Lecture 3 Flashcards
Measuring Body composition II
Anthropometry -BMI
Body Mass Index (BMI)= weight (kg) / height (m)^2
BMI Advantages
- Easy, quick, non-invasive
-Correlates with percent body fat at group level
-Useful for individuals- with additional information
BMI: limitations
-Doesn’t tell you what weight is measuring
-High BMI could be high lean, fat, oedema
Anthropometry- Mid-upper-arm circumference
High income countries
Decrease can reflect reduction in muscle or fat (or both)
Anthropometry- Mid-upper-arm circumference
Low income countries
-Subcutaneous fat small so MUAC changes parallel changes in mucle mass
- Diagnosis of Protein Energy malnutrition
Mid-upper-arm circumferance =Can be used to calculate muscle estimates
-Mid-upper-arm muscle circumference
-Mid-upper-arm muscle area
Anthropometry -Skinfolds (what is used)??
-Calipers measure fold of skin & underlying fat
Calipers: Data used
-By comparing to percentiles
-To calculate % fat using equations
Common skinfolds
-Triceps
-Biceps
-Subscapular
-Suprailiac
-Thigh, calf, abdomen
Skinfolds: Advantages
-Simple
-Cheap
-Can be precise and accurate
Skinfolds: Limitations (assumptions)
-Constant skin and subcutaneous fat compressibility
-Constant skin thickness
-Chosen sites are good estimate of total fat
-Subcutaneous fat represents constant proportion of fat in every person
Oedema
Extra cellular fluid building uo in the skin
- leaves a dent
-Wet beri beri
-Low income countries this can occur due to a lack of nutrition
Why cant you measure fat mass with someone who has oedema
You wont be able to take a useable measurement because it wont be able to measure your exact fat mass because you have to much fluid in the way
Three-component model
Fat mass(FM) + Fat-free mass (FFM)
Fat free mass
- Bone mineral content
-Lean soft tissue
Dual-energy X-ray Absorptiometry (DEXA)
-Medical imaging technique
- Small amount of radiation used to measure lean mass, fat mass and bone mineral content
Dual-energy X-ray Absorptiometry (DEXA)
Principles
-Dense tissues (e.g bone) absorb more X-rays, less dense tissue (e.g fat) absorb less
-Detectors under the body measure intensity of X-rays that have passed through
-Dexa then generates a 2 dimensional image
-Software algorithms calculate body composition
DEXA:advantages
-Accurate (2% error)
-Precise (grams FM, lean mass, BMC)
-Suitable for almost all ages
-Relatively quick
DEXA: Disadvantages
-Expensive
-Size limits
-Requires trained technician
Four main types of approach for regional body composition
1-Computed tomography (CT)
2-Magnetic resonance imaging (MRI)
3-DEXA- Calculated estimate
4-Anthropometry- circumferences
Computed Tomography
-Medical imaging technique
-Rotating X-ray tube and detectors -> detailes cross-sectional images
Computed tomography is used to measure
Lean mass, fat mass and bone mineral content
- Can also discriminate between visceral and subcutaneous fat
-“Gold standard”
Computed Tomography: advantages
-“Gold standard”
-Accurate
-Measures visceral fat
Computed Tomography: limitations
-Expensive
-Higher levels of radiation than DXA so limitations on use
MRI
-medical imaging technique
-Strong magnetic field & radio waves -> detailed cross sectional images
MRI used to measure
Lean mass, fat mass and bone mineral content
-Can also discriminate between visceral and subcutaneous fat
- Different tissues emit different radio waves when H atoms align by magnetic field
MRI: Advantages
-Very accurate
-No radiation, so multiple scans on same person possible
-Measure visceral fat
MRI: Limitations
-Expensive
-Slow
-Strong magnetic field
DEX- calculated estimate
-Can measure regions
-But cant discriminate between visceral and subcutaneous fat
Estimate visceral fat from an automated image-processing algorithm using raw 2D images from DXA
Anthropometry- circumferences
Waist (proxy for central fat)
Hip (proxy for peripheral fat)
Calculate waist:hip ratio (WHR)
Calculate waist:hip ratio (WHR)
BUT ……
Many now consider that waist circumference alone is better indicator of visceral fat
Calculating abdominal visceral fat level from body composition
To evaluate the merit BMI, % body fat, waist circumference and WHR as predictors of abdominal visceral fat level
what was the best measure for AVF
waist circumference was the best overall indicator of abdominal visceral fat level- with correlation coefficients >-0.75 for younger and older women and men. WHR was a poor measure of AVF especially in women and should not be used as a surrogate measure of visceral obesity
Circumferences: Advantages
simple
Circumferences: Disadvantages
-Not direct measure of visceral fat
-Difficult in very obese
Body fat % Obese Men
> 25
Body fat % Obese Women
> 35
when measuring regional body fat and its relationship to health we ideally want a measure of
Visceral, central fat
