BMI and Body Composition

BMI (Body Mass Index)

BMI is a calculation using your height (meters) and weight (kilograms). BMI= kg/m2

BMI categories

Underweight <18.5

Normal 18.5-24.9

Overweight 25-30

Obese >30

1 kg= 2.2 lbs.

1 inch= 1.54 cm (height in inches, convert to cm, divide by 100 to convert to meters, then square it)

Body Composition

Body composition measurements estimate the amount of fat mass and fat-free mass. Your body weight (total body

mass) is fat mass+ fat-free mass. Fat-free mass is everything in your body other than fat: organs, tissues, bones,

muscles, etc.

Body fat percentage is the fat mass/total body mass.

Body fat categories

Women Men

Risky (high fat) > 40% Risky (high fat) >30%

Excess fat 31-40% Excess fat 21-30%

Moderately Lean 23-30% Moderately Lean 13-20%

Lean 19-22% Lean 9-12%

Ultra Lean 15-18% Ultra Lean 5-8%

Risky (low fat) <15% Risky (low fat) <5%

Ways to measure body composition

Underwater or Hydrostatic weighing- One method of body composition analysis in which a person is weighed while

submerged in a large tank of water. This method of determining body composition relies on Archimedes' Principle of

displacement which states:

1. The density of fat mass and fat-free mass are constant 2. Lean tissue is more dense than water 3. Fat tissue is less dense than water. 4. Therefore person with more body fat will weigh less underwater and be more buoyant.

Underwater weighing has been considered the gold standard for body composition assessment, however new, more

sophisticated methods may make underwater weighing obsolete in the future.

BodPod- With results comparable to hydrostatic weighing, the BOD POD Gold Standard Body Composition Tracking

System uses patented Air Displacement Plethysmography for determining percent fat and fat-free mass. The simple, 5-

minute test consists of measuring the subject's mass (weight) using a very accurate electronic scale, and volume, which

is determined by sitting inside the BOD POD chamber. From these two measurements, the subject's body composition is

calculated.

http://www.bodpod.com/products/howWorkBodpod

Skinfold measurements- Because underwater weighting it is complicated and cumbersome and requires special

equipment, most exercise physiologists use simple skinfold measurements to determine body fat percent. The American

College of Sports Medicine says that when performed by a trained, skilled, tester, they are up to 98% accurate.

Bioelectrical Impedance is another method of assessing body fat percentage. There are a variety of body composition

and body fat analyzers and scales available for home use that provide more than just total weight measurements. These

devices determine total weight, the percent and amount of body fat, muscle mass, water, and even bone mass. While

the readings can be affected by hydration levels, food intake, skin temperature, and other factors, if you follow the

directions and take the reading under similar conditions, you will obtain the best results.

http://sportsmedicine.about.com/od/fitnessevalandassessment/a/Body_Fat_Comp.htm

BMI vs. Body Composition

It is important to recognize that although BMI is often used to measure body composition due to lack of equipment and

time for other measurements, it is not a true measure. The equation used to calculate BMI only considers weight and

height, not what your weight actually consists of.

You may have two women who are 5’7” and weigh 145. Since they are the same height and weight, they will have the

same BMI of 22.7. This indicates a normal range. However, if these same women were to measure their body fat

percentage using a BodPod, the first woman actually has 17.9% body fat, which is considered “ultra lean” for a woman.

The second woman finds out that she has 32% body fat, indicating that she needs to lose some fat, not necessarily

weight. Therefore, weight is not always a good indication of a person’s body composition. It is important to maintain

muscle mass through regular strength training and exercise. Those who only consider their weight as an indicator of

healthy levels should consider body composition measurements to see actual fat and fat-free mass.