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Chapter: Introduction to Human Nutrition: Body Composition

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Direct methods - Body composition techniques

The (chemical) analysis of carcasses is a time-consuming exercise and requires very precise approaches to the task.

Direct methods


Carcass analysis

The (chemical) analysis of carcasses is a time-consuming exercise and requires very precise approaches to the task. The carcass has to be carefully dissected into the different tissues that are then exactly weighed, after which the chemical analyses have to be performed. To avoid errors it is important that no unaccounted water losses occur during the analytical work. As early as the nineteenth century, it was rec-ognized that the variation in chemical body composi-tion was reduced when results were expressed as a fraction of the fat-free body. The data on the chemical composition of only a few human cadavers form the basis for the assumptions that are normally used in indirect methods. These chemical analyses were per-formed in five men and one woman. It was concluded that, on the basis of FFM, the mean amounts of water, protein, and minerals in the body are 72.6%, 20.5%, and 6.9%, respectively. The variability in these figures is about 13% for protein and minerals and 4% for water. Although one can question the quality of these data as a basis for other methods (low number, high variation in age, variation in gender, some carcasses were not analyzed immediately after death), they form the basis for many indirect and doubly indirect body composition methods. Chemical carcass analy-sis also revealed that the amount of potassium in the FFM is fairly constant. This fact is used as the basis for the calculation of the amount of FFM or for body cell mass from total body potassium, determined by 40K scanning.

In the 1980s, cadaver studies were performed again in the “Brussels study.” Unfortunately, only informa-tion at a tissue level and not at atomic or molecular level was collected. However, the need for cadaver studies has greatly diminished given that the same information can now be obtained in vivo by IVNAA.


In vivo neutron activation analysis

 

IVNAA is a relatively new body composition tech-nique that allows the determination of specific chemi-cal elements in the body. The body is bombarded with fast neutrons of known energy level. The neutrons can be captured by chemical elements (as part of mol-ecules) in the body, resulting in a transition state of higher energy for that element – energy that is finally emitted as gamma rays. For example, capture of neutrons by nitrogen results in the formation of the isotope 15N, which will emit the excess energy as gamma rays:

 

14+ 1 15N* + gamma rays

 

where 14N is nitrogen with atomic mass 14, 15N is nitrogen with atomic mass 15, and 1n is a neutron.

 

With IVNAA, many elements in the body can be determined, including calcium, phosphorus, nitro-gen, oxygen, potassium, and chlorine.

 

The information obtained at the atomic level can be converted to more useful information. For example, from total body nitrogen total body protein can be calculated as 6.25 times the total nitrogen, assuming that body protein consists of 16% nitrogen. The advantage of the method is that the chemical body composition can be determined in vivo and can be compared with other, indirect, techniques. For fundamental studies and for validation of existing techniques in special groups of subjects, for example in different ethnic groups, elderly subjects, obese sub-jects, or in the diseased state, the methodology can be of great importance. The disadvantage of IVNAA is not only the price. The subject is irradiated, with the radiation dose used depending on the number and kind of elements to be determined. It is relatively low for nitrogen (0.26 mSv) but high for calcium (2.5 mSv).

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