Why BMI is in the toilet.

BMI is as outdated as your Granny’s favourite pair of underpants. But before we talk about why BMI (Body Mass Index) is an imprecise measure of how fat a person may be and an imperfect predictor of a person’s health risks, lets take a closer look at what your body weight is actually made up of.

Body Weight— What is it measuring?

Your total body weight is the sum of your:

  • Fat Mass – the sum of all the fatty tissue in the body, and
  • Fat-free Mass
    • Bone mineral content – the weight of your skeleton
    • Lean mass– muscle tissue, soft tissue, organs, fluid (blood + water)

Total mass = fat mass + fat free mass

Body Mass Index (BMI)

Body Mass Index (BMI) is an obesity classification scheme established in 1832 used to estimate how overweight a person is by dividing their weight in kilograms by height in meters squared.

< 18.5 kg/m²Underweight
18.5 – 24.9 kg/m²Normal Weight
25 – 29.9 kg/m²Overweight
> 30 kg/m²Obese

BMI has been used for hundreds of years in population health studies, and by health care professionals, doctors, dietitians & nutritionists and personal trainers to help determine whether a patient/client is a healthy or unhealthy weight.

However, BMI has some major flaws. Firstly, it doesn’t distinguish between lean mass (muscle + soft tissue + fluid) and fat mass and therefore often leads to confusion and misinformation. The general assumption when someone has a high BMI is that the extra weight is fat, however this is not always the case. Individuals who have a high lean mass, or a lot of muscle are therefore generally classified as overweight or obese, when in fact the opposite is true.

A second flaw is that BMI is not gender specific. Clearly, males and females have different body compositions, so it doesn’t make sense to measure both by the same scale.

DEXA & Total Body Composition Analysis

Dual-energy X-ray Absorptiometry (DEXA) is the gold standard in total body composition (TBC) assessment and bone mineral density (BMD). It uses low dose x-ray technology to detect bone loss or changes in the body’s soft tissue composition (muscle mass and fat mass). It also determines and monitors distribution of body fat, bone and lean muscle mass with 99% accuracy.

Fat Mass Index (FMI)— The New BMI

A DEXA total body composition scan can help determine your Fat Mass Index (FMI), which is gaining more attention by healthcare professionals as an obesity classification scheme. It is measured by dividing a person’s fat mass in kilograms by their height in meters squared. That means, no matter how muscular or “big boned” you are, your FMI will still accurately classify the amount of fat you are carrying.

Unlike Body Mass Index body mass index, fat mass index:
  • Is gender specific
  • Measures excess fat, not excess mass
  • Is not confounded by lean tissue
Severe Fat Deficit< 2.0< 3.5
Moderate Fat Deficit2.0 – < 3.03.5 – < 4.0
Mild Fat Deficit2.3 – < 3.04.0 – < 5.0
Normal3.0 – < 6.05.0 – < 9.0
Excess Fat6.0 – < 9.09.0 – < 13.0
Obese Class I9.0 – < 12.013.0 – < 17.0
Obese Class II12.0 – < 15.017.0 – < 21.0
Obese Class III> 15.0> 21.0