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

8
The test measures the production rate of 13 CO 2
in
expired air, followed by oral ingestion of 13 C-labeled
urea. Breath tests for H. pylori using stable isotopes
are reliable and noninvasive tools that can be safely
applied in children from developing areas, where high
rates of infection and malnutrition are observed.
Isotope dilution methods are used in the assessment
of vitamin A status. The principle relies on labeled
carotenoid conversions to vitamin A, which can be
traced with 13 C carotenoids. Vitamin A pool sizes are
measured by the dilution of an oral ingested tracer into
the different body pools. This technique has potential
applications in measuring the effectiveness of vitamin
A and carotenoid supplementation and fortification
regimes in nutrition studies.
Another powerful use for stable isotopes is in the
assessment of bioavailability and pool sizes of trace
elements such as iron ( 57 Fe and 58 Fe) and zinc ( 67 Zn
and 70 Zn). The uptake of these labeled micronutrients
can be traced in vivo and which has been used widely
as a valuable tool for measuring the effectiveness
of supplementation or fortification trials in several
developing countries.
Estimation of total energy expenditure
The doubly labeled water ( 2 H 2 18 O) method is the only
technique that can accurately determine the energy
needs of people in their own environments and is one
of the most reliable methods for determining food
energy intake. This method is gaining wider acceptance
because it is inexpensive and accurate, and can be
applied under field conditions. When doubly labeled
water is administered to a subject, both isotopes mix
with the body water and are eliminated in the body
fluids over a period of days. The turnover of body
water can be estimated from the daily measurements
of 2 H concentration in urine or saliva samples. When
the samples are analyzed for 18 O, the values will reflect
a more rapid excretion rate than that for 2 H (deuterium),
because the 18 O is also incorporated into
exhaled carbon dioxide. The difference in excretion
rates between 18 O and 2 H tracers thus reflects the
volume of carbon dioxide produced over the period
of observation. This value can be used to calculate the
total energy expenditure of a subject.
Determination of lean body mass
A tracer dose of water labeled with 2 H or 18 O is administered
and allowed to equilibrate for 4 to 6 hours. The
isotope concentration in saliva or urine will reflect
the dilution undergone by the isotope. When the lean
body mass is calculated, the difference in body weight
is the amount of adipose (fatty) tissue.
Measurement of overall nitrogen flux
The nitrogen flux balance stumbles in periods of
stress, and then the catabolic processes predominate
over synthetic processes and a negative balance is the
result. Whole-body protein turnover is measured
by administration of a single oral dose of an amino
acid, or preferably a protein, labeled with 15 N [e.g.,
yeast grown in medium containing ( 15 NH 4
)SO 4
].
Urine is collected for 9 to 12 hours and the amounts
of tracer nitrogen in urinary NH 3
and in the urea are
determined. These two values provide a reliable estimation
of whole-body protein turnover that is insensitive
to changes in nonprotein-nitrogen metabolism.
Nutrient absorption and utilization after
diarrhea
Weaning infants often have periods of infection leading
to diarrhea. During these periods, nutrient intake is
insufficient to maintain infant growth, and regeneration
of small-intestinal capacity is essential. When rice
labeled with 13 C (by exposure to 13 CO 2
during periods
of photosynthesis) is cooked (rice water for rehydration)
and consumed, digestion and absorption of the
starch can be detected from the appearance of labeled
13 CO 2
in breath samples. The degree of malabsorption
can be estimated from the recovery of tracer carbon
in total stool carbon [17].
Isotopic techniques in support of
preventive measures
V. Iyengar
Preventive measures are based on the knowledge that a
clean environment and a healthy lifestyle are essential
for a productive existence. From the public health
perspective, preventive health care begins with the
newborn and extends across the entire life span. Isotopic
tools for strengthening health and nutrition
monitoring are being used extensively in industrialized
countries to analyze human energy requirements,
body composition, and the metabolism of important
nutrients such as protein, fat, vitamins, and minerals.
The information acquired has led directly to many
improvements in nutrition and health. These tools are
thus well suited for determining the success of foodsupplementation
programs and other interventions
aimed at combating the many forms of malnutrition.
Several strategic applications of isotopic techniques
are supported by the International Atomic Energy
Agency (table 3).
The International Atomic Energy Agency, through
coordinated research projects and technical cooperation
projects in the areas of health, nutrition,
and environment, is eminently positioned to provide