F&N Bulletin Vol 23 No 1b - United Nations University

Nuclear and isotopic techniques
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TABLE 3. Overview of selected isotopic techniques in human nutrition studies
Technique
Radioisotopic tracer study
Radioimmunoassay
Nuclear analytical techniques
Applications
Body composition (tritium-labeled water)
In vivo study of iron uptake and bioavailability ( 59 Fe/ 55 Fe)
In vitro study of iron dialyzability ( 59 Fe)
Iron status (serum ferritin), iodine status (T 3
, T 4
, TSH)
Trace element content of foods, diets, and human tissues
Body composition (DEXA)
Whole-body counting Body composition (lean body mass)( 40 K)
Uptake and bioavailability of essential micronutrients ( 59 Fe and 65 Zn)
In vivo neutron activation analysis Body composition (total body nitrogen, calcium, etc.)
Stable isotopic tracer study
Body composition and breastmilk intake (deuterium-labeled water)
Substrate metabolism ( 13 C- and 15 N-labeled amino acids, fat)
Energy expenditure (deuterium- and 18 O-labeled water)
Uptake and bioavailability of essential micronutrients
(Fe, Zn, and vitamin A using appropriate stable isotopes)
Bacterial colonization ( 13 C-urea breath test)
available to provide the needed neutron source.
Clinical applications. Isotopes are used in the detection
of Helicobacter pylori infection, measurement of
gastric emptying, and measurement of macronutrient
assimilation in many laboratories around the world.
Measuring pollutants in air has become a matter of
global concern, particularly in some of the world’s
largest cities. Nuclear and related analytical techniques
are uniquely suited for conducting nondestructive
multielement analyses of air particulate matter collected
on filters. Commonly used techniques include
neutron activation analysis, particle-induced x-ray
emission (PIXE), particle-induced gamma-ray emission
(PIGME), energy dispersive x-ray fluorescence
analysis (EDXRF), total reflection x-ray fluorescence
(TXRF), and ICP-MS [15].
Stable isotopes in nutritional studies
supported by the International Atomic
Energy Agency
Measurements of breastmilk intake, energy expenditure,
micronutrient status, macronutrient utilization,
body composition, and many more indicators are
important in assessing the nutritional status of infants,
children, pregnant women, and nursing mothers, as
well as that of individuals who subsist on marginal
food supplies. Stable isotopic tracers are completely
safe and noninvasive and can be used in free-living
humans. They emit no externally measurable radiation,
and their presence in excess of natural levels
is detectable only by changes in the ratio of minor
isotope to major isotope. The ratio is measured by an
isotope ratio mass spectrometer in which heavy and
light forms of the same molecule undergo separation
and quantification. The required technique can be
obtained from the resources available [16].
Cost of analysis is indeed a cause for concern, since
the instrumentation and expertise required are not
available in many countries. However, stable isotopes
for the studies supported by the International Atomic
Energy Agency are made available under the project,
and in some cases assistance is provided for analysis in
an International Atomic Energy Agency–recognized
laboratory. The underlying idea is that these studies
would encourage strengthening of the infrastructure
of national facilities for subsequent investigations. The
cost of the equipment for carbon-13 breath tests used
for infection studies is not very high and can be within
the reach of many developing countries.
The major isotopes (hydrogen-1, carbon-12, nitrogen-14,
and oxygen-18) are always accompanied by a
constant proportion of minor heavier isotopes whose
individual abundances range from 0.02% to 1.11%.
An inventory of the human body shows that a 50-kg
person has a total of 225 g of hydrogen-2, carbon-13,
nitrogen-15, oxygen-17, and oxygen-18 [17]. Although
there are variations in the proportions of 1 H to 2 H, 13 C
to 12 C, 14 N to 15 N, and 16 O to 18 O, each has a characteristic
baseline abundance to which tracer measurements
are referred. The enriched form (e.g., 2 H 2
O or H 2 18 O)
may be used directly, 13 CO 2
may be incorporated
into plants by biosynthetic procedures, or the isotope
may be transformed through organic syntheses into
labeled fats, carbohydrates, or amino acids. These
stable isotopes can be administered orally, and the
metabolic products into which they enter (e.g., body
water, respiratory carbon dioxide, and urea) can be
sampled in breath, saliva, milk, urine, and stool.
13 C-urea breath tests are used to examine bacterial
colonization in the stomach by Helicobacter pylori.