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Understanding BMI and Body Fat Testing  

You've seen them in print and heard them spoken – Body Mass Index (BMI) and body fat percentage – but what do they mean and why are they important?

BMI and body fat percentage are measurements we use to gauge our body's fatness. More than looking in a mirror or tugging on our pants, BMI and body fat % assign a numerical value, a number, to our ‘fatness' level so we can quickly reference it against valuable data from decades of scientific studies.

  • Are you at an increased risk for weight-related ailments, such as high blood pressure, diabetes, heart disease and stroke?
  • Do you need help setting a healthy weight goal?
  • Is your diet and fitness plan effective?

BMI and body fat testing helps you answer these questions and more. What's a good body fat percent? Use this Body Fat Chart to find out.

BMI    BMI is a simple weight-to-height ratio (weight/height2).   Because no specialized equipment is necessary and it is easily performed by physicians and clinicians, this method gained much popularity over the past twenty-five years as obesity has become a world-wide crisis.  For adults, a good BMI reading is between 18.5 and 24.9.

Body Fat Testing    There are many ways to test one's body fat, including skin fold calipers, hydrostatic weighing (dunk tank), and bioelectrical impedance (example: BodySpex scale).   Body fat testing via one of these methods takes into account one's weight, height and other additional personal values based on the testing method.   As such, body fat testing is an individualized assessment.

BMI vs Body Fat Testing

BMI and body fat testing have similar goals, indicating body fat levels, but can issue meaningfully different results for some individuals.

The BMI formula is designed to provide a generalized indicator of a person's health level.   If you are sedentary (inactive) with a normal body composition (not highly muscular or big-boned, for example) a BMI reading can be very useful.

BMI's main drawback is that it does not differentiate between muscle and fat or take body type into account.  To BMI, weight is weight. But, not all weight is equal or unhealthy.  Bodybuilders and athletes, for example, often register as ‘obese' when this is clearly not the case.

Body fat tests indirectly measure a person's fat level by exploiting one of several differences between fat tissue and muscle tissue.  For hydrostatic weighing, this difference is tissue density.   For bioelectrical impedance analysis, the difference is tissue hydration level (conductivity).

When properly performed, a body fat test will provide a more accurate picture of your health level than BMI.  As you progress down your health and fitness path, you will invariably gain a greater appreciation for body fat testing over BMI.

Body Fat is Essential

Do not be overly obsessed about having some body fat.  Everyone needs body fat for good health and reproduction.  There are two kinds of fat:  essential fats and storage fats.  Because women carry most of the burden of reproduction, their level of essential fat is much higher than men's.

Essential body fat levels for men are between 2-5% and for women 12-15%.  Storage fat is excess fat that we work to keep to a minimum.

Why are BMI and Body Fat % Important?

They help you:

  • Set a baseline health assessment – do you have fat-related health risks?
  • Determine a healthy weight goal – how much, if any, should you lose?
  • Provide progress assessment – is your diet and fitness plan working?

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