Understanding 體重指數
What this number does and doesn’t tell you.
What BMI Measures
Body Mass Index is calculated from height and weight alone: weight in kilograms divided by height in metres squared. It was developed as a population-level screening measure, giving researchers and clinicians a quick, low-cost way to categorize weight status across large groups of people.
What BMI Doesn’t Measure
BMI is a useful starting point, but it has well-documented blind spots:
Muscle vs. fat vs. bone
BMI cannot distinguish fat mass from lean body mass. Two people with the same BMI, say, an active older adult with more muscle and someone with more body fat, can have very different body compositions the number won’t show.
CDC, 2025
Where fat is distributed
BMI can’t indicate where fat is located in the body, even though fat distribution is independently linked to cardiovascular risk. In practice: a care team may also look at waist circumference alongside a BMI result.
National Academies, 2023
Bone density
BMI doesn’t reflect bone density, which becomes especially relevant for older adults screening for osteoporosis risk. A DEXA scan, not BMI, is the appropriate tool for that.
Age-related body changes
BMI wasn’t designed with the body composition changes that naturally occur with aging in mind, such as gradual muscle loss even when weight and therefore BMI stays stable.
Why the Categories Exist
The standard BMI categories offer a general reference point based on population data. They’re useful for flagging when a conversation about weight, nutrition, or activity might be worthwhile, but they aren’t diagnostic thresholds for any individual.
Watch: The Problem With BMI
Two short explainers on why BMI can be a misleading measurement on its own, and what a more complete picture looks like:
Mayo Clinic Minute: The Problem With BMI
A Mayo Clinic obesity researcher explains why BMI alone can be misleading, particularly for people who don’t necessarily look overweight. (1:10)
What BMI Doesn’t Tell You About Your Health — Vox
A clear breakdown of how BMI was developed, and why it can be an unreliable measure of health for some individuals, including people with higher muscle mass.
Using This Result Well
The most useful way to use a BMI result is as one input among several, brought to a care team who can interpret it alongside other information: blood pressure, activity level, existing conditions, and how the number has changed over time rather than a single snapshot.
- Centers for Disease Control and Prevention. BMI Frequently Asked Questions (2025).
- National Academies of Sciences, Engineering, and Medicine. “The Science, Strengths, and Limitations of Body Mass Index.” Translating Knowledge of Foundational Drivers of Obesity into Practice (2023).
- Yale Medicine. “Why You Shouldn’t Rely on BMI Alone” (2023).
This page provides general health information and is not a substitute for personalized medical advice. BMI results should be interpreted by a qualified healthcare professional in the context of a patient’s full health picture.
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