STRONTIUM-90 IN MILE
the gut
a, since
| in the
tissue,
ughter,
Id be a
y. The
absorp-
olloidal
dangerhe geochat the
10,18e ).
latively
es with
It has
of Sr-90
ye same
is about
qination
re bones
23 of the
ch may
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nts diseium if
mals, a
calcium
J0sition
t. Sr-90
0 milk.
‘rations
yn unit
omicroite that
af it is
by ase from
plant to man vary, aceording to the investigator. Most authorities estimate that
the discrimination factor for cow’s milk is about 0.09 to 0.14; that is, that there
are seven to 11 times more Sunshine Units present in the cow’s ration than appear
in the milk (5,8, 15,190) (see Appendix for Discrimination Ratio definition).
Studies in rats and rabbits indicate a discrimination ratio of about 0.5 between
the mother’s blood and the fetus (5,191). The rato of Sr-90 to calcium present
in the milk of lactating goats was found to be 0.88 of that present in the blood
(19a, 22). It should be noted that discrimination factors or ratios may be expressed either as whole numbers or as their reciprocal fractions.
In the animal, the principal sites of discrimination are the gut, kidney,
placenta, and mammary gland. Neuman (191) has schematically summarized
the discrimination scheme in the body and uses a factor of two for eachsite.
This scheme is shown in Figure 1. The discrimination by the gut is apparently
FECES
/” |
URINE
BLOOD
a
ran
Sr/Ca = X<—+—Sr/Ga#l/2 X—+-—Sr/Ca = 1/4 xl Sr/Ca = 1/8 X
DIET
!
BLOOD
!
FETUS
!
GUT
KIDNEY
||
BONE
'
PLACENTA
OR
MAMMARY
GLAND
Fie, 1. Schematic summarization of major discrimination sites against Sr-90.
crimination factor of two is assumed for each site (191).
OR
MILK
A dis-
influenced somewhat by the diet. It has been reported that if the diet is primarily milk the discrimination by the gut is lowered (5,792). The discrimination
ratio from diet to bone shows considerable variation, but the more recent work
indicates a value of four (8, 15).
These discrimination values indicate that the Sr-90 content of milk per gram
of calcium should be less than vegetables or cereals, because of several discrimination sites, and also less than other animal foodstutfs because of the additional
discrimination by the mammarygland. However, the possibility that human and
animal foods differ in their Sr-90 content and that the human gut does not discriminate as well against Sr-90 on a diet high in milk must also be considered
in any evaluation. This will be considered more fully later.
Lhe levels of Sr-90 in milk, Apparently, it has been only since April of 1954
that milk has been routinely analyzed for its Sr-90 content. These analyses are
under the supervision of the Atomic Energy Commission and are a part of
Project Sunshine. One of the aims of Project Sunshine was to find an index by
which the Sr-90 levels in human bone could be readily estimated. Milk and its
products were a natural choice, since in the United States the majority of the
dietary calcium comes from milk products and, thus, the levels in milk should
parallel those found in human bone. If the discrimination factor from milk
to bone and the Sr-90 level in milk is known, the Sr-90 content in human bone
can be readily estimated. Also, the monitoring of milk is facilitated by the fact
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