Body Composition Hydrostatic Weighing

What is Body Composition? Body composition makes reference to a number of things including –Total amount of lean mass (very closely related to muscle mass) –Total amount of fat mass –Percent lean mass – lean mass/total body mass –Percent body fat – fat mass/total body mass

Two-Component Model Human Body Composition = Fat Mass + Fat-Free (Lean) Mass i.e. Siri Formula Density of FM = 0.9 g/mL Density of FFM = 1.1 g/mL

Fat and Lean (Fat-Free) Mass Lean Mass (FFM): All of the body’s non-fat tissues –Bone, muscle, organs, and connective tissue Fat Mass –Essential fat Men (3%), Women (12%) –Storage fat: Found in adipose tissues Subcutaneous or visceral Functions: Insulator, substrate, padding

Body Composition is Calculated From Body Density A relatively small number of studies have been done in which cadavers are dissected and the lean mass and fat mass are separated, weighed, and their density is determined From these studies regression equations were developed to determine body composition from body density

Why Are There 3 Different Formulas? Each formula has been invented using different “population.” Therefore, each formula uses different assumptions (Density of FFM vs FM). Which formula should you use?

Ways to Determine Body Density Direct Measurements –Hydrostatic Weighing –Plethysmography (Bod Pod) Indirect Measurements –Standard Tables –Body Mass Index (BMI): Wt (kg)/Ht 2 (m 2 ) –Skinfolds –Bioelectrical Impedance Analysis (BIA)

Hydrostatic Weighing A direct measurement of body density Considered the gold standard of body density measurement

Density… Density = Mass / Volume In order to obtain Density, you have to measure Mass and Volume. Mass: easy to measure…get on a scale. Volume: get in the water! (volume of water displaced = volume of your body)

A Little Physics… Archimedes’ principal: Buoyancy Force (Fb) = Weight of Fluid Displaced

Body Mass Body Volume Body Density = = Volume of Water Displaced Mass of Water Displaced Density of Water = Buoyancy Force Underwater Weight – Dry Weight Acceleration Due to Gravity

A Complication: Air in Your Body ↓ ↑ Volume & ↑ Buoyant Force Where is it? AIR

Air in Your Body Lungs & Gastrointestinal Tract How do you control them? Air in Lungs:test at RV Air in GI Tract: minimize it by fasting

Problems Is RV accurate? Some people do not feel comfortable submerging in the water at RV. GI gas is not measurable. (could be anywhere from 0.1 to 1.5 L) Densities of FM & FFM are not always as presented earlier. (i.e. Osteoporosis patient?)

For Your Interest… Dry Weight = m b *g F b = V H 2 O *D H 2 O *g = (V b + V air )*D H 2 O *g Under Water Weight = Dry Weight – F b = m b *g – (V b + V air )*D H 2 O *g since g is constant throughout this derivation, let’s ignore it. UWW (in kg) = DryW (in kg) – (V b + V air )*D H 2 O V b = DryW / D b UWW = DryW – [ (DryW / D b ) + V air ) ]*D H 2 O UWW = DryW – (DryW / D b )*D H 2 O – (V air )*D H 2 O (DryW / D b )*D H 2 O = DryW – UWW – (V air )*D H 2 O D b = (DryW)*D H 2 O / [ (DryW – UWW) – (V air )*D H 2 O ] DryW [ (DryW – UWW) / D H 2 O ] – (V air ) D b =