Body Composition Flashcards
What are some of the reasons that we measure body composition?
1) Eliminates the limitation of BMI: BMI cannot distinguish between fat mass and fat free mass. So a very fit individual with lots of muscle mass is heavier, so classified as overweight or even obese. Which is then associated with co-morbidities and metabolic complications. But it does not apply in this case. Body composition can solve this.
2) A method needed by professional athletes: for particular sports, they need to maintain a particular physique e.g lean for marathon runners, this measure helps them to ensure it is maintained. Also, they will need to maintain the minimum amount of muscles just enough for their sport, so no extra energy expenditure occurs.
3) Health indicator is adiposity.
Highlight the differences between fat mass and fat free mass?
Fat mass: lower density, contains no water i.e. Anhydrous, no potassium
Fat free mass: higher water content (72-73%), density higher, contains potassium.
Describe the basic principle behind underwater weighing?
It is based on Archimede’s principle: the buoyancy force of water acting on a body submerged in it is equal to the weight of the volume of displaced water. Since density=mass/volume, we can measure one’s weight in air, then submerge the body completely into water, it will reduce in mass due to water’s buoyancy force upwards. This reduction in weight divided by density of water gives the volume of displaced water, which is equal to volume of the body. Using body weight in air/ its volume just calculated, minus gut gas and residual volume, we can get density of the body. Then use Siri equation or Brooke to convert it to fat percentage.
Briefly describe how underwater weighing is performed?
1） Prior to experiment, subject will wear tight swim wear, preferably the speedos type. Water is heated to 25-28 degree Celsius, bromine added.
2) Subject weighted before entering water.
3) Enters water and submerged sitting on a chair. Subject is asked to exhale maximally with a nose lip on. Then hold it while their whole body is submerged with weight in water measured.
How is the body density measured using the bod pod?
The equation manipulations same as that of the underwater weighing. But the volume is determined using change in air pressure. Once the body sits within the chamber, the room available to air molecules has reduced. Based on Boyle’s Law, this means pressure increases when volume of the gas decreases (have less room to move around), and vice versa. This change in pressure is detected by comparing it with a reference chamber at the back.
List strengths and limitations associated with skinfold anthropometry?
Strengths: inexpensive, field-based, harmless, in a way easy to do (but difficult to be accurate), convenient (does not require much sophisticated and heavy machinery as opposed to others such as UWW).
Weaknesses: many possible sources of errors (3 stages, adjustments of calculations based on different populations, differences across calipers leads to varying results, used to have different protocols, assumptions making it less accurate, potential for embarrassment, many factors need to be considered regarding to how the clients feel.
Specify the different sources of errors associated with skinfolds?
1) More than 100 progression equations available. Which one to choose to match our populations? It is difficult.
2) Since progression equations developed from the measurements obtained from underwater weighing method, they are affected by the errors in underwater weighing. (These include estimation of residual volume and gut gas, accuracy of the scales and temperature of the water that affects its corresponding density)
3) Results of UWW needs to be converted to % fat using either Brozek or Siri equations. These equations also have certain errors.
4) Different calipers give different values.
5) Used to have different calipers at obtaining skin folds.
List and explain assumptions made in converting anthropometric measurements into body density and then body fat.
1) Compressibility & thickness of skin remains constant across individuals: different peoples’ fat can be different in terms of how easily they compress i.e some skin are very hard to compress, resulting in higher skinfold readings. Also, thicker the skin, greater the skinfold will be, potentially over-estimating amount of fat.
2) Few thicknesses are measured: not enough to cover all areas where fat are distributed. Some people can have little adiposity in a particular part, while large adiposity in some other areas. So may not be accurate representation of the whole body.
3) Ratio of subcutaneous fat: visceral fat is fixed: some people have very unequal amounts of these 2 types of fat. So skinfolds only indicates subcutaneous fat, which may not be enough to indicate overall adiposity.
Why is it important to use bony landmarks to identify anthropometric sites?
Ensure exact location of measurement site. I.e. Does not change the accurate marking.
What is the 3 compartment model? What about four?
3 compartment model: fat mass, fat free dry mass and total body water.
4 compartment model: fat mass, total body water, bone mass and residue.
Describe dual energy x-ray absorptiometry? Include pros &cons?
Low dose of x-ray emitted and penetrates through the subject’s body. Denser tissue will allow less x-ray to pass through hence detected.
Pros: simultaneous detection of bone, fat and non-bone tissues, low radiation dosage, quick 15-30 mins.
Cons: expensive, table may be too small for large subjects, still radiation, accessibility issue, errors with the equations.
Describe MRI and its pros & cons?
Pros: highest definition (shows many different tissues, can be used to calculate accurate cross-sectional areas of the tissues), relatively rapid i.e. 30 minutes, no radiation dosage.
Cons: some exclusions of subjects with implants like pacemakers, expensive, accessibility issue.
Bio-electrical impedance method & pros and cons?
Either connected to electrodes or standing on a special scale (domestic use), small amount of electrical current passed through the body. FFM contains 70-75% of water, hence conducts electricity better. This can give us FFM%, hence FM%.
Pros: wide range of equations available.
Cons: equations need to be population-specific, results can be inaccurate due to varied hydration and exercise levels, electrode placements is important, so possible source of error.
