Final - Body Composition Methodology Flashcards
Body composition assessment is quantification of what body structure components?
muscle (protein)
bone
fat
water (body fluid compartments)
Overweight
a body mass greater than some standard (usually for a given stature)
Overfat
body fat greater than a standard for sex and age (requires body composition analysis)
Is it possible to be overweight without being overfat?
yep
Essential fat
bone marrow stores & stores in viscera and nerves - needed for normal function - 3% in males - 12% in females Includes sex-specific fat
Storage Fat
- accumulation of lipid in adipocytes
- nutritional reserve
- subcutaneous and visceral (intraperitoneal)
Lean Body Mass (LBM)
includes essential fat, muscle, water, bone, mineral
Fat-Free Mass
All lipid contributions excluded
BMI and obesity classification
20-24.9: desirable
25-29.9: Overweight (grade 1 obesity)
30-40: Grade 2 obesity
>40: grade 3 obesity
Average % fat in young adults:
Men: 12-15%
Women: 25-28%
Overview of Body Composition Methodologies
- Hydrostatic weighing
- Skinfolds
- Bioelectric Impedance Analysis
- Imaging Technologies
- Air Displacement Plethysmography (bod-pod)
Overview of Densitometry
Measure Body Volume (hydrostatic weighing, skin folds, bod-pod) + measure body mass on a scale
mass/volume = density
conversion equation from density to %fat (Siri, Brozek)
Yields estimate of % fat
Hydrostatic Weighing
- 2 compartment body composition model of FFM and fat
- Goal: estimate body density (d=m/v)
- We have mass, we need body volume
Body Density
g/cm3 (mass per unit volume)
Fat and FFM have different densities
- Fat-free body tissue: 1.100 g/cm3
- fat in adipose tissue: 0.900 g/cm3
How does hydrostatic weighing work?
- determines body volume by Archimedes’ principle
- object placed in water is buoyed up by a counterforce equal to the water it displaces
- Amount of water displaced (spillover) equals loss of weight completely submerged
When using hydrostatic weighing, what must you taken into account?
air in lungs and GI tract
failture will overpredict %fat
Calculation of body density from underwater weight
Population specific formulas to calculate % body fat from body density all have some error bc of biological variability
- bone mineral density
- partitioning and volume of water
- mineral components of the FFM
Siri’s Equation
% body fat = (495/D) - 450
Brozek’s Equation
% fat = (457/D) - 414.2
Hydrostatic Weighing Validity
when residual volume is actually measured, hydrostatic weighing is the standard to which other methods are compared (within 2.0% of true value)
Advantages of hydrostatic weighing
- highly reproducible
- can be highly accurate
- valid for many populations (except small children and those who won’t be submerged)
Disadvantages of Hydrostatic Weighing
- best accuracy in highly technical experienced labs
- bulky equipment
- methodological concerns (prediction of RV, accuracy of weighing procedure, subject cooperation)
Skinfold Thickness Anthropometric Measurement
- relies on the observation that within a certain population, a certain fraction of total body fat lies just under the skin (prediction of total body fatness from subcutaneous fat)
- skin fold thickness highly correlated with % body fat
Most important variables contributing to measurement error in Skinfold measurements
investigate technique and experience
reproducibility and accuracy
Methodology for Skinfolds
- sites can vary (triceps, subscap, suprailiac, abdominal, thigh, chest)
- Measurement duration is very important
- reading should be taken within 2 seconds of applying caliper jaws
Skinfold Caliper
Best results are obtained with a specifically- calibrated skin fold caliper
- constant tension exerted by spring loaded caliper jaws
- tension exerted affects how much extracellular fluid is pushed out of the area
Prediction equations for skin folds
- choose equation develop and verified for subjects demographic
- subcutaneous fat to total body fat ratio is altered with age and sex
- failure to use appropriate equations can introduce huge errors into the estimate of density and % BF
Sum of Skinfolds
- helpful for following weightless in an individual
- in obese individuals, it may be more accurate to use prediction equations that use circumferences than skin folds
Advantages of Skinfolds
- inexpensive and quick - mass screening
- good for following changes in body comp
- fairly reproducible and accurate in experienced hands
disadvantages of skin folds
- population specificity in prediction equations can lead to large error
- error highly dependent on investigator technique
- caliper used should be the same used in prediction equation
Bioelectrical Impedance Analysis
- noninvasive, fast, accessible, relatively cheap method to evaluate body composition
- determines electrical impedance of body tissues (measures opposition of tissues to flow of small alternating electrical current and provides estimate of total body water)
- less impedance in FFM (due to greater electrolyte concentration)
BIA estimates FFM and body fat from TBW from predication equations that use what?
- impedance
- weight
- height
- sex
- age
BIA accuracy
- reliable estimate of TBW under most conditions
- limited ability to predict body fat in severely obese
- not useful in measuring short-term changes in body composition
- significant differences in % fat prediction between manufacturers
- different prediction equations used
- accuracy equivalent to skin fold technique if protocol is followed
Standard recommendations for BIA
- attempt to maintain normal hydration, plasma osmolality, body fluid distribution
- dry skin
- no eating/drinking within 4 hrs, exercise within 12 hrs, alcohol within 48 hrs, diuretic agents
- void before assessment
Near-Infrared Interactance
- uses amount of light absorption vs reflection: typically at a single site to predict body fat, uses prediction equations
- not for human body composition analysis currently
Air-Displacement Plethysmography (Bod-Pod)
- similar to hydrostatic weighing, it is densitometric
- body volume directly measured
- body density and % body fat calculated using same equations used to HW
Bod Pod vs Underwater Weighing Advantages:
- Fast (5 mins)
- Can use with various populations (obese, elderly, pediatric)
- Good compliance ( no need for maximal exhalation or immersion, mobile)
- less operator training required
Body Composition Assessment Sources of Error:
- measurement error (intra and inter-investigator error)
- subjectivity (training and experience)
- fixed equipment or machine error
- Underlying assumptions error (constants, models and predictive equations used, estimates of physiological variables, **normal biological variability)
- experimental conditions (hydration status, subject cooperation, environment)
