BMI: More Like BM-Why Is This Still a Thing?
The Limits of BMI and Consumer Body Scans
BMI, or body mass index, was developed in the 1830s by mathematician Adolphe Quetelet as a population-level statistical tool, not a measure of individual health. BMI does not account for age, sex, race, body composition, or metabolic variability. Despite this, it has been widely applied to individual bodies for nearly two centuries.
In 2023, the American Medical Association formally acknowledged these limitations, recommending that BMI be used only alongside additional measures of health risk, including body composition, waist circumference, relative fat mass, visceral adiposity, and metabolic factors. This shift was an important step forward. The question that remains is how these recommendations are being applied in real-world clinical and commercial settings.
In practice, most people are not being referred for gold-standard body composition testing such as DEXA scans, nor are they undergoing metabolic assessments as part of routine care. Instead, BMI logic has migrated into consumer-facing devices that promise more precision while often relying on the same underlying assumptions.
When BMI Logic Shows Up in Consumer Tech
Smart scales, gym-based impedance machines, med spa assessments, and 3D body scanners frequently claim to estimate body fat percentage, basal metabolic rate, and overall health risk. Many of these tools require only height and weight as inputs, immediately signaling that BMI-based equations are involved, regardless of how sophisticated the interface appears.
To better understand the accuracy of these devices, I compared results from a Fit3D body scan with same-day measurements from resting and active metabolic testing and a DEXA scan.
The results were clinically meaningful:
Basal metabolic rate was underestimated by more than 500 kcal
Fat mass was miscalculated by approximately 4.5 pounds
Percent body fat was overestimated by 4 percent
These discrepancies are not trivial. Errors of this magnitude can meaningfully affect how someone is counseled about nutrition, exercise, and health risk.
One additional output from the body scan was a “body shape rating,” generated by comparing measurements to an undisclosed population sample. This type of ranking lacks transparency and clinical relevance. This risks reinforcing harmful comparisons without offering actionable insight.
Where credit is due, circumference measurements produced by the scanner appeared precise. Because manual circumference assessment is subject to user error and variability, this is one area where technology may offer a practical benefit when used appropriately.
Why This Matters
BMI remains widely used because it is simple, inexpensive, and easy to standardize. However, simplicity does not equal accuracy. When population-level tools are applied to individual bodies, especially through consumer technologies, the results can be misleading at best and harmful at worst.
Health is not defined by a single number. Body composition, metabolic function, bone density, and functional capacity all provide more meaningful information than weight alone. Without context, BMI-based estimates risk obscuring these factors rather than clarifying them.
What to Use Instead
No single metric can define health. A more responsible approach considers:
Body composition rather than total weight
Preservation of lean mass alongside fat loss
Metabolic rate and energy needs
Functional movement and physical capacity
Individual health history and goals
BMI can offer limited insight at the population level, but it should not be treated as a diagnostic tool for individual bodies, nor should it be repackaged through consumer devices as a precise measure of health.
Nutrition and training decisions are best made using tools that respect biological variability rather than flatten it into a single equation.
