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1 From the Obesity Research Center, St Luke's-Roosevelt Hospital, Columbia University College of Physicians and Surgeons, New York (ZMW, WS, DPK, SH, RNP, and SBH); the Department of Applied Science, Brookhaven National Laboratory, Upton, NY (LW); the Department of Academic Affairs, Winthrop University Hospital, Mineola, NY (JFA); the Jean Mayer US Department of Agriculture, Human Nutrition Research Center on Aging at Tufts University, Boston (MEN)
2 Supported by Knoll WRISC award Prop-797-03 (MER-1296-89) and National Institutes of Health grant NIDDK 42618. 3 Reprints not available. Address correspondence to ZM Wang, Weight Control Unit, 1090 Amsterdam Avenue, 14th Floor, New York, NY 10025. E-mail: zw28{at}columbia.edu.
ABSTRACT
Background: Protein is an important body component, and the
presently accepted criterion method for estimating total body protein (TBPro) mass—in vivo neutron activation (IVNA) analysis—is unavailable to most investigators and is associated with
moderate radiation exposure.
Objective: The objective was to derive a theoretical cellular level TBPro mass and distribution model formulated on measured total body potassium, total body water, and bone mineral and to evaluate the new model with the IVNA method as the criterion.
Design: The new model was developed on the basis of a combination of theoretical equations and empirically derived coefficients. TBPro mass estimates with the new model were evaluated in healthy women (n = 183) and men (n = 24) and in men with AIDS (n = 84). Total body nitrogen was measured by IVNA, total body potassium by whole-body 40K counting, total body water by tritium dilution, and bone mineral by dual-energy X-ray absorptiometry.
Results: The group mean (± SD) TBPro mass estimates in healthy women and men and men with AIDS (8.2 ± 0.9, 11.0 ± 1.8, and 10.5 ± 1.1 kg, respectively) with the new model were similar to IVNA criterion estimates (8.9 ± 0.9, 11.1 ± 1.6, and 10.9 ± 1.2 kg, respectively). TBPro mass estimates with the new model correlated highly with the IVNA estimates in all subjects combined (r = 0.92, P < 0.001). The new model suggests that the composite TBPro mass within each group consists mainly of cellular protein (75-79%) and, to a lesser extent, protein in extracellular solids (19-23%) and extracellular fluid (2%).
Conclusion: The new model provides a non-IVNA approach for estimating protein mass and distribution in vivo.
Key Words: Body composition • nutritional assessment • total body nitrogen • total body potassium • total body water • whole-body counting
INTRODUCTION
Protein is a functionally important component at the molecular level of body composition. Protein mass in healthy adults
is relatively large, representing 10.6 kg, or 15.1%, of body mass in
the reference man (1). The actual amount of protein found in
living humans is based on 2 study sources, in vivo neutron
activation (IVNA) analysis and non-IVNA methods (2, 3).
The chemical formula for protein is assumed to be
C100H159N26O32S0.7 with a ratio of nitrogen to protein of 0.16
(4). Assuming that all body nitrogen is incorporated into protein, a total body protein (TBPro) model that can be measured
by IVNA was derived from total body nitrogen (TBN) (5, 6):
SUBJECTS AND METHODS
New model
All protein exists within the fat-free mass (FFM) compartment. On the cellular body-composition level, FFM can be
divided into 3 components: body cell mass (BCM), extracellular fluid (ECF), and extracellular solids (ECS) (15):
RESULTS
Subject characteristics
A total of 291 adult subjects (183 healthy women, 24 healthy
men, and 84 men with AIDS) were evaluated in the study
(Table 1). The healthy women ranged in age from 23 to 81 y,
in body mass from 48.0 to 82.0 kg, and in body mass index (in
kg/m2) from 18.4 to 29.8. The healthy men ranged in age from
25 to 78 y, body mass from 57.6 to 89.1 kg, and body mass
index from 20.2 to 28.4. The men with AIDS ranged in age
from 22 to 62 y, in body mass from 46.6 to 77.2 kg, and in body
mass index from 16.7 to 25.9.
There were significant differences between healthy women and healthy men in all measures (P < 0.01-0.001), except age and body mass index (Table 1). The men with AIDS weighed on average 5.8 ± 7.4 kg (P = 0.009) less than the healthy men. There were no significant differences between the 2 groups of men in TBN, TBK, TBW, or bone mineral (Table 1; all P > 0.05).
Correlations between TBPro and TBK
There were good correlations between TBPro (kg) by IVNA
and TBK (mmol) in all 3 groups.
DISCUSSION
IVNA criterion method
Because the "true" value of body protein is not measurable
in vivo, a reference method with high accuracy is necessary to
evaluate less accurate methods. The applied TBPro reference
method should ideally meet 2 criteria: the method should avoid
major assumptions and have maximal precision. Because of its
unique role as an essential component of protein, the assessment of TBN by IVNA has been used as the well-validated
criterion for TBPro (5, 6).
The mean measurement error associated with the IVNA
model can be estimated for the healthy subjects by assuming an
average TBN of the healthy men and women as shown in Table
1 and measurement precision as stated in Subjects and Methods. Accordingly,
ACKNOWLEDGMENTS
We gratefully acknowledge the National Institutes of Health and Knoll
Pharmaceuticals for their support of this study.
ZMW and SBH designed the study; ZMW and SH analyzed the data; ZMW, SH, WS, and SBH wrote the manuscript; and DPK, LW, JFA, MEN, SBH, and RNP collected the data. None of the authors had any financial or personal interest in any company or organization sponsoring the research, including advisory board affiliations.
REFERENCES