucking in the abdomen

• Height: measured standing, without shoes

Interpretation

Hazard Ratio is a statistical measurement used to define the frequency of an outcome event in one group versus another, over a given period of time. In a recent study by Zhang et al (2024), the period of time was 20 years.

Unlike BMI, there is no single universally standardized BRI cutoff for classifying health risk. Different studies have used different, population-specific thresholds:

• A CKD-focused study found J-shaped associations between BRI and mortality, with a BRI >10 associated with significantly increased all-cause mortality risk (adjusted HR 1.82) (Yang et al., 2026)

• An osteoporosis-focused study used a cutoff of BRI >4.07 to define its high-risk group, identifying a threshold effect around BRI=5 (Ding et al., 2025)

• Large NHANES-based cohort studies have generally used quintiles specific to their study population rather than fixed cutoffs (Zhang et al., 2024; Ibrahim et al., 2026)

Given this lack of standardization, any single numeric range should be treated as an approximate guide rather than a validated clinical threshold.

What the Evidence Shows

• U-shaped relationship with mortality: In a study of 19,535 participants with metabolic syndrome using 10 consecutive NHANES cycles (1999–2018), BRI demonstrated a U-shaped association with mortality risk, with the highest quintiles showing increased all-cause mortality (HR 1.29) and cardiovascular mortality (HR 1.39) (Wang et al., 2026). Similarly, a nationally representative cohort of 32,995 US adults found that both the lowest and highest BRI groups experienced significantly increased all-cause mortality risk relative to the middle quintile (Zhang et al., 2024).

• Elevated cardiovascular mortality risk: An NHANES analysis of 31,351 adults followed for a median of 10.1 years found that participants in the highest BRI quintile had a 54% higher risk of cardiovascular mortality compared to the middle quintile (reference group), a risk that persisted even among those with normal BMI (Ibrahim et al., 2026).

• More granular risk stratification than BMI: A retrospective cohort study of 778,812 individuals from a Japanese claims database found that BRI and ABSI identified mortality risk differences across four categories, compared to only three categories for BMI (Kimura et al., 2026).

• L-shaped (inverse) relationship in specific populations: In a cohort of 1,596 osteoporotic individuals aged 50 and older, higher BRI was associated with better long-term survival, showing an L-shaped inverse relationship with a threshold at BRI=5 (Ding et al., 2025) — illustrating that the "obesity paradox" documented with BMI in some chronic-disease populations may also apply to BRI.

• Chronic kidney disease population: Among 6,240 US adults with CKD followed for a median of 6.6 years, restricted cubic splines demonstrated J-shaped associations between BRI and mortality, with higher BRI independently associated with increased all-cause and cardiovascular mortality risk, offering greater prognostic value than BMI or waist circumference alone in that population (Yang et al., 2026).

Advantages Over BMI

BRI directly incorporates fat distribution rather than treating the body as a simple cylinder, as BMI implicitly does (Thomas et al., 2013). Regression models in the original derivation study found that body roundness modestly improved predictions of percent body fat and percent visceral adipose tissue compared to traditional metrics of BMI, waist circumference, or hip circumference (Thomas et al., 2013).

Limitations

No standardized clinical cutoffs yet exist for BRI, unlike BMI's widely accepted categories — studies vary in the thresholds they apply, from population quintiles (Zhang et al., 2024) to specific numeric cutoffs like >4.07 (Ding et al., 2025) or >10 (Yang et al., 2026), which limits consistent clinical application. Additionally, the scaling constants in the formula itself have been characterized as arbitrary rather than physically derived (Cook, 2024).

Thomas DM, Bredlau C, Bosy-Westphal A, Mueller M, Shen W, Gallagher D, Maeda Y, McDougall A, Peterson CM, Ravussin E, Heymsfield SB. Relationships between body roundness with body fat and visceral adipose tissue emerging from a new geometrical model. Obesity (Silver Spring). 2013 Nov;21(11):2264-71.