Variability of body density in ambulatory subjects

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60 days. (group. II). It was assumed that data obtained from group. I which included volunteers familiar with the densitometric procedure would yield more.
Variability subjects Jose

Mendez,

of body measured Ph.D.

and

Henry

ABSTRACT density was physical

activity

from

group

more

reliable

for

days

and

density

lung

from

group

volume

Body

long Nutr.

Body

of body

were

composition,

because

it is a safe,

noninvasive technique and is accepted as the most reliable. However, in this method there are two sources of error: 1) the intrinsic errors involved in the experimental determination of body density, and 2) the errors related to the assumptions of constancy of the “biological values” used in the calculation of body

composition. described and the 78

Mendez latter.

the

Durnin former

and and

and Koffias The standard The

Taylor Brozek

(1) have et al. (2)

(3) have discussed error of a single

American

the

most

body

II).

the

measurements.

as indicated

density

that

body

levels

reliability,

The

obtained

would

yield

subjects.

Weight

precision

in body

0.73% body fat in groups I and did not influence the reliability

by

and

data

procedure

of inexperienced

to 0.32 and the technique diet

assumed

densitometric

consisted variable

It was

volume and body diet and level of

density

appears

of physical

stability

to remain

activity

of

body

are

con-

composition,

density

occurred. Although there are several methods available for the estimation of body composition in vivo, we have selected body density determinations by underwater weighing as a rou-

method

II which

composition

achieved by controlling diet and physical activity. Unfortunately, body weight alone does not determine changes in the fat and the fatfree components of the body. Therefore, the determination of body composition becomes necessary to establish if any changes have

research

(group with

periods during which 34: 78-81, 1981.

In metabolic studies of healthy adults, it is necessary to make observations on man over long periods of time under conditions of stable body weight and body composition. In general, it is assumed that stable conditions of body weight and body composition are

tine

60 days familiar

0.0008 to 0.0018 g.mf’, equivalent data suggest that familiarity with

measurement.

WORDS

I) and

volunteers than

over relatively Am. J. Clin.

variability

of weight in air and underwater, residual lung men who were advised to maintain a constant (group

included

residual from The

Ph.D.

Journal

of Clinical

observation of the body density determination by underwater weighing has been shown to be in the order of 0.002 g m1’. This error has been reported for measurements of density repeated in the same day (4), and on different days (1, 5). The present study was designed to evaluate the variability of the different measurements required to calculate body density by underwater weight, i.e., body weight, underwater weight, and residual lung volume, as well as the variability of body density in ambulatory subjects advised to maintain a constant diet, and a constant level of physical activity for varying lengths of time from 4 to 60 days. -

Materials

and

methods

Two groups of healthy subjects participated in the study. The first group included 17 males, who were graduate students and staff members of the Noll Laboratory, The Pennsylvania State University and whose ages ranged from 22 to 52 yr. Two density determinations ‘From the Human Performance Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802. 2Supported in part by Public Health Service Grants AM083 11 and RR07082. ‘Address reprint requests to: J. Mendez, 101 Noll Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802.

Nutrition

34: JANUARY

1981,

pp.

78-81.

Printed

in U.S.A.

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KEY

four

information

density varied II, respectively. constant trolled.

variability in ambulatory

I which

underwater

of body

C. Lukaski,

The studied

density in ambulatory at different days13

VARIABILITY were would

made. 4 days apart. It was assumed that adhere closely to the required conditions

of their with the

relation procedures.

to

the laboratory and These measurements

this

OF

BODY

group because

the familiarity would be cx-

=

Ma/(((Ma-Mw)/Dw]

-Vr-0.l)

Db and Dw are the densities of the body and water, respectively; and 0.1 is the estimated gas volume (100 ml) in the gastrointestinal tract (7). The comparisons between time periods of Ma and Mw, Vr and the calculated body density were made using analysis of variance with repeated measures. Because body weight and body density were not significantly different between time periods (p > 0.05), an estimation of the error of repeated observations was obtained using the values from each period as replicates.

Results

and

discussion

The Ma and (Db) obtained TABLE Ma and

I Mw,

Mw, Vr, for group

Vr and

Db

and body densities I and II are pre-

of subjects

in group

I, (n Mw

Ma(kg) 14

14

75.0 3.2 18.2 0.53

3.12 0.19 26.0

t)

Mean SE CV SEm

75.2 3.2 18.1

to

and

observation,

=

initial

J42/2N;

and r

4-day =

values

correlation

yr

(kg) 14

14)

4.76 0.968 0.974 0.135 respectively; coefficient;

CV b

=

=

% variation

slope;

and

a

coefficient; =

intercept.

Db(g.ml’)

(L)

1.63 0.11 27.6

3.18 0.19 25.8 0.15

0.71 0.998 1.001 -0.241

% Error r b a *

17)

=

14

SEm

14

1

1.0632 0.0022 0.9

1.61 0.08 21.1 0.16 9.88 0.868 0.650 0.550 =

standard

1.0635 0.002 1 0.8 0.0008 0.08 0.990 0.962 0.041 error

of a single

Downloaded from www.ajcn.org by guest on September 27, 2011

sented in Tables 1 and 2, respectively. No significant differences (P > 0.05) were obtamed when initial values were compared with subsequent determinations. In group I, the difference in mean Ma over 4 days was only 0.2 kg which was similar to the difference observed between the first 1 1 days in group II. However, after 60 days, the difference in average Ma was greater, about 0.8 kg. The SE and the variation coefficient (CV) are presented in Tables 1 and 2 to indicate the degree of dispersion of values. It was shown in both groups that the SE and the CV were very close, with the exception of CV for the Vr which appears more variable. This variability was expected as it is very difficult to replicate the exact conditions for the determination of Vr. No significant differences (p > 0.05) were obtained between initial and other days of observation for the different variables studied, and the pairing of values was made to calculate the standard error of a single observation (SEm). These values indicated how close replicate determination were obtained, and were given in absolute amounts and as percentage error. The greatest deviation between replicate observations was obtained for Mw and Yr. This is reasonable since Mw would vary inversely with Yr. Despite the variability of Mw and Yr, the Db values calculated for 4 different days were similar. These results strongly support the need to determine Yr simultaneously with Mw in order to obtain valid and reproducible determinations of Db. The SE of a single observation in the determination of Db varied from 0.0008 to 0.0018 g.ml’, or 0.08 to 0.17%

pected to give the best estimates of precision for the density procedure. The 2nd group included 14 young male students of The Pennsylvania State University whose ages ranged from 20 to 30 yr. This group was assumed to be less motivated and to represent the general student population. Three density determinations were made: initially and after 1 1 and 60 days on a controlled diet. The diet provided per day an average of 3565 ± 86 kcal, and 99 ± 3 g protein, or 49 ± I kcal.kg and 1.4 ± 0.01 g protein . kg’ as determined from a 7-day dietary record and interviews given by an experienced nutritionist. Body density was determined by using the underwater weighing system and method of Akers and Buskirk (4) which require the measurement of body weight in air (Ma), body weight underwater (Mw), and residual lung volume (Vr). This system is unique in that Mw is measured electronically with strain gage transducers which measure to the nearest 20 g. Further, Vr is determined at the time of the underwater weighing using an opencircuit for nitrogen washout from the lungs as described by Darling et al. (6). Before the hydrostatic weighing, Ma was determined to the nearest 20 g on platform scale with the subjects in shorts. Body density was calculated by using the following equation: Db

79

DENSITY

80

MENDEZ

AND

LUKASKI

in groups

.9

I and

II, respectively.

Differences

a

.1

1-),o r’ -

r--

-

-

,OOO--0 00 O\

I UI-8

m

The correlation (r), regression in Db of 0.0008 0.0018 ml’and make difference of0.32 andcoefficient the calculated percentage of body fat,0.73% using formula Brozek periods, etrespectively. al. (2), and over the ing4-the 60-day ofa

-

.

-

I -;

1’-

-

o -oo

t

coefficient

m ni

.0 0

-

U

#{149}ll

i

,,-

0r-

r

N

0c’

00

c.1 000

0\

O

0

:

o-..

.1

rt

N

(4rs



re

o

r

mtercept words, tained identity ments,



ception

r- r-

r-

O

-

0

-

0

the

intercept

(a)

For a perfect agreement between replicates or days, a regression coefficient of 1 , and an

0.t,.EI-U

of days zero are required; or in the the data should fall measureon other the were line. 0.944 The ofpoints regression toobservation 1.067 forcoefficients all oband with cx-

of

Yr

which

varied

from

0.650

to

9II

0.753. In authors most thereplicability regression cepts coefficients small deviations from werecases, zero. very4,therefore, unity tomade thewith butinter-the determinations Most vary have (1,from reported 5,close 7) working aunity, Db

C

of approximately 0.002 g m1’ which is less than the standard of 0.005 g ml’ estimated by Siri (8) as acceptable in using the densi-

.Qaa

r-

C.

-r--.o..ri

t-

and

-

-

.9

(.1

tometric

>

method

for

determination

of body

a

L)

fatness. The present study agrees with this precision in observations extended to 60 days. There is, however, a report by Durnin and

.. U0.. -

Rahaman The The (9) inof three that the experienced determinations body density for periodgroupg.of ml’. 1 yr. Noassumption details reproducibility ofrepeated the subjects study cited were was a given. 0.0008

la o

‘r

a

--r.--’)-

I

;

.9

r

e; ooo o-:

r.4 rs q

U

5 3

2

I‘

n

I

00

:

ri N

ri

-

0

-

.

©

a

,.

o a

*0

.,

m

o o

-

0

o

-

r’

.. .

ii a

would roven false, provide as more the precision reliable information of measurement was determined in the inexperienced and experienced groups were similar. Thus, this mdicates that reliable information tained in an open population instruction and practice given tual

.

determination

of body

can be obwith minimal before the ac-

density

is made.

Un

a The ao

expert

is gratefully

0”

assistance

of H.

Barlett

and

F. Weyandt

acknowledged.

References 0 a

U

*

U rn

‘-

.0

u

1. Durnin ments

JVGA, of density

Taylor A. Replicabihity of the human body

of measureas determined

a

Downloaded from www.ajcn.org by guest on September 27, 2011

.1

(b),

are given in Tables 1 and 2 to show how close a measure made on one day relates to the measurements on the subsequent days.

:

ri -

or slope

VARIABILITY

2.

3.

4.

5.

6.

OF

by underwater weighing. J App Physiol l960;l5: 1424. Brozek J, Grande F, Anderson JT, Keys A. Densitometric analysis of body composition: Revision of some quantitative assumptions. Ann NY Acad Sci l963;l 10:113-40. Mendez J, Kollias J. Diet and starvation on the composition and calculated density of fat-free body mass. J Appl Physiol 1977;42:73l-4. Akers R, Buskirk ER. An underwater weighing system utilizing “force cube” transducers. J Appl Physiol l969;26:649-52. Keys A, Brozek J, Henschel A, Mickelsen 0, Taylor HL. The biology of human starvation. Minneapolis: University of Minnesota Press, 1950;1080. Darling RC, Cournand A, Richards DW. Studies on the intrapulmonary mixture of gases. III. An open-

BODY

DENSITY

81

circuit method for measuring residual air. J Chin Invest 1940; 19:609-18. 7. Buskirk ER. Underwater weighing and body density: A review of procedures. In: Brozek J, Henschel A, eds. Techniques for measuring body composition. Washington, DC: National Academy of SciencesNational Research Council, 196 l;90-l06. 8. Siri WE. Body composition from fluid spaces and density: Analysis of methods. In: Brozek J, Henschel A, eds. Techniques for measuring body composition. Washington, DC: National Academy of SciencesNational Research Council, 196 l;223-4. 9. Durnin JVGA, Rahaman MM. The assessment of the amount of fat in the human body from measurements of skinfold thickness. Br J Nutr l967;21: 68 1-9.

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