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What Is BMI? | Ask The Fitness Nerd

What is BMI and is it an Accurate Measure of Weight and Health?

Dear Fitness Nerd,

Can you explain exactly what BMI is? I recently bought a Wii Fit and according to the BMI calculation I’m fat! I checked some other BMI calculators online and am getting similar results. But I’ve never really considered myself overweight. I exercise daily and think I look pretty toned, but according to the BMI charts I’m overweight! Should I be worried? — Erika (Atlanta, GA)

Ericka, the Body Mass Index, or BMI is one of those things that I wish would just go away, since it causes all kinds of anxiety for people who may actually be in very good shape. It also makes people who may actually have high body fat levels focus on the wrong thing: losing scale weight, when they should be focusing on reducing body fat and encouraging muscle building.

While BMI can be a tool for gauging a person’s body composition in relation to height and weight, it tells you nothing about a person’s percentage of body fat. So for people who have low body fat levels with substantial amounts of lean tissue (muscle), it can be very misleading. 

But before we get into this in greater detail, let’s take a look at what BMI is.

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What Does Body Composition Mean? | Fitness, Health and Exercise Glossary

What Is Body Composition and How Is It Measured?

Definition of Body Composition

In physical fitness, body composition is defined as the percentage of fat, muscle, and bone in the body. Usually it’s expressed as a ratio of lean mass to fatty mass. Lean mass includes muscle, bone, skin, internal organs and body water. Fatty mass is mostly composed of body fat (subcutaneous fat) as well as internal essential fat surrounding organs. Body composition will typically be displayed as either a percentage of fat (body fat percentage or %fat) or as a percentage of lean body mass (LBM).

Why Is Body Composition Important?

Body composition is a much more accurate representation of a person’s leanness than scale weight or Body Mass Index (BMI), because it does not rely on height and weight alone to measure leanness.  It measures the ratio of body fat to lean tissue and bone in the body, not scale weight. 

This is important, because a person may have a high-scale weight (even for their height), yet have also have a high muscle-to-fat ratio which makes them extremely lean. That same person might be labeled overweight using the standard BMI calculation, which does not take into account body composition, only mass (weight) relative to your height, weight, age and gender. 

Excess body fat, or a body composition with a high fat-to-muscle ratio is unfavorable because it increases the risk of cardiovascular disease, Type II diabetes, Metabolic Syndrome and certain cancers. Excess body fat, especially at levels considered obese, can also put stress on the joints and interfere with mobility and the ability to perform everyday activities.

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