Body Composition

What are the five major components of body weight?

1. extracellular water

2. intracellular water

3. minerals

4. protein

5. fat

Of the five components of our weight, what represents fat free mass (FFM)? What represents fat-mass (FM)?

• FFM (4)

  • extracellular water

  • intracellular water

  • minerals

  • proteins

• FM

  • fat

Of the five components of our weight, what gives us energy?

protein and fat

What are the BMI cutoffs?

What does waist circumference/Waist:Hip Ratio (WHR) measure?

measures central obesity (WC) and distribution of intra- abdominal (visceral) to subcutaneous fat (WHR)

What is WC/WHR highly correlated with?

highly correlated to BMI. % body fat, subcutaneous and intra-abdominal fat in women and men

What is a better predictor of diabetes, WC or BMI?

WC

• in general WC are better predictors of adverse effects of excess adipose such as CVD

What are the cutoffs for WC in terms of measuring central obesity for both men and women?

• men = > 102 cm (40 in)

• women = > 88 cm (35 in)

What is the difference between visceral and subcutaneous fat?

• Subcutaneous fat is the pinchable "jiggly" fat located just beneath the skin, acting as energy storage and insulation.

• Visceral fat is hidden fat surrounding and within internal organs (liver, intestines) deep within the abdominal cavity

What percent of our total fat is subcutaneous vs visceral?

• subcutaneous = 90%

• visceral = 10%

What is intramuscular fat?

adipose tissue that has accumulated within and between muscle tissue

What is fat-free mass?

all residual, lipid-free tissues and extracellular materials (water, muscle, bone, connective tissues, and internal organs

What are the cutoffs for waist-to-hip ratio for women and men?

• men = > 1.0

• women = > 0.8

What are three conditions that affect body composition?

1. sarcopenia

2. osteoporosis

3. cachexia

What is sarcopenia?

progressive loss of muscle mass (primarily losing protein)

• typically associated with aging as it affects 10 - 50% of individuals > 65 years old

• increases risk of falls by 5x (and falls result up to 9% of mortality rate amongst adults > 65 years)

What is sarcopenic obesity?

when sarcopenia (loss of muscle mass) co-exists with obesity

• fat can infiltrate the muscle tissue

What is osteoporosis?

when bones become weak and brittle, this is typically characterized by loss of bone mineral density and low bone mass

• increased risk of hip, spine, and wrist fracture

How many women vs men > 50 years re impacted by osteoporosis?

• women: 19.6%

• men: 4.4

tends to be higher amongst women just in general

What is cachexia?

weight loss (especially in FFM but also some FM) due to diseases, leading to severe wasting

What diseases can lead to cachexia?

1. cancers

2. advanced HIV/AIDS

3. tuberculosis

4. heart failure

5. chronic kidney disease

6. chronic obstructive pulmonary disease

What happens when we have too much extracellular water?

edema

What happens when we have too low mineral levels?

could result in osteoporosis, osteopenia, and rickets

What is the two compartment model?

assumes that body is compromised of fat mass + FFM

• method is to measure one aspect of FFM and calculate the others based on assumptions of ratios and then determine the fat mass by taking the difference

What assumptions are made under the two compartment model?

• subject has constant hydration

• constant ratio of bone to muscle mass

What is the caveat of the two compartment model?

assumptions are likely to be incorrect for those who are ill, malnourished, or have metabolic disturbances

What is fat mass?

sum of all adipose tissue depots

What are five methods for estimating body composition?

1. anthropometric (skinfold)

2. bioelectrical impedance (BIA)

3. stable isotope dilution

4. radiologic methods

5. densitometry

What does skinfold thickness typically estimate?

subcutaneous fat

• helps us to calculate density which then helps us to calculate % body fat

• most widely used method to estimate subcutaneous adipose tissue as it is a good estimate compared to DXA

What are six assumptions made under the skinfold thickness measurements?

1. double thickness of skin and subcutaneous adipose tissue has a constant compressibility

2. thickness of skin is negligible/constant

3. thickness of subcutaneous adipose tissue is constant/predictable → in reality the thickness of subcutaneous adipose tissue varies among different sites within the same individual

4. fat content of adipose tissue is constant

5. proportion of internal to external fat is constant

6. body fat is normally distributed

What are the four different skin fold thickness measurements?

1. bicep

2. tricep

3. subscapular (back)

4. suprailiac (hip area)

What is the most common single-site skin fold measurement?

tricep

What is a single-site skin fold measurement good for?

can be used to estimate risk of wasting in combination with MUAC

• cannot be used to estimate total percent body fat

What are the two equations that are relevant when we just have one skin fold site measurement?

1. arm fat area (AFA): correlates with total body fat

1. arm muscle area (AMA): index of muscle mass

What measurement is accurate in assessing muscle wasting in the elderly?

mid-upper arm muscle area (MAMA)

A minimum of how many skin fold measurements are recommended when using skin fold thickness to assess body fat?

minimum of 3

When calculating the density and % body fat from skin fold measurements, what are some assumptions we make about the FFM?

• constant density of 1.10g/cm³ (more than water)

• constant hydration

• constant proportion of bone mineral to muscle

When calculating the density and % body fat from skin fold measurements, what are some assumptions we make about the FM?

• constant density of 0.9g/cm³ (less than water)

What is the Durnin-Womersley equation used for?

used to calculate body density given skin fold measurements

What are the percent body fat cutoffs for men and women?

What are the advantages associated with the skin fold measures for body composition assessment? (4)

1. inexpensive

2. non-invasive

3. relatively quick and simple to take the measurements

4. can provide a more accurate estiamte of body fat compared to BMI

What are the limitations associated with the skin fold measures for body composition assessment? (2)

1. predictive equations may not be appropriate for all population groups

2. requires practice to take measurements accurately and precisely

What is Bioelectrical Impedance Analysis (BIA)?

an electrical current is passed through the body and the impedance to the current is measured

• this impedance helps us to determine FM vs FFM as electrical conductivity differs in both

• fat and cell membranes are relatively non-conducting whereas electrolytes in water (found in FFM) are more conductive

• mainly estimates total body water (TBW) with high precisions which can then help us calculate FM and FFM

What are the two components found in BIA measures?

1. resistance

2. reactance

both allow for estimations of intra and extracellular water

What is resistance?

body fluids resist the electrical current at different rates

What is reactance?

cell membranes act as capacitors, storing electrical energy

What are some requirements/criteria that make a BIA test more accurate?

1. ideally performed after a fast

2. 30 minutes after voiding

3. normal hydration

4. traditionally done with subjects in a supine position, some instruments allow standing position

What are the three different segmental BIA measurements?

1. hand to foot

1. hand to hand

1. foot to foot

What percent of our TBW is FFM?

73.2%

How do we determine body composition given BIA?

1. measure TBW from BIA

2. estimate FFM from TBW (FFM = TBW/0.732)

3. calculate the FM from FFM and body weight (FM = body weight - FFM)

4. calculate % body fat

What are the advantages of BIA?

1. simple and fast to perform

2. safe

3. relatively inexpensive

4. reliable in healthy populations

5. can be used in large studies

6. can be used easily in outpatient clinical settings

What are the disadvantages of BIA?

1. affected by the subject’s position, state of hydration, exercise, meals, alcohol and caffeine consumption

2. affected by electrode placement

3. predictive equations may not be applicable to certain populations

What are two different methods that fall under densitometry that is applicable to measuring body composition?

1. hydrostatic weighing (underwater weighing; this is typically the gold standard)

2. air displacement plethysomography (bod pod)

What is the methodology behind densitometry that helps it estimate one’s body composition?

attempt to assess body composition by measuring the density (mass/volume) of the entire body

What are some characteristics associated with hydrostatic weighing?

• determines whole body volume based on Archimedes’ Principle (helps us to obtain volume of a person)

• if the mass and volume of an object are known, density can be calculated which can then be used to calculate body fat percentage via the siri equation

What is the Archimedes’ Principle?

states that the volume of an object submerged in water equals the volume of the water the object displaces

What are the assumptions made about FFM and FM when talking about hydrostatic weighing?

The same assumptions are made when considering skin folds

What are the different variables involved in the body density equation used for hydrostatic weighing?

What are the different steps involved in hydrostatic weighing?

1. record body weight in air

2. subject expels all air from their lungs

3. submerge chair in the water (subject holds their breath for 10-15 seconds)

4. record body weight in water

5. measure residual lung volume (RV) or calculate using predictive equations

6. apply correction factor to the volume of air in the GI tract (assumed to be 100 mL)

What are the advantages of underwater weighing?

has historically been the method upon which other indirect measures of body composition were compared

What are the disadvantages of underwater weighing?

• subject’s ability to cooperate

• residual lung volume determination and GI gas assumption

• not practical for large studies

• underestimation of body fat in non-whites and athletes

• overestimation of body fat in elderly

• requires special facilities

What are some characteristics of air displacement plethysmography (bod pod)?

• used to measure volume in a similar sense compared to underwater weighing but instead of using water we use air

• when a subject enters a chamber of a known volume, the subject’s body volume = the reduction in the chamber volume

What are the different steps involved air displacement plethysomography?

1. when the bod pod is empty measure the interior volume of the chamber

2. when the subject is seated inside measure the volume of the test chamber again

3. measure or apply a correction factor for residual lung volume

What is a pea pod?

a bod pod but for infants (1-8 kg)

• same principles as the bod pod

• accommodates most infant behaviors

What are the different assumptions about FFM and FM made in regard to air displacement plethysmography?

same as both skin fold and underwater weighing assumptions

What are the advantages of the bod pod in comparison to hydrostatic weighing?

• more acceptable to most subjects

• greater precision

• residual lung volume is not as much of a factor

What are the disadvantages of the bod pod in comparison to hydrostatic weighing?

• equipment cost ~$32 k

• still assumes a constant density of FFM and FM

• calibration can be challenging

What are two radiologic methods used to determine body composition?

1. dual-energy x-ray absorptiometry (DXA)

2. computed tomography (CT) scans

in general these are more accurate methods for calculating body composition

How does dual-energy x ray absorptiometry work?

have two low dose x ray beams of differing energy pass through the body that can differentiate between bone and soft tissue and thus determines the relative proportion of tissues

• separate estimates of bone mineral, FM and lean, soft-tissue mass

• tends to be the gold standard for assessing body composition

• highly correlated with underwater weighing

• takes less than 5 minutes for a very low dose of radiation

What was dual energy x ray absorption originally used for?

originally developed for bone density determinations (ex: osteoporosis/osteopenia screening)

• later became a widely used method of body composition assessment

Besides body composition what can DXA also be used to identify?

used to identify sarcopenia based on the ASMM index

What is Appendicular skeletal muscle mass (ASMM)?

sum of the muscle mass of the arms and legs

How do you calculate the ASMM index? What do the cutoffs represent?

ASMM index = ASMM/height²

• determine presence of sarcopenia via DXA

• cutoff values defined in comparison to young adult populations

What are the advantages of DXA?

• compares well with other techniques (ex: underwater weighing)

• reproducible measurements

• pediatric and adult references are available

• is the gold standard measure of bone mineral content and bone density

• quick

• can measure body composition

What are the limitations of DXA?

• variability of fat mass at different depths (there can be inaccuracies among obese patients are those who are losing or gaining weight)

• calibration can be problematic

• movement artifacts can cause uninterpretable measure

• expensive

• small radiation exposure

How does a CT scan work to measure body composition?

measures the transmission of an x-ray beam through the body tissues and typically produces a detailed cross sectional image of the body in which where we can measure regional muscle mass and adipose tissue

• can distinguish between visceral and subcutaneous fat

• gold standard to measuring visceral fat (aka intra-abdominal fat)

• primarily used for research

What can a arm or leg CT scan tell us?

can identify muscle wasting or intramuscular adipose tissue accumulation

What are the advantages of CT scans?

• can measure regional muscle mass and adipose tissue

• can differentiate between types of lean body mass (muscle vs organs)

• can assess distribution of subcutaneous and visceral fat

What are the limitations of CT scans?

• high cost

• limited availability

• radiation makes multiple scans of same individual not desirable but multiple scans are needed to estimate total body fat (hence why it is rarely done)

What are the two isotope dilution methods that help us determine body composition?

1. deuterium dilution method

2. doubly-labeled water

What isotope is used in deuterium dilution? Which one is used in doubly-labeled water?

• deuterium: ²H₂O

• doubly-labeled water: ²H₂¹⁸O

Describe the mechanism behind the isotope dilution method and how it helps to determine body composition?

assess total body water (TBW) in order to determine FM

• after you take a baseline saliva sample you give a person a known amount/dose of the isotope (typically a water containing ²H) and you give it time to mix evenly throughout all the water in the body (~ 3 - 5 hours) equilibrating it

• afterwards you measure how diluted the isotope is in a sample of saliva

• the more water the body contains, the more diluted the isotope will be

• once you have the total body water you can calculate the FFM (as TBW is typically 73.2% of our FFM)

What is the one caveat with the isotope dilution method?

have to take into account that the isotope not only mixes with body water but also with other hydrogen molecules in fats and proteins

• saliva sample taken typically makes it look like the total body water volume is about 4.1% larger than it actually is

How do we take into account that the isotope dilution method doesn’t only mix in with one’s total body water?

to get the true TBW you have to divide the measured dilution space by 1.041 (to remove the 4.1% overestimation)

What is the equation used in isotope dilution?

C₁V₁=C₂V₂

• C₁V₁ = dose of initial isotope given

• measure the ending C₂ level

• solve for volume of water in the body as TBW = V₂

Once we determine TBW from isotope dilution, how do we use it to calculate FM?

since we know that TBW makes up 73.2% of our FFM, we can use that to calculate our total FFM and use that FFM to calculate FM

What are some assumptions made from isotope dilution?

• isotope is exchanged from the body as water

• isotope does not alter normal physiologic process of homeostatic regulation

• fat is free of water

• fat free mass is assumed to be 73.2% water in healthy non-pregnant adults

What are three major sources of error while measuring total body water using isotope dilution?

1. failure to administer accurately measured isotope does

2. consumption of beverages during the equilibration period

3. sample contamination by atmospheric water before analysis

What are the advantages of isotope dilution?

• accurate precise measure of TBW

• isotopes are stable therefore no radiation exposure

• safe for children and women

• more affordable than mass spectrometry

What are the limitations of isotope dilution?

• administering a precise amount of the tracer dose can be challenging

• high cost and high level of expertise needed

• collection of saliva from young children can be difficult

• saliva samples must be stored properly until analyzed to prevent contamination from atmospheric water