17, US. rane

Onlythe man whois familiar with the art andscience ofthe past is
competent to aid in its progress in the future.

- Theodore Bulroth


Chassic Editor:
Harold Sandstead, MD

Professor, Department of Preventive Medicine and Community Health, University of Texas Medical Branch,

Galveston, Texas, USA


Body Composition from Fluid Spaces and Density:
Analysis of Methods
William E. Siri
Reprinted with permission from
Techniques of Measuring Body Composition. J. Brozek. ed. National Academy of Sciences National Research Council, Washington, DC, 1961:223-244

... Prospective Overview

Henry C. Lukaski, PhD
From the U.S. Department of Agriculture, Agricultural Research Service, Grand Forks Human Nutrition Research

Center. Grand Forks, North Dakota. USA

Because methods for the assessment of human body

composivon are indirect and based on assumptions

regarding the chemical or physical characteristics of
various components of the body, an important issue ts
the variability to expect for estimates derived with anyof
various methods available. Siri convincingly addressed
this fundamental issue more than 37 vears ago when he
evaluated the densitometric and isotope dilution methods, initially in a laboratory report! and later in a
contribution in a conference proceedings.” The contributon serves as one of the landmarks in the field of bodycomposition methodology.
In the early 1950s, two techniques, hydrodensitometry
or underwater weighing and hydrogen isotope diluuon.
were used routinely to assess body fatness in adults.*

Each of these methods relies on assumptions regarding

en aee


a unique chemical or physical property of the fat-free
body ¢e.g.. constant hydration and protein-to-mineral
raticy) and fat (e.g., densityoffat is less than that of bone.
muscle. and protein) determined from chemical analy-

ses. Although other investigators, e.g.. Behnke et al.tand

Keys and Brozek. acknowledged the limitations of these
assumptions, Siri? Challengedthe validity of these basic
premises and formulated estimates of error in the prediction of body fatness based on the variability of the
chemical composition of the fat-free body and adipose


Siri was troubled bythe reliance on a “reference bady”

for which the relative chemical composition ofthe fat-

free body was assumed to be constantor at least constant
within narrowlimits. Furthermore, it was assumed that.


when body weight changed, fat was ether added to or
removed from the reference body without disruption of

the basic assumption of constant composition of the fat-

free body. Direct chemical analyses of animals indicated
that these assumptions were valid.’ However, studies of
the body composition of humans during weight change
associated with dietary changes suggested that adipose
tissue, or the Ussue that is gained or lost. is not onlyfat
per se but also consists of water and cellular materials.*”
These obser ations led Sir to assess the components of
efror associited with the densitometric method.
The error associated with any indirect method of hodyCOMposition assessment has two components. Measurement error and biological uncertainty Sint concluded
that measurement error is minor relative to the uncertainty associated with the interindividual sariabilicy in the

assurmptions of the chemical constancy of the fat-free

body. He used a propagation-of-error model and determined that the error in estimating percentuge bodyfat
determined with densitometry was ~4°o because of
contributions from variability (standard des tation: in the
water content (2.7) and protein-to-mineral ratio (2.190)
of the fat-tree body and adipose ussue composition

1.) inthe general population, Siri indicated that this

degree of uncertainty could be decreased if total-body
water was used along with densitometry lo esumate body

Siri emphasized the importance of treating measurements of componentsof the fat-free bodyas independent
variables in the assessment of body fatness. This

(Continued on Page 492)

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