Lecture 3: Measuring Body Composition 2 Flashcards
BMI calculation:
= weight (kg) / height (m2)
What are the advantages of measuring BMI?
- easy, quick, non-invasive
- correlates with percent body fat at group level
- useful for individuals with additional information
What are the limitations from measuring BMI?
- doesn’t tell you what weight is measuring
- high BMI could be high lean, fat, oedema
What does a decrease in mid-upper-arm circumference reflect in high income countries?
Reduction in muscle or fat (or both)
What does a decrease in mid-upper-arm circumference reflect in low income countries?
Subcutaneous fat is low shows changes in muscle mass
What is Protein Energy Malnutrition (PEM)?
a condition caused by a severe deficiency of protein and calories, leading to a range of health problems including muscle wasting, stunted growth, and weakened immune function, commonly seen in developing countries
What muscle estimates can mid-upper-arm circumference show?
- Mid-upper-arm muscle circumference (MUAMC)
- Mid-upper-arm muscle area (AMA)
What are skinfolds?
Callipers measure fold of skin and underlying fat
How are skinfolds data used?
- by comparing to percentiles
- to calculate % fat using equations
What are common skinfold areas?
- triceps
- biceps
- subscapular
- suprailiac
- thigh, calf, abdomen
What are the advantages of skinfolds?
- simple
- cheap
- can be precise and accurate
What are limitations/assumptions made of skinfolds?
- Constant skin and subcutaneous fat compressibility
- Constant skin thickness
- Chosen sit is good estimate of total fat
- Subcutaneous fat represents constant proportion of total fat in every person
What is an oedema?
Oedema is the abnormal accumulation of extra-cellular fluid in tissues, causing swelling, often in the legs, feet, or hands
What is Dual-energy X-ray Absorptiometry (DEXA)?
Medical imagine technique - small amount of radiation used to measure lean mass, fat mass and bone mineral content
How does DEXA work (the principles)?
- Dense tissues (e.g. bone) absorb more X-rays
- Detectors measure intensity of X-rays passing through
- W-dimensional image generated
- Software algorithms calculate body composition
What are the advantages of DEXA?
- accurate (2% error)
- precise
- suitable for almost all ages
- Relatively quick
What are the limitations of DEXA?
- expensive
- size limits
- requires trained technician
What are the 4 main types of approach for regional body composition?
- CT
- MRI
- DEXA
- Anthropometry
What is computed tomography (CT)?
A rotating X-ray tube and detectors take detailed cross-sectional images that discriminate between visceral and subcutaneous fat
What are CT’s used to measure?
Lean mass, fat mass and bone mineral content
What are the advantages of CT’s?
- “Gold standard”
- Accurate
- Measures visceral fat
What are the limitations of CT’s?
- Expensive
- Higher levels of radiation than DXA
What is magnetic resonance imaging (MRI)?
Strong magnetic field & radio waves take detailed cross-sectional images that discriminate between visceral and subcutaneous fat
What are MRI’s used to measure?
Lean mass, fat mass and bone mineral content
How do MRI’s work?
Different tissues emit different radio waves when H atoms aligned by magnetic field
Which has better discrimination, MRI’s or CT’s?
MRI’s
What are the advantages of MRI’s?
- very accurate
- no radiation!
- measures visceral fat
What are the limitations of MRI’s?
- expensive
- slow
- strong magnetic field
Can DEXA measure visceral fat?
From an automated image-processing algorithm using raw 2D images - cannot discriminate but can measure regions
What is waist circumference a proxy for?
central fat
What is hip circumference a proxy for?
peripheral fat
Many now consider that waist circumference alone….
is a better indicator of visceral fat
What are the advantages of anthropometry circumferences?
Simple
What are the disadvantages of anthropometry circumferences?
- Not direct measure of visceral fat
- Difficult in very obese
What percent body fat is considered normal?
Men: 15-22
Women: 18-32
What percent body fat is considered overweight?
Men: >22
Women: >32
What percent body fat is considered obese?
Men: >25
Women: >35
