96
RADIATION STANDARDS, INCLUDING FALLOUT
STRONTIUM 90
It is generally accepted that the movement of strontium through the food
chain is to a large extent interrelated and governed by the simultaneous movement of calcium. Stable strontium is normally present in the food chain and
an understanding of the behavior of radioactive strontium added by man to the
food chain can most easily be gained by consideration of the behavior of stable
strontium and calcium.
As will be pointed out later, Sr” will not exactly parallel the behavior of stable
strontium until steady state conditions leading to comparable physical distribution are attained in the future. As time passes after the cessation of testing or
even after constant testing at about the samerate, there will be an approach to
identical behavior.
Chart 2 shows some actual data primarily from extensive British surveys
giving typical daily intakes and body contents of calcium and stable strontium
expressed in milligrams. The sources of calcium are broken down into milk,
plant foods, and “other,” which includes fish, eggs, meat, and mineral sources.
Values for the United States would be similar except that the “other” category
would contain about 250 milligrams less and the milk about 250 milligrams
more calcium; the relative strontium to calcium relationship in milk would be
about the same as indicated. This difference comes about because the British enrich their bread with mineral calcium, whereas in the United States milk solids
are used for this purpose.
The meaning and implications can better be grasped from chart 3, which presents the same information normalized to a Sr/Ca ratio in the diet of 1.
It is first noted that the Sr/Ca ratio of milk is one-tenth that of the plant
foods. This is because the cow preferentially utilizes calcium over strontium
by a factor of 10 for milk secretion. It is obvious from this diagram that if
an. individual reduced milk consumption to zero and derived all of his calcium
from plant sources, the Sr/Ca ratio of his diet would be doubled. Conversely, if
an individual derived all of his calcium from milk, his diet would have about
one-fifth the typical Sr/Ca ratio. The significance of this is emphasized by
recognition of the fact that the amounts of Sr” and calcium in the total diet
determine the body burden of Sr™.
It should also be noted that values of the Sr® content or the Sr”/Ca ratio
of any individual dietary constituent cannot by themselves be used to assess the
degree of exposure: for such an evaluation it is necessary to know the total
amounts of Sr” and calcium in the diet.
The ratios of Sr/Ca in the body, the fetus and mother’s milk reflect the usual
biological discrimination against strontium in favor of calcium. This behavior
is usually expressed in termsof the observed ratio—for example,
OR
-
body/diet = 0.25,
The variability in differential behavior. the processes involved and the application of these concepts have been studied most intensively in the laboratory, documented in the literature, and generally accepted.
The discussion so far has been concerned with the steady-state behavior and
it is of interest to consider the actual state of affairs in the last 2 years. Chart
4 presents values for Sr”/Ca expressed as in previous diagrams. The differences between plant foods and milk are not as great as will be attained under
steady-state conditions for reasons that will be discussed presently. Nevertheless, plant foods have a higher Sr”’/Ca ratio than the typical) diet, and milk has
a lower value.
The next three charts present data that demonstrate this by the comparison
in three cities of levels of Sr® per gram of calcium in a typical total diet, in che
milk contained in the diet, and in the nonmilk components.
This concept is of such importance that I should like to restate it in terms of
practical application. Human beings and animals of all ages must have a certain amount of calcium in the diet to build new bones and teeth or to remodel
and rebuild bones already formed. Calcium in the diet comes primarily from
dairy products and plant foods, both of which contain Sr”. The calcium from
dairy products will most always have less Sr” than the calcium from plant
foods because of discrimination by the cow. If the consumption of dairy prod-
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