1. Body Composition Assessment
Reference Methods

2. Reference Methods
Laboratory methods provide reference or criterion measures for the derivation and evaluation of body composition field methods and prediction equations.

3. Reference Methods Generally more expensive, more inconvenient,
and more time-consuming than field methods,
but have a greater accuracy.

4. Reference Methods
Nevertheless, all laboratory methods make certain assumptions and are still subject to some measurement error.
Thus, a true “gold standard”, or perfect reference method for in vivo body composition assessment does not exist.

5. Densitometry
Refers to the general procedure of estimating body composition from body density.

6. Density
Db = m/V

7. Densitometry
The primary requirement for accurately estimating body density is to obtain an accurate measure of body volume.

8. Densitometry
Body volume can be measured using either hydrodensitometry (underwater weighing) or air displacement plethysmography (Bod Pod)

9. Hydrodensitometry
Also know as hydrostatic weighing, or underwater weighing, provides an estimate of total body volume from the water displaced by the body when it is fully submerged.

10. Hydrodensitometry
When combined with a measure of residual volume, this method provides a good measure of body volume from which body density can be easily calculated.

11. Hydrodensitometry
Long considered a “gold standard”, hydrodensiometry often has been used as the criterion method in validation studies of new body composition assessment methods

12. Hydrodensitometry
There are limitations to hydrodensitometry, especially when applied across a wide range without adjustments for the changes that occur with growth, maturation, and aging.

13. Hydrodensitometry
Although density can be estimated with acceptable precision and accuracy in most groups, the assumption of an invariant fat-free composition, commonly used to convert density to composition, may not be valid for many individuals.

14. Hydrodensitometry
The magnitude of the deviation from the assumed fat-free composition, more than measurement errors in body density, ultimately determines the accuracy of densitometric estimates of body composition for any individual or group.

15. Body Composition Models
The density of any material is a function of the proportions and densities of its components.

16. Body Composition Models
In the classic two-component model of body composition, body weight is divided into fat (F) and fat free fractions (FFM).

17. Body Composition Models
The FFM is a heterogeneous compartment that can be further divided into its primary constituents of water (W), protein (P), and mineral (M) making up a 4 component model.

18. Body Composition Models
The two most commonly used density equations are the:
Siri
Brozek

19. Body Composition Models
Except for the very lean and obese, for whom the Brozek equation is better suited, the two equations give similar results.

20. Assumptions and Validity
1. The separate densities of the body components are additive.
2. The densities of the constituents of the body are relatively constant from person to person.

21. Assumptions and Validity
3. The proportions of the constituents other than fat (or adipose tissue in the case of the Brozek equation) are relatively constant from person to person.
4. The person being measured differs from a standard reference body only in the amount of body fat or adipose tissue.

22. Assumptions and Validity
The assumption of an invariant nonfat compartment is tenuous.
Studies based on chemical and anatomical models have demonstrated considerable variation in FFM composition and density due to growth and maturation, specialized training, aging, and race.

23. Assumptions and Validity
Even within a population, there is considerable inter-individual variation that challenges the assumption of FFM “chemical constancy”.

24. Assumptions and Validity
Although the multi-component (3C or 4C) approach is preferred, it is commonly not possible to measure water and mineral due to a lack of equipment, time constraints, or expense.

25. Underwater Weighing
Based on Archimede’s principle that a body immersed in a fluid is acted on by a buoyancy force, which is evidenced by a “loss” of weight equal to the weight of the displaced fluid.

26. Underwater Weighing
Thus, when a subject is submerged in water, body volume is equal to the loss of weight in water, corrected for the density of water (Dw) corresponding to the temperature of the water at the time of submersion:
V = (Wa-Ww)/Dw
where Wa and Ww are the subject’s weight in air and water, respectively.

27. Underwater Weighing Therefore, body density can be calculated by:
Db = ____Wa________
(Wa-Ww) - (RV +100) Dw

28. Underwater Weighing
Residual volume is commonly measured using either the closed-circuit approach, where there is a dilution and eventual equilibration of an inert gas or helium, or the open circuit approach where nitrogen is “washed-out” of the lungs during a specified period of oxygen breathing.

29. Underwater Weighing
Both approaches yield precise estimates of residual volume and with appropriate equipment and procedural modifications can be used to estimate residual volume with the subject either inside the tank (simultaneously with underwater weighing) or outside the tank.

30. Subject Preparation
Ideally UWW should be measured with the subject having fasted at least four hours and having refrained from strenuous exercise and other situations than can cause unusual dehydration or over hydration.

31. Subject Preparation
The subject should avoid gas producing foods for at least 12 hours and there should be no smoking for at least 3 hours prior to weighing.

32. Subject Preparation
Prior to weighing, subjects are instructed to void the bladder and defecate.
Subjects should be nude or wearing lightweight, tight-fitting nylon swimsuits or their equivalent to minimize trapped air.

33. Subject Preparation
Bathing caps should not be worn since they trap air bubbles.
If possible, subjects should shower prior to entering the tank to remove organic wastes such as perspiration and body oils that tend to cloud the water.

34. Underwater Weighing
There are several methodological issues to consider when estimating body volume from underwater weight.

35. Underwater Weighing These include subject position, residual volume,
number of trials and selection criteria
alternative lung volumes,
and head placement.

36. Underwater Weighing
The measurement of RV at the time of underwater weighing is
time-efficient,
easier on subjects for whom multiple trials are burdensome,
and contributes to more valid estimates of body density.

37. Underwater Weighing RV can be estimated by multiplying VC by:
24% in males
28% in females

38. Underwater Weighing To increase the accuracy of the results use:
1. The highest weight obtained if it appears more than twice.
2. The second highest weight if it is observed more than once and the first criterion is not satisfied.
3. The third highest weight if neither 1 or 2 are met.

39. Underwater Weighing
Doing four or five trials and using the average of three trials that agree within 100 g is an acceptable alternative in subjects for whom 10 trials is burdensome.

40. Underwater Weighing Expect an error of + 2%.
All variables must be measured as accurately as possible to minimize the combined total error.

41. Other Methods
There are several other methods for estimating body volume, including
water displacement with a whole-body volumeter,
gas dilution,
whole-body plethysmography,
and the buoyancy method.

42. Other Methods
With the exception of the Bod Pod ( a form of whole-body plethysmography), these methods are not used commonly for body composition assessment, due either to expense or to the difficulty of obtaining precise and accurate estimates of volume.

43. Water Displacement
Similar to underwater weighing except that the actual volume of water displaced by the subject is measured rather than the loss of weight in water.

44. Gas Dilution
Body volume can be estimated from gas dilution using an inert gas such as helium as a tracer.

45. Gas Dilution
To do so, a known volume of helium is allowed to mix freely with the air in a small closed chamber of constant volume in which the subject is enclosed.

46. Gas Dilution
This technique has the advantage of not requiring a measurement of residual lung volume since the lungs comprise a part of the difference between the chamber and subject volumes.

47. Gas Dilution
Also helium dilution is applicable to individuals from infancy through old age, whether healthy or ambulatory, and it requires relatively little subject cooperation and effort.

48. Gas Dilution However, the technique is more complex than others,
has a relatively high initial cost,
and requires continuous calibration checks and very precise measurements of helium concentrations to discriminate between subjects of varying volumes.

49. Plethysmography
Body volume can be estimated using a plethysmograph, which eliminates the need for total immersion of the subject.

50. Plethysmography
This method used a closed vessel (e.g., Bod Pod) in which the subject stands in water or sits in air.

51. Plethysmography

52. Plethysmography
The volume of the subject is determined by measuring pressure changes produced by a pump.
The Bod Pod uses a pressure-volume relationship between two chambers to estimate body volume.

53. Plethysmography

54. Plethysmography
Plethysmography is based on a combination of Boyle’s Law and Poisson’s Law.
They both relate to the inverse relationship between volume and pressure.

55. Plethysmography
Boyle’s Law assumes isothermal conditions (air temperature remains constant as its volume changes).
Poisson’s Law allows for changes in air temperature as are found when the human body gives off heat in an enclosed environment.

56. Plethysmography
It’s important to remember this difference because clothing, hair, thoracic gas volume, and body surface area must be controlled since they act isothermically.

57. Plethysmography
To control for these factors, clients are tested wearing minimal clothing (spandex swimsuit) and a swim cap to compress the hair.

58. Plethysmography
An estimate of body surface area, calculated from the height and weight of the client, is used to correct for the isothermal effects of the body’s surface.

59. Plethysmography
Thoracic gas volume, or the volume of the air in the lungs and thorax, is either directly measured or estimated by the Bod Pod to account for the isothermal conditions in the lungs.

60. Plethysmography
Compared to UWW, the Bod Pod has excellent validity, with 2C comparisons showing the Bod Pod to have a difference of only -0.3% body fat.

61. Bottle Buoyancy
Involves having a subject hold a bottle filled with water and air against their chest and submerge in a pool.
Water is added until the subject is completely submerged and suspended just below the water surface.
Once this occurs, the water in the bottle is measured and volume calculated.

62. Recommended Procedures
Based on considerations of expense and the precision and accuracy of measurement, the underwater weighing technique continues to be the most wide-spread and useful method for estimating body volume leading to the assessment of body composition.