Body composition (Unit 1) Flashcards
What is the difference between the two compartment model and three compartment model for assessing body composition?
Two Component Model:
Fat Mass + Fat free
Three component:
Fat Mass + Bone mineral content + bone-free fat-free mass
What are the different methods for measuring body composition?
- Anthropometry
- Potassium Count
- Creatine excretion
- CT
- Contour Photograph
- MRI
- Underwater weighing
- Bioimpedance analysis
- metabolic balance
- TB conductivity
- DXA
- Air displacement
- Plethysmography
Why do we measure Body composition?
+ monitor growth
+ assess response to therapy/ change
+ assess health
+ create reference values
What are the 2 primary anthropometry measurements of interest?
- Total Body composition
2. regional Body composition
How do you calculate body fat %?
(FM/BW) x 100
What does regional body composition show us?
If fat is stored centrally or peripherally (body shape)
Why is knowing regional body composition important for health?
Visceral/ central adiposity has much greater health risks –> greater associations with disease
What diseases does central adiposity increase the risk of?
- diabetes
- CHD
- CVD
- Hypertension
- Hyperlipidemia
- Arthritis
- Cancers
Central fat includes both visceral & subcutaneous fat; which one increases the risk of disease more?
Visceral
What does the two-compartment model assume?
FFM composition is always constant
Assumes density is 1.1 g/cm3 & FM’d density = 0.9 g/cm3
What are the assumptions underlying densitometry?
- Density of FFM is constant
- Density of Fat does not vary between people
- The water content of fat free mass = constant
- proportion of bone to FFM = constant
What are the advantages of under water weighing?
accurate (~2% error)
What are the limitations of under water weighing?
+ density of FFM may not be uniform
+ bulky/ expensive equipmeny
+ need to repeat
+ not suitable for all
How do you calculate BF% from underwater weighing?
Weigh normally & underwater; use this to calculate BV followed by body density.
Using estimates of fat & FFM densities can calculate composition
What is the difference between Air displacement plethysmography & underwater weighing?
Calculate displaced air instead of water
How is bio impedance analysis used to measure body composition?
Electrical current allows to estimate TBW from impedance (resistance & reactance) & as FFM is 73% water can estimate FFM using Lukaski & Bolonchuk’s formula for total body water and dividing it by 073.
What are the advantages of BIA?
- Simple,
- cheap
- portable
- suitable for all ages
What are the limitations of BIA?
- have to be fasted
- have to control PA
- Have to control Hydration
- need appropriate equation for subjects
How do we interpret anthropometry methods such as BMI, skinfolds, circumferences & breadths
- Compares with reference data & uses cut offs that relate to morbidity & malnutrition risk.
What does anthropometric reference data need to be representative of?
A healthy population (including age, gender, ethnicity)
what are the advantages of anthropometry measures?
- simple
- safe
- Non-invasive
- Cheap
- Portable equipment
- Good for large studies
What are the limitations of anthropometry measures?
- less accurate
- less sensitive to change
- more sources of error (measurement error; variation in tissue compositions; not valid in diseased/ obese)
What measure is combined with BMI to classify disease risk?
Waist circumference
What are the cutoffs for waist circumference (when do they increase disease risk)?
Men: >102
Women: >88
What are the advantages & limitations of BMI?
Advantages:
Simple; Good for groups
Limitations: Measure of weight only; High BMI may = high FFM of FM
what is fat free mass?
Mixture of water, minerals & Protein
What is total muscle mass strongly correlated with?
- Mid-upper-arm muscle circumference (MUAMC)
2. Mid-upper-arm muscle area
When is Mid-upper-arm muscle circumference (MUAMC) useful as an anthropometry measure?
In low income countries indicates parallel losses in fat and muscle and can be used to diagnose protein energy malnutrition
How are skinfolds used to measure anthropometry?
Callipers measure skin & fat; from this equations are used to calculate body density.
What are the advantages & limitations of using skin folds?
Advantages:
- Simple + safe
- Cheap
- Precise & accurate
Disadvantages:
Relies on assumptions:
1. Constant skin & fat compressibility
2. Constant skin thickness
3. Sites are good estimate of total BF
4. Subcutaneous fat represents constant proportion of total fat
5. Constant fat fraction in adipose tissue
How is DXA used to measure body composition?
Uses a small amount of radiation to measure lean mass, fat mass & bone mineral content
What are the advantages & disadvantages of DXA?
Advantages:
Accurate & precise
Suitable for all ages
Limitations:
expensive & size limits
How can we measure regional body composition
- Computed tomography (CT) -> radiation (differentiates between visceral & subcutaneous fat)
- Magnetic Resonance Imaging (MRI)
- DXA -> measures regions but can’t discriminate between fat types
- Waist circumference
What are the advantages & limitations of CT scans?
Advantages:
Accurate & measures visceral fat
Disadvantages:
Expensive; more radiation
What are the advantages & limitations of MRI’s?
Advantages:
- No radiation
- Measures Visceral
Disadvantages
- expensive
- Slow
- strong magnetic field
What are the advantages & limitations of body circumference?
Advantages:
simple
Disadvantages:
Not direct measure of fat
Difficult in obese
What are the ~BF% recommendations for health ? (NO AGREEMENT
Normal: W: 18-32; Men 15-22 Overweight W >32; M>22 Obese W: > 35; M >25
What does the anthropometric method used depend on?
Time, resources, cost & participants
What are 6 measurements of body size?
- Head circumference
- Length
- Height
- Weight
- Mid-Upper arm Circumference (MUAC)
- Elbow breadth
What are growth indicies used for?
Interpret measurements by comparing to a reference population
How do we measure head circumference?
Measure Occipital frontal circumference (priminent part of back of head & above eyebrows) using flexible, non-stretch tape
What does head circumference for age indicate?
If chronic protein energy deficiency is present (small head circumference = reduced brain development)
What is weight for age used for?
Assess under & over nutrition in children aged between 6 months & 7 years
What is the main limitation?
- Under-nutrition may b underestimated if stunted
2. Weight may be appopriate for height
What is recumbent length?
measures infants & children less than 85cm using a calibrated length board lying down
What does a low weight for height and high weight for height indicate, respectively?
Low = wasting High = overweight
What is weight for height used for?
Identify children who would benefit from an intervention
Issues with weight for height?
- May underestimate malnutrition as doesnt identify children experiencing stunting (need to us both weight for height & Height for age)
- Oedema implicates how it is interpreted (use height for age)
What is height for age an index of?
Past nutritional status
Low = potential stunting
What is the main limitation of height-for age?
Can’t identify wasted children
What are the causes & ramifications of stunting?
Causes:
Periods of inadequate food intake & poor diet quality & increased morbidity
Ramifications:
Reduced size, work capacity & poor reproductive outcomes
Wasting indicates ____ malnutrition; whilst stunting indicates ____ malnutrition
Acute
Chronic
How should growth indicies be selected?
- Weight-for-height & Weight-for-age should be used in populations with high stunting & low wasting
- equipment availability
- availability of trained examiners
Why is BMI used in large studies?
- Easy
- Quick
- Non-invasive
- Precise
- correlates well with obesity & mortality risk
What are the limitations of BMI?
+ Doesn’t distinguish between muscle, fat & oedema
+ no indication of body fat distribution
BMI has a strong ____ dependance in childhood
Age
What is sensitivity?
How good a measure/ test is at correctly identifying people who have the disease
What is specificity?
How good a measure/ test is at correctly identifying people who are well
Why is childhood obesity a reason for concern?
- May track into adulthood
2. Risk factor for hyperlipidemia, hyperinsulinemia, hypertension & early atherosclerosis & diabetes
What are the uses of reference data?
+ facilitate international comparisons of anthropometric indices
+ identify people with abnormal indicies
+ evaluate trends
+ evaluate the effectiveness of intervention programmes
+ Used in Clinical setting to monitor growth, identify people with under & over nutrition & assess the response to treatment
What is the difference between reference & standard data?
- Reference is descriptive of what the population’s status is
- Standard is prescriptive -> what it should be
What are some sources of reference data?
- USA CDC 2000
- European
- UK90
- NZ
- WHO
Why did WHO develop International Reference Data (growth standards)?
+Growth in infancy was not well characterised
+ Exclusively BF babies grow differently (may cause concern & stop mothers BF) so wanted portray BF as the norm
+ develop ideal growth rates on healthy populations (don’t smoke; single birth; no morbidity or health constraints)
When are percentile’s used & when are Z-scored used?
Percentiles are used in industrialised countries, and z-scores are mainly used in disadvantaged countries
When are percentiles used?
For evaluation of well-nourished populations from industrialised countries
What percentile’s are used to classify people at risk?
Below the 3rd/5th
Above the 97th/95th
What is a z-score?
tells us how many standard deviations a measurement is away from the many
What does standard deviation tell us?
Tells us about the spread of our data
around the mean.
How do you calculate z-scores?
Z-score = (subject’s measurement - reference mean) / reference SD
