Body Composition Assessment

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 Assumptions • 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.