BMI
Meaning of BMI: A population statistic borrowed for individual use, which is the root of most complaints about it, including where the category lines fall.
Definition of BMI
BMI, short for body mass index, is a number calculated from weight and height: weight in kilograms divided by the square of height in meters. It was devised to sort populations into broad weight bands for screening rather than to measure any one person's body fat, and the calculation cannot separate fat from muscle or bone.
What is it used for?
BMI places people into broad bands running from underweight through healthy weight and overweight to obesity. Across large groups those bands track reasonably well with rates of type 2 diabetes, high blood pressure, cardiovascular disease, several cancers and joint disease. That makes BMI useful for comparing populations, following trends over time, and defining who is eligible for a study or a treatment program. It is quick, needs only a scale and a tape measure, and produces the same number wherever it is calculated, which is precisely why it became standard.
What does it miss?
| Factor | Effect on the reading |
|---|---|
| Heavy muscle | Classes a lean, muscular person as overweight |
| Muscle lost with age | Leaves an older adult in a healthy band while fat has replaced muscle |
| Where fat sits | Cannot distinguish fat around the organs from fat on hips and thighs, though the difference matters |
| Frame and bone density | Contributes to weight without contributing to fat |
| Pregnancy, fluid retention, limb loss | Weight no longer represents the same thing |
None of this makes the number useless. It makes it a starting point, which is why waist measurement, the ratio of waist to height, and direct measurement of body composition are used alongside it.
Do the thresholds apply to everyone?
Not identically. The relationship between BMI and the risk of conditions such as type 2 diabetes differs between population groups, and lower thresholds are applied for people of some South Asian and East Asian backgrounds because risk begins to climb at a lower index. The bands are also drawn for adults; in children and adolescents BMI is interpreted against age and sex charts, since normal body composition changes throughout growth. The same figure can therefore carry different meaning depending on who it describes.
Also known as
Used in a sentence
The patient's BMI was calculated and recorded as part of the routine health assessment.
Good to know
BMI can class a muscular person as overweight and leave an older adult with reduced muscle inside a healthy band, since the calculation cannot distinguish fat from muscle.