trace substance is used to measure the extent of
uptake of the substance into the tissues. Increased
tissue uptake due to cellular deficiency results in
decreased excretion of the load.
Tissue deposits that do not serve as stores or
supply reserves have been investigated as possible
markers of the long-term trend in the concentration
of transport forms of trace minerals. Scalp hair is
by far the best studied of the sites. Trace minerals
are more concentrated in hair, making the elements
easier to assay, and hair specimens can be obtained
without trauma, require no special preservatives, and
can be stored without deterioration (Gibson 1990).
However, the concentrations of trace minerals in
hair are influenced by hair color and the rate of hair
growth and by contamination from water supplies,
shampoos, hair treatments, and hair grooming agents
(Taylor 1986). Consequently, hair analysis does not
have a place in the clinical evaluation of nutritional
status. Fingernails and toenails have been proposed
as potential tissues for the analysis of trace mineral
deposition but they are also highly susceptible to
environmental contamination. It is unlikely that nail
analysis will ever be used clinically.
As a summary of the many different laboratory
approaches that can be used in the assessment of
nutrition status of a trace substance, the laboratory
studies pertinent to the evaluation of iron, cobal-
amin, and folate deficiency are listed in Table 8.3.
Not all of the studies are clinically available: those
Nutritional Status
8-7
Table 8.3
Laboratory Studies for the Assessment of Iron, Cobalamin, and Folate Deficiency
(TC, transcobalamin; dU, deoxyuridine)
Pathophysiology
Iron
Cobalamin
Folate
↓
Transport form
plasma iron
plasma holo-TC II
plasma folate
↑
Transport protein
plasma iron binding capacity plasma TC II (as protein)
↓
Tissue stores
plasma ferritin (as protein)
plasma cobalamin
↑
Tissue receptors
plasma transferrin receptor
↓
Tissue supply
red cell ferritin
red cell cobalamin
red cell folate
marrow erythroblast iron
↑
Tissue precursor
red cell protoporphyrin
↓
Tissue product
individual red cell hemoglobin
mean red cell volume
mean red cell hemoglobin
total red cell hemoglobin
blood hematocrit
blood hemoglobin
↑
Tissue enzyme substrate
plasma methylmalonate
↓
Tissue enzyme product
anemia
anemia
blood hematocrit
blood hematocrit
blood hemoglobin
blood hemoglobin
blood cell megaloblastosis blood cell megaloblastosis
marrow megaloblastosis
marrow megaloblastosis
dU suppression test
dU suppression test
+
Therapeutic response
reticulocyte count
reticulocyte count
reticulocyte count
blood hemoglobin
blood hemoglobin
blood hemoglobin
