Unformatted text preview: 9/13/2010 Objectives Type II Approaches: Densitometry and 2C Models
Session #4 Thursday September 2nd, 2009 Introduce the Type II approach Explain body composition models Present reference man and the two component (2C) model Provide overview of densitometry and how to calculate body density Explain Archimedes principal, hydrodensitometry (hydrostatic weighing), how to use hydrodensitometry to calculate body density Present the Siri 2C equation Discuss the underlying assumptions of 2C methods Present Total Body Potassium (TBK) and Total Body Water (TBK) methods of estimating body composition Discuss chemical constancy (chemical maturity) Type II Approach
constant constant or relatively constant ratios or proportions model model based methods ComponentComponent-based, property-based, propertyor combined
Atomic Molecular Body Composition Models
Body Composition Model
BM = H + O + N + C + Na + K + C + P + Ca + Mg + S BM = FM + TBW + TBPro + Mo + Ms + CHO BM = FM + TBW + TBPro + M BM = FM + TBW + nonfat solids BM = FM + FFM BM = cells + ECF + ECS BM + FM + BCM + ECF + ECS BM = AT + SM+ bone + organs + other BW = head + trunk + appendages # of components
11 6 4 3 2 3 4 5 3 Cellular Tissue Whole Body Reference Man Model Reference Man Models Reference man becomes very important when we consider model-based approaches to body composition assessment He represents the simplest 2-component model – fat and fat free mass 2C 4C 3C 3C 1 9/13/2010 Densitometry
The The density of any material is a function of the proportions and densities of its components. Density Density (D) = Mass (M)/Volume (V) Thus, Thus, body density (Db) = BM/BV
• BM = body mass; BV = body volume In the absence of body volume measurement
Db = f + ffm 1 f ffm or =+ f ffm Db df dffm + df dffm Db = Body Density f = the proportion of bodyweight that is fat the ffm = the proportion of bodyweight that is ffm fat free mass df = density of fat dffm = density of fat free mass Body Density of Reference Man
f + ffm 1 f ffm Db = =+ or f ffm Db df dffm + df dffm Density of the FFM?
1 wpm = ++ dffm dw dp dm
dffm = density of fat free mass w = the proportion of ffm that is water the p = the proportion of ffm that is protein the m = the proportion of ffm that is mineral the dw = density of water dp = density of protein dm = density of mineral Need Need the densities of fat and FFM Given: Given: Density of fat: 0.9 kg/L Density Density of FFM? Reference Man Solving Solving for Dffm
1 wpm .738 .194 .068 = ++ = + + dffm dw dp dm .9937 1.34 3.038 1 = 0.9098 dffm Dffm = 1 ≈ 1.100 kg L 0.9098 2 9/13/2010 Db of Reference Man
Db = f + ffm 1 f ffm or = + f ffm Db Df Dffm + df dffm Hydrostatic Weighing 1 f ffm 0.15 0.85 = + = + = 0.9393 Db Df Dffm 0.9 1.100 Db = 1.0645 kg L Hydrostatic Weighing (Underwater Weighing - UWW)
What W hat is actually being measuring? Body Body Volume What W hat do we really want to measure? Body Body Density Let’s Let’s first look at how volume is measured Archimedes Principle
Archimedes Archimedes Principle states that the buoyant force on a submerged object is equal to the weight of the fluid that is displaced by the object. OR A body immersed in a fluid is acted on by body a buoyancy force, which is evidenced by a loss of weight equal to the weight of the displaced fluid. Archimedes Principle Weight in the water Weight in the air What is the volume of the object? 3 9/13/2010 Calculating Volume
V= Wair - WH2O DH2O V = volume Wair = Weight of object in the air WH2O = Weight of object in the water DH2O = Density of Water Weight in the water Weight in the air V= Wair - WH20 7 kg − 4kg = = 3.02L DH20 0.9937 kg L Applying Archimedes Principle to Body Density
Db = mass Wair = Wair - WH2O volume − ( RV + GI) DH2O Reference Man and Hydrostatic Weighing
Reference Reference Man
• Weight = 70 kg • WeightH20 = 3.06 kg 3.06 • Residual Volume = 1.5 L • GI Volume = 0.1 L Db = Body Density (kg/L) Wair = Weight of body in the air (kg) WH2O = Weight of body in the water (kg) DH2O = Density of Water (kg/L) RV = Residual volume of the lungs (L) GI = Gas volume in the gastrointestinal tract (L) Calculate Calculate Reference Man’s Db Plug and Chug
Db = Wair Wair - WH20 − (RV + GI) DH20 Air Plethysmography – Bod Pod
Uses air pressure to measure body volume Volume of empty chamber Volume of occupied chamber Subtraction of occupied from empty Only need average lung volume of normal breathing Db = 70 kg 70 kg = = 1.0645 kg L 70 kg - 3.06 kg 65.76 L − (1.5 L + 0.1 L) kg 0.9937 L 4 9/13/2010 Siri 2C equation
Use Use body density to estimate fat fraction So you have body density, now what? How can you use Db to estimate %fat?
%Fat %Fat Fat = 4.95 − 4.50 Db %Fat = ( 4.95 − 4.50) × 100 Db Simplest solution for %fat when using body density
Densities of the components are known and additive The components of the FFM have constant densities The components of the FFM have constant proportions The person being measured differs from reference man only by percent fat Df What if Db varies? Racial differences in FFM densities
Density of the FFM is greater in African-Americans than in Whites 1.113 kg/L vs. 1.100 kg/L Body densities differ What component has the greatest impact on the density of FFM? MINERAL! – density of 3.038 kg/L 5 9/13/2010 Changes In Components - Children
Components of FFM Age Weight(kg) Birth 3.5 1-month 4.2 6-month 7.7 1-year 10.1 5-year 18.1 10-year 32.0 Ref Man 70 %Fat 14.0 15.0 26.0 23.0 15.6 16.5 15.0 TBW (%FFM) 80.6 80.5 79.5 79.0 77.0 76.0 73.8 Protein Mineral (%FFM) (%FFM) 15.0 3.7 15.1 3.7 16.0 3.7 16.6 3.7 18.2 4.0 19 4.3 19.4 6.8 Changes In Components How do we get around differences in FFM density?
When using a density based equation such as the Siri 2C equation there needs to be equations developed for each specific population Other 2C Methods Population specific equations! 2C Methods: Total Body Potassium (TBK)
40K 40 40 Concentration of 40K Emits Emits minute amounts of gamma radiation Found Found primarily in the FFM Whole Whole body counter Property? Property?
• Radioactive Population
Children Adult Males Adult Females [40K]
2.30 g/kg of ffm 2.66 g/kg of ffm 2.55 g/kg of ffm
These become the conversion factors used to convert TBK to kg of FFM 6 9/13/2010 Using TBK to estimate FFM
Calculate Calculate FFM Given Given
• Male subject • Bodyweight = 50kg • Total Body Potassium (TBK) = 110 g ffm = TBK 110g of K = = 41.4kg of ffm g of K g of K 2.66 2.66 kg of ffm kg of ffm This male subject has 41.4 kg of FFM How much fat does he have? What is his %fat?
Fat = BW - ffm Fat = 50 kg - 41.4 kg = 8.6 kg %Fat = Fat × 100 BW 8.6 kg %Fat = × 100 = 17.2% 50 kg ♂
TBK ffm = 2.66 g of K kg of ffm ♀
TBK ffm = 2.55 g of K kg of ffm ♂♀
TBK ffm = 2.30 g of K kg of ffm Assumptions
40K 2C Methods: Hydrometry (Isotope Dilution)
Used Used to estimate total body water (TBW) ViCi = VfCf ViCi Solve Solve for Vf Vf = is found only in the fat free mass Cf Property? Property?
• Dilution Isotopes (Tracers)
Two Two different isotopes are used: Tritium Tritium (3H, Hydrogen 3, nucleus contains one proton and two neutrons) in the form of tritiated water (3HOH) Deuterium Deuterium (2H, Heavy Hydrogen, nucleus contains one proton and one neutron) in the form of 2H2O. Tritium Tritium is radioactive and emits beta rays where as deuterium is stable and not radioactive. Characteristics of a useful isotope
It has to be non-toxic The isotope distributes only in the compartment of interest (TBW, ECF) The isotope distributes equally in compartment of interest It is not metabolized 7 9/13/2010 Assumptions associated with this method
Water is neither lost nor gained during the equilibration period No amount of the isotope is lost nor gained during the equilibration period Water is a constant compartment of the fat free mass
1. In reference man water is 73.8% of fat free mass 2. Children and youth: 74%-77% 3. Elderly: 72% Estimating FFM by TBW
Male subject Weight: 75 kg TBW by isotope dilution: 44.2 kg kg of ffm = TBW(kg) 0.738 kg of ffm = TBW(kg) × 1.36 kg of ffm = = TBW(kg) 0.738 Review
What is the Type II approach? What is meant by 2C, 3C, and 4C models? What level (from the 5-level model) are the reference man’s models based on? What is densitometry and how are the results applied to estimating body composition? The Siri 2C equation uses denistometry as its foundation, what are the assumptions? Hydrostatic weighing measures what body property? What principle underlies this measurement? What is the importance of RV and GI values? How do they effect the meausurement? Describe the total body potassium and hydrometry methods. What are the assumptions? 44.2kg = 59.9kg of ffm 0.738 kg of ffm = TBW(kg) × 1.36 = 44.2kg × 1.36 = 60kg of ffm How much fat does this subject have? What is his %fat? If TBW was given in L, what would you do? 8 ...
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