Lecture 3 - Measuring Body Composition Flashcards
what are the advantages to BMI
- easy, quick, non-invasive
- correlates with percent body fat at group level
- useful for individuals - with additional information
what are the limitations to BMI
- doesn’t tell you what weight is measuring
- high BMI could be high lean, fat, oedema
in high income countries mid upper arm circumference a decrease can reflect
a decrease can reflect reduction in muscle or fat (or both)
in low income countries mid upper arm circumference a changes show what and why
subcutaneous fat small so MUAC changes are parallel changes in muscles mass
in low income countries mid upper arm circumference can lead to diagnosis of
Protein Energy Malnutrition (PEM)
middle upper arm circumference can be used to calculate muscle estimates such as…
- middle upper arm muscle circumference (MUAMC)
- mid upper arm muscle area (AMA)
in what countries is mid upper arm circumference more useful than others
more useful in low income countries than high income ones
- due to less subcutaneous fat so would show more changes in muscle mass
what is a skin fold
calipers measure fold of skin and underlying fat
data from skinfolds is used to
comparing to percentiles
or
to calculate % fat using equations
what are common skinfolds
- triceps
- biceps
- subscapular
- suprailiac
- thigh, calf, abdomen
what are the advantages to skinfolds
- simple
- cheap
- can be precise and accurate
what are the limitations / assumptions to skinfods
- constant skin and subcutaneous fat compressibility
- constant skin thickness
- chosen sites are a good estimate of total fat
- subcutaneous fat represents constant proportion of total fat in every person
what kind of technique is dual energy x-ray absorptiometry (DEXA)
medical imaging technique
how does the dual energy x-ray absorptiometry (DEXA) work
small amount of radiation used to measure lean mass, fat mass and bone mineral content
what are the principles of dual energy x-ray absorptiometry (DEXA)
- dense tissues (e.g bone) absorb more x rays, less dense tissues (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
what are the advantages to dual energy x-ray absorptiometry (DEXA)
- accurate (2% error)
- precise (grams FM, lean mass, BMC)
- suitable for almost all ages
- relatively quick
what are the limitations to dual energy x-ray absorptiometry (DEXA)
- expensive (100-150 bucks per session)
- size limits (some people may be too wide)
- requires trained techniciain
what are the 4 main types of approach for measuring regional body composition
- computed tomography (CT)
- magnetic resonance imaging (MRI)
- DEXA - calculated estimate
- anthropometry - circumferences
what type of technique is computed technology
medical imaging technique
how does computed technology work
rotating X-ray tube and detectors, gives detailed cross sectional images
what is computed technology used to measure
lean mass, fat mass and bone mineral content
what can computed technology discriminate between
between visceral and subcutaneous fat
what is computed technology considered
“gold standard”
what are the advantages to computed technology
- “gold standard”
- accurate
- measures visceral fat
what are the disadvantages to computed technology
- expensive (more expensive than DEXTA)
- higher levels of radiation
what kind of technique is magnetic resonance imaging (MRI)
medical imaging technique
how does magnetic resonance imaging (MRI) work
strong magnetic field and radio waves, gives detailed cross sectional images
what is magnetic resonance imaging (MRI) used for
to measure lean mass, fat mass and bone mineral content
what can magnetic resonance imaging (MRI) discriminate between
visceral and subcutaneous fat
in MRI, different tissues emit
different radio waves when H atoms aligned by magnetic field
what are the advantages to magnetic resonance imaging (MRI)
- very accurate
- no radiation, so multiple scans on same person possible
- measures visceral fat
what are the limitations to magnetic resonance imaging (MRI)
- expensive
- slow
- strong magnetic field
dual energy X ray absroptiometry (DEXA) - calculated estimate is lots cheaper than
CT or MRI scans
dual energy X ray absroptiometry (DEXA) - calculated estimate can measure
can measure regions
dual energy X ray absroptiometry (DEXA) - calculated estimate can not discriminate between
visceral and subcutaneous fat
dual energy X ray absroptiometry (DEXA) - calculated estimate can estimate visceral fat from
an automated image processing algorithm using raw 2D images from DXA, this can be incorrect though
in anthropometry waist circumferences are a proxy for
central fat
in anthropometry hip circumferences are a proxy for
peripheral fat
waist and hip circumference can be used to calculate
waist : hip ratio
what do many now consider about waist circumference
many now consider that waist circumference alone is a better indicator of visceral fat rather than waist : hip ratio
what are the advantages to using circumferences
simple
what are the disadvantages to using circumferences
- not direct measure of visceral fat
- difficult in very obese
what are the cutoffs for obese % body fat commonly used in diagnostic textbooks
men >25%
women >35%
more accurate methods of measuring body composition are usually
more expensive or difficult
choosing a method for measuring body composition will depend on
time, resources, cost and participants
