Body Mass Index (BMI)

Last Updated: April 4, 2023

BMI is not a highly accurate measure of obesity. That being said, it can be useful as a complementary datum. BMI has a high rate of false negatives, particularly among females, with nearly half of obese people being classified as normal or overweight in some studies. The amount of false positives, on the other hand, is surprisingly small: less than 5% in men and 1% in women, according to one study.

What is BMI

BMI, or Body Mass Index, is a simple formula using a person's height and weight to calculate a number which is supposedly representative of their level of body fat.

The formula itself is:

  • Weight (kg) / Height (m)2

or

  • (Weight (lbs) * 703) / Height (in)2

And the numerical value fits into the following categories:

  • Less than 18.5 is seen as underweight
  • 18.5-24.9 is seen as average weight and is usually a target range
  • 25-29.9 is seen as overweight
  • 30 and above is seen as obese

In the obese category, it is further divided into 30-34.9 (Class I Obesity), 35-39.9 (Class II Obesity) and 40+ (Morbid Obesity). The first two classes are seen as at-risk populations whereas health complications associated with obesity are assumed in a state of Morbid Obesity.

Uses of BMI

BMI tends to be used in large-scale population research and surveys as it can be calculated from height and weight, either self-reported or taken quickly and non-invasively by a researcher.

It has the benefits of being quick, easy to calculate, and a measure of body fat in which most of the population will consent to (unlike calipers which are invasive due to touching skin with cold metal objects, and hydrostatic weighing which dunks people in water without oxygen in their lungs).

An exception to this is the NHANES series of studies, a large scale series of studies conducted in the US that assesses persons by calipers.

Summation

If you are normal weight or overweight according to BMI (18.5-29.9) there is still a chance you are actually obese, and thus is primarily due to low levels of lean mass (muscle, water, and glycogen).

If you are obese according to BMI, you are most likely obese according to body fat percentage as well. When sampling from the general population, over 95% of men and 99% of women identified as obese by BMI were obese via body fat levels.[6]

Outliers to this dataset, those who have enough lean mass to be classified as obese by BMI but not by body fat percentage, are far and few in society. These persons would normally be highly active athletes or dedicated 'weekend warriors', and it is unlikely sedentary persons or those with infrequent exercise habits would be these outliers.

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