Body Mass Index (BMI) is a number calculated from height and weight: weight in kilograms divided by height in metres squared. It categorises adults as underweight (< 18.5), normal weight (18.5–24.9), overweight (25–29.9) or obese (≥ 30). It was developed in the 1830s as a population-level statistical tool — not as an individual health diagnostic.
What BMI is useful for
At a population level, BMI correlates reasonably well with body fat and with risk of conditions including type 2 diabetes, cardiovascular disease and some cancers. This makes it a useful screening tool in large studies and in public health policy.
In a clinical setting, it can flag patients who warrant further investigation. For an individual with a BMI of 35, it is reasonable to discuss cardiovascular and metabolic risk. For an individual with a BMI of 17, it is reasonable to discuss nutritional status.
Where BMI fails as an individual measure
BMI cannot distinguish between muscle and fat. A professional athlete with very low body fat may have a BMI in the 'overweight' range purely because muscle is heavier than fat. Conversely, a sedentary person with a 'normal' BMI may carry a high proportion of visceral (abdominal) fat — the type most strongly associated with metabolic disease.
BMI also does not account for age, sex, ethnicity or where fat is distributed on the body. For South Asian, East Asian and Middle Eastern populations, health risks associated with excess adiposity begin at lower BMI thresholds than those originally defined for European populations.
"BMI is a population tool wearing a clinical costume. It can open a conversation, but it should never close one."
Dr. Lina Haddad, CMO, Looms
Measures that complement or replace BMI
- Waist circumference — a strong predictor of visceral fat and metabolic risk; risk increases above 94 cm in men and 80 cm in women (European guidelines)
- Waist-to-height ratio — dividing waist circumference by height; a ratio above 0.5 is associated with increased cardiometabolic risk across all ethnicities
- DEXA scan — measures fat mass, lean mass and bone density precisely; used in clinical research and specialist settings
- Body fat percentage via bioelectrical impedance — widely available in consumer scales; less precise but useful for tracking trends
The 2023 AMA position shift
In 2023 the American Medical Association formally updated its policy on BMI, acknowledging its 'significant limitations' and its history of being 'used for racist exclusion'. The AMA now recommends using BMI alongside other risk factors — including waist circumference, body composition and cardiometabolic markers — rather than treating it as a standalone diagnostic.
This doesn't mean ignoring BMI. It means using it as one data point among several, rather than the primary label defining a person's health.
The practical takeaway
If your BMI is in a category that concerns you, pair it with a waist circumference measurement and discuss with your doctor whether a full metabolic panel — fasting glucose, lipid profile, blood pressure — is appropriate. Context and trends matter far more than a single number.