Body Composition

Question 1

Body Composition: The relative amounts of fat and fat-free mass?

Answer 1

– Fat-free mass: all the body’s non-fat tissues: 
• Bone 
• Water 
• Muscle 
• Connective  tissue 
• Organ tissues 
• Teeth 

– Body fat:
• Essential fat
• Nonessential (storage fat)

Question 2

Essential fat?

Answer 2

• Lipids incorporated in the nerves, brain, heart,
lungs, liver and mammary glands

• Crucial for the body functioning
• 3-5% body weight for men
• 8 –12% body weight for women

Question 3

Nonessential Fat?

Answer 3

• Extra fat or fat reserves
stored in the body

• Exists primarily in fat cells
or adipose tissue

• Located below the skin
and around major organs

• The amount of nonessential fat depends on many factors: gender, age, heredity, metabolism, diet and activity level.

Question 4

Assessing body mass index, body composition and body fat distribution

Answer 4

• Body mass index (BMI):
– Measure of body weight that is useful for classifying the health risks of body weight

– Based on the concept that weight should be proportional
to height

– BMI doesn’t distinguish between fat and muscle.

– Calculated dividing your body weight (Kg) by the square of your height (m).

Question 5

Assessing body mass index, body composition and body fat distribution…

Answer 5

• NICE guidelines
–Consider body fat distribution and other disease risk factors in addition of BMI

–Uses the measure of the waist circumference

Question 6

Two-component Model: Fat and Fat Free

Answer 6

Siri and Brozek et al. –Different equations with slightly different estimates, both use body density to estimate % BF.

Question 7

Two-component Model: Assumptions?

Answer 7

• The densities of the fat and fat free components are additive and are the same for all individuals.
• The proportions of water, mineral and protein in the fat free body or reference body are constant within and between individuals.
• The individual being measured differs from the reference body only in the amount of body fat (triglyceride) or obesity (adipose) tissue. Conversion equations can be used for specific populations

Question 8

Multi-component Models: Multi-component (3C and 4C) molecular model?

Answer 8

• Multi-component (3C and 4C) molecular model
equations take into account interindividual variability in the hydration or mineral content (or both) of the fat free body. Generally more
accurate. Adjust Db for TBW or TBM variability.

Question 9

Multi-component Models: Multi-component tissue-level model?

Answer 9

Multi-component tissue-level model. Used with DXA divides into bone, bone-free lean tissue and fat mass.

Question 10

Methods for Measuring Body Fat?

Answer 10

• Hydrodensitometry

• Air Displacement
Plethsmography

• Bioelectrical Impedance
Analysis

• Skinfolds

• Dual-Energy X-Ray 
Absorptiometry
\:
Siri Equation
% Body Fat = 
(495/Body Density) -
450

Question 11

Percentage Body Fat: Hydrodensitometry?

Answer 11

–Hydrostatic weighing or Underwater weighing

• Archimedes’ principle –the volume of an object
submerged in water equals the volume of water displaced by the object.

• The individual is submerged and weighted under water

• The percentages of fat and fat-free weight are estimated
from body density

• Considers that muscle has a higher density and fat a lower density than water.

Db = Ma/{[(Ma –Mw)/Dw] –(RV + GI volume)}Where Ma = Mass in air, Mw = Mass in water, Dw = Density of water, RV = residual lung volume, GI volume = 100ml.

Question 12

Percentage Body Fat: Air Displacement Plethysmography/The Bod Pod?

Answer 12

– Small chamber containing
computerized sensors

– Measures body composition by using air displacement and pressure-volume relationships to derive body volume and density.

– %Body fat is then estimated using models discussed earlier.

Question 13

Percentage Body Fat:

Bioelectrical Impedance Analysis (BIA)

Answer 13

• Works by sending a low level electrical current through the body and measuring the body’s resistance/impedance to it.
• Fat-free tissues, where most water (~73%) is located, are good conductors of electrical current.
• Fat is a bad conductor.

• The amount of resistance to electrical current is related to
the amount of fat-free tissue in the body.

• Uses prediction equations for TBW, FFM, and %BF.
• Different models including segmental and multifrequency BIA and upper and lower body analysers.

Question 14

BIA technique (SEE 2.7 –6.3%)?

Answer 14

Pre-test guidelines:

• No eating or drinking within 4 hours of the test
• No exercise within 12 hours of the test
• Urinate prior to the test

• No alcohol consumption within 48 hours of the
test

• No diuretic medication within 7 days of the test
• Note stage of menstrual cycle in females.

Question 15

Percentage Body Fat: DEXA or DXA - Dual-Energy X-Ray Absorptiometry?

Answer 15

–Measures the tissue absorption of high- and low-
energy X-ray beams.

–Can give whole body or regional body composition.

–Also often used to measure bone density at various
sites.

–Slight differences in how different manufacturers
software and models work.

Question 16

Other Methods?

Answer 16

• Near-Infrared Interactance (NIR)
Estimates %BF from the optical density measured at the biceps.

• Neutron Activation Analysis (NAA)
6C model. Direct chemical analysis at atomic level.

• Computerised Tomography (CT)
Uses X-ray beams. Usually not used for whole body.

• Magnetic Resonance Imaging (MRI)
Uses hydrogen nuclei and radio frequency to produce an image.

• Whole body Potassium (TBK)
Often used to measure Body Cell Mass. Useful in disease states.