Gilbert B. Forbes, M.D.
Univerrity of Rochester School of Medicine and DentLitry, Rochester, New York
SIGNIFICANT
ISOTOPES
IN
FOODS
207
J
r is the purpose of this report to reviewsome of the information available on
isotopic contamination of foods. There is a
vast amount of data on this subject, for
1)0th the Atomic Energy Commission and
the U. S. Public Health Service have
oper-ated extensive monitoring programs for a
number of years. Certain states and non-governmental agencies have also
partici-pated. The result is that we know much
more about fallout accumulation in foods than about many of the other, and more nu-tritious, constituents.13
Scores of radionuclides are produced in a nuclear explosion. Their potential
sig-nificance for man-and by this I mean
possible radiation hazard-depends upon a
number of physical and biological factors. In addition to the amounts produced these include physical half-life, stratospheric resi-dence time, chemical reactivity, transmis-sion through the food chain, gastrointestinal absorption, tissue localization, turnover rate in the body, and excretion. When the sum total of these is considered, only four
radio-nuclides, J131, Sr59, Sr90, and Cs137, qualify
under ordinary circumstances as possibly
significant in foods (Table I).
The first in this group is of importance only for brief periods following the bomb explosion and hence is likely to be found only in those foods which are eaten fresh. The half-lives of the latter two comprise an appreciable portion of the human life span.
Generally speaking, fallout products are
detectable in most foods, but when one
considers the usual American diet it turns out that milk and other dairy products are prime contributors. These are the major
source of iodinel3l and account for 40 to
50% of strontium9#{176} intake and about 60% of
cesium’37 intake. And of all the routes by
which fallout products can enter the body-air, water, food, and skin
contamination-food stuffs are by far the most important. In this connection, it is well to remember
that Americans are vigorous consumers of
dairy products. Surveys reveal an average milk consumption by 6-month-old infants of
850 ml daily. Older children consume
ap-proximately 450 to 700 ml of milk daily,
and consumption by adolescents shows a
wide range-200 to 1,000 ml daily,
de-pending on regional habits. The average values of milk consumption for adult men are 350 to 500 ml daily and for women 130 to 260 ml daily.4’5
A recent compilation shows that the
year-ly per capita consumption of fluid whole
milk in the United States is 253 pints. Other
dairy products, such as cream, cheese,
butter, ice cream, and low fat and evapo-rated milks, together account for an addi-tional 100 pounds. Dairy products contrib-ute 15% of total calories, and there can be no doubt that milk is a staple of the
Ameri-can diet.6
Table II illustrates the milk level of the
four radionuclides during the past few
years. These are averages for the United States; there are rather wide regional
varia-tions, the range being 3 to 10 times the
mean.’ The effect of atmospheric testing is clearly evident, the short-lived isotopes waxing and waning as testing is resumed
and then stopped.
The importance of diet is clearly shown
by studies which have been made on
ces-ium’37 body burdens in man. This isotope
can be assayed in the whole body
scintil-TABLE I
CHARACTERISTICS OF RADIONUCLIDES
Radio-nuclide
Physical Half-Life
Tissue
Distribution
Principal Dietary
s
IodinelM Strontium8’ Strontium’#{176} Cesium”7
8 (Ia 53 da yr
30 yr
Thyroid Bone Bone General
TABLE II
AVERAGE MILK CONCENTRATIONS, UNITED STATES
Date !odu,e’31 Strontium89 Strontium’0 (‘esiu I37 Remarks
July 1960 - 5 8 10
,Ian. 1961 - - 6 5
()(t. 1961 100 - 8 10 Atmospheric testing
resumed, Sept. 1961
Jati. 196 - 5 8 55
July 196 40 60 15 70
Jan. 1963 10 35 15 70
July 1963 - 100 30 170 Test Ban Treaty, fall
of 1963
Jan. 1964 - 5 25 130
July 1964 - - ‘30 100 Chinese bombs Oct.
1964, May 1965
JaIl. 1965 - - 0 70
Apr. 1965 - - 0 70
Values are in picocuries per liter (1 pCi= 10’ j.oCi).
lation counter; and, since it is distributed in the body much as is potassium, it is cus-tomary to express the results as picocuries Cs/gram K. Hesp found a striking linear relationship between milk intake and Cs/K ratio in British adults. Some recent pre-liminary observations of ours indicate that
it is also true of children. Of six 8 to 10
year olds whose average milk intake was
30 oz per day the body Cs/K ratio was 43
pCi/gm, while the value was 33 for six who drank less than 19 oz.
Strontium metabolism is qualitatively similar to that of calcium. It so happens that most of the processes involved in the ultimate transfer of the isotope from atmos-phere to food and in turn to human tissue involve a discrimination factor in favor of calcium. The ratio Sr’#{176}/Ca progressively
falls as the isotope proceeds through the food chain: soil-plant--*man. The result is
that human bone contains about 5% as
much Sr’#{176}/gm Ca as the soil which re-ceived the fallout.
The ratio (Sr’#{176}/gm Ca bone) to (Sr’#{176}/ gm Ca diet) has been the subject of much study, and has been established at about 0.25 for adult man. Since skeletal calcium
is under physiologic control, the Sr#{176}#{176} con-tent of bone at any given age is largely a
function of the dietary 5r90/Ca ratio. Thus, the bone level of Sr’#{176}will be less either when dietary Sr#{176}#{176}is low or dietary calcium is high. Though cow’s milk is an important source of Sr’#{176},it has a higher content of
calcium than most plant foods; the latter taken together with the discrimination fac-tor provided by the cow means that
indi-viduals in milk-drinking countries have
lower bone levels of Sr’#{176}than those whose diet is largely from plant sources. Theo-retically, one can further improve the sit-uation by increasing dietary calcium from
uncontaminated sources, but the level
would have to be raised by a factor of two to four to achieve even a modest effect, and such an addition to the already
gen-erous American intake might well have
deleterious effects which far outweigh the most pessimistic predictions for Sr’#{176} radia-tion.
Of interest to the pediatrician is the fact
that certain infant formula products are
prepared in such a way as to lower the
Sr’#{176}/Ca ratio. Preparation which involves ion-exchange resins of electrodialysis re-sults#{176}in a considerable reduction of both
SUPPLEMENT
Sr’#{176}8 and Cs137.9 Soya formulas, which are
fortified with calcium, have a low Sr90/Ca
ratio, as do some of the prepared infant cereals. Breast milk contains one-half as much Sr#{176}#{176}/gmCa as does cow’s milk, and about one-fourth as much Cs’37.’#{176}’1’Thus, the human mother provides a discrimina-tion factor against Sr, a phenomenon which
accounts for the somewhat lower Sr90/Ca ratio in the deciduous teeth of breast-fed children. 12
A few examples of special situations are of interest. Alaskan Eskimos and Finnish
Lapps have much higher body burdens of
both Sr’#{176}and Cs’37 than Americans or Eu-ropeans. Moreover, certain radionucides,
such as Cs134 and Na22, are present in these peoples which are rarely, if ever, found in others. The reason is ecological. There is little opportunity for fallout products to
mix in the frozen soil of these regions and they are trapped in plants; consequently, the principal food animals-caribou and rein-deer-accumulate large amounts.
Some foods avidly acquire certain trace
materials. Brazil nuts have a high radium content and tea and coffee a high content
of both Sr’#{176}and Cs’37 as well as certain other fallout products.f
A question has recently arisen with re-gard to certain infant foods containing chicken. Deboning of chicken, formerly done by hand, is now done mechanically, with the result that the final product con-tains up to 13% bone powder. Since chick-ens accumulate Sr’#{176}in skeleton in the same fashion as other vertebrates, this change in processing technique results in an increased Sr#{176}#{176}content of the infant food. However, the calcium content is increased even more, so there is a slight fall in the Sr’#{176}/Caratio.t
The Utah episode of July 1962 illustrated
an unfortunate combination of factors,
which will be discussed more fully by other
+Since the usual daily consumption of tea and coffee (as dry power) is only a few grams, these foods constitute minor source.
Subcommittee on Radionuclides in Foods, Food Protection Committee report to the National
Research Council, November 19, 1964.
speakers. Atmospheric conditions produced a high level of fallout, the result of the
Ne-vada tests. Much of the milk consumed
came from local or nearby sources; the
cows were fed fresh feed and their milk
was drunk fresh. The result was a rather high consumption of J131
It should be noted here that the
adminis-tration of stable iodine will appreciably re-duce I’s’ uptake by the thyroid gland.13”4
How does the infant situation compare with that of the adult? There are a few fac-tors, both in diet and host, which operate to
the advantage of the infant, and some
which put him to a disadvantage. Since the latter are more numerous they will be
dis-cussed first.
Generally speaking, infants and children
have a higher food intake per unit body weight than the adult, so that proportional-ly more fallout products are consumed. The
disparity in milk intake is even greater than that for other foods.
Growth is another factor. A portion of in-gested Sr’#{176}is incorporated along with
calci-um in newly formed bone; this phenome-non taken together with the greater reactiv-ity of young bone accounts for the
observa-tion that the 1-year-old infant skeleton con-tains about 10 times as much Sr’#{176}/gm Ca as the adult.15 In other words, the infant discrimination ratio is less favorable. In view of this the use of breast milk or one of the proprietary infant formulas mentioned earlier might be of some use on these occa-sions when fallout contamination is high.
Under today’s circumstances, however, lit-tle if any benefit would follow.
With respect to iodin&32, the infant and child are the critical sectors of the popula-tion. Since thyroid uptake, in terms of per-cent of dose, is about the same in all age groups, the much smaller size of the infant
gland results in a larger accumulation of
J131 per gram of tissue. Radiation dose is
approximately proportional to isotope
con-centration within a tissue; hence, the young thyroid gland will be irradiated more
com-pounded by the larger milk intake (the principal source of I’s’) of the former.
It is generally thought that young tissue
is somewhat more radiosensitive than old
tissue. Furthermore, the long life expectan-cy of the modern infant means that expo-sure to long-lived isotopes such as Sr9#{176}and Cs’37 will inevitably be greater than that of tile older adult.
Acting to the advantage of the infant is the factor of placental discrimination, for the fetal skeleton has a Sr’#{176}/Ca ratio which is about one half of the mother.’6
The rate of cesium’37 turnover is about five times higher in the infant and about
two and one-half times higher in the child
than the adult. Pendleton, et report a
biological half-life of 20 days in the human infant in contrast to one of 100 days in the adult. The result is somewhat lower Cs137/ K ratio, and hence a lower radiation dose from this isotope, in the younger subject.18
It is an easy task to review the published
data on radionuclide concentration in
foods. A far more difficult question is the
connotation of the word “significant.” How hazardous are the present levels of food contamination?
In the present state of knowledge this is clearly a matter of opinion, for the levels
encountered by the average American are
such as to provide for only a small
incre-ment to natural background radiation, a
background to which man has always been
exposed. Opinions are largely a function of attitude and hence they are apt to vary, even among the expert and well-informed.
One has only to recall the controversy which ranged over the subject of contagion in the nineteenth century’9 or that over the
potential value of radar in the twentieth century.2#{176}
Yet, these were simple situations com-pared to the evaluation of small-dose radia-tion hazard. The hazards of large doses are
well known, but those from the minute
amounts which accrue from fallout (less
This can be measured in the whole body counter: turnover rate is equal to the daily in-take (or excretion) divided by the body burden.
than 0.01 r/year) are impossible to
esti-mate except on theoretical grounds. The
situation is compounded when the
produc-tion process has war-like overtones and
when the product distribution is not
re-stricted to the producers but is worldwide. Thus, we find Senator Bartlett2l expressing great concern over the Alaskan situation, where Sr9#{176}and Cs137 body burdens far ex-ceed those in other states, while the Federal Radiation Council appears unconcerned.22 Nuclear physicists, for example, have fre-quently voiced alarm as to potential fallout hazard. Yet, the radiation from
inadequate-ly shielded cyclotrons can be far greater.
Although certain dietary manuevers are
helpful, particularly with respect to
short-lived isotopes,I clearly the only way to
effectively reduce fallout is to stop above-ground weapons tests.
Atomic energy is a fact of modern life, and radiation exposure is increasing year
by year. The biological risk from present levels of fallout, though it cannot be esti-mated accurately, is, in my opinion, an ex-ceedingly small one-so small that it will escape detection by even the most meticu-lous observer. From the quantitative stand-point, more attention should be directed at other sources of manmade radiation, name-ly diagnostic roentgenography, since it con-tributes a much greater average dose than
does fallout. Radiation in any form is of particular concern to the pediatrician, since
his subjects have a much longer life expec-tancy and will contribute more to future
genetic burdens, whatever these may be,
than the adult.
REFERENCES
1. Joint Committee on Atomic Energy, Congress of the United States: Hearings before the Subcommittee on Research, Development, and Radiation, June 29 and 30, 1965. 2. Radionuclides in Foods: Washington, D.C.:
National Academy of Science National Re-search Council Publ. 988, 1962.
IIThese include the feeding of cattle on stored feed, a reduction in consumption of fresh milk, and the administration of stable iodine to both
3. Forbes, C. B., Nutrition in relation to prob-lems of radioactivity. Pediat. Clin. N. Amer., 9:1009, 1962.
4. Filer, L. J., Jr., and Martinez, C. A.: Intake of selected nutrients by infants in the United States, Clin. Pediat., 3:633, 1964. 5. Morgan, A. F.: Nutritional Status U.S.A.
Cali-fornia Agriculture Exp. Station Bulletin 769, 1969.
6. Butz, W. T.: How Americans use their dairy foods. National Dairy Council, Chicago 1966.
7. Hesp, R.: Uptake of cesium-137, due to nu-clear weapon fall-out, in subjects from West Cumberland. Nature, 206:1213, 1965. 8. Filer, L. J., Jr., and Sarett, H.: Unpublished
data.
9. Forbes, C. B.: Unpublished data.
10. Lough, S. A., Hamada, C. H., and Conar, C. L.: Secretion of dietary strontium-90 and calcium in human milk. Proc. Soc. Exp. Biol. Med., 104:194, 1960.
11. Straub, C. P., and Murthy, C. K.: A com-parison of Sr90 component of human and cow’s milk. PEDIATRICS, 36:732, 1965. 12. Rosenthal, H. L., Cilster, J. E., and Bird,
J. T.: Strontium90 content of deciduous hu-man incisors. Science, 140:176, 1963. 1:3. Saxena, K. M., Chapman, E. M., and Pryles,
C. V.: The minimal dosage of iodide
re-quired to suppress uptake of iodme-131 by normal thyroid. Science, 138:430, 1962. 14. Cuddihy, R. C.: Thyroidal iodine-131 uptake,
turnover, and blocking in adults and adoles-cents. Health Phys., 12:1021, 1966. 15. KuIp, J. L., Schulert, A. R., and Hodges,
E. J.: Strontium-90 in man IV. Science, 132:448, 1960.
16. Comar, C. L., and Wasserman, R. H.: Stron-tium and barium. In Coniar, C., and Bron-ner, F., ed.; Mineral Metabolism: An Ad-vanced Treatise, Vol. II. New York: Aca-demic Press, pp. 523-572, 1962.
17. Pendleton, R. C., Mays, C. W., Lloyd, R. D., and Church, B. W.: A trophic level effect on Cs”7 concentration. Health Physics, 11: 1503, 1965.
18. Onstead, C. 0., Oberhausen, E., and Kearv,
F.: Cesium-137 in man. Science, 137:508, 1962.
19. Holmes, 0. W.: Tile contagiousness of puer-Peral fever. In Medical Essays. Boston: Houghton Muffin, 1861.
20. Snow, C. P.: Science and Government. Cam-bridge, Massachusetts: Harvard University Press, 1961.
21. Senator E. L. Bartlett: Tile Congressional
Record, August 3, 1965. 2.2. Federal Register, May 22. 1965.
DISCUSSION
DR. Lis: Opening the discussion on Dr. Forbes’s paper is Dr. Bernd Kahn.
Dn. KAHN: Dr. Forbes mentioned that it would be of interest to discuss specific
problems encountered in measuring
ra-dioactivity levels in food and relating them to human exposure. The four major
prob-lems that occur to me are obtaining reliable analyses of food samples, representative samples, accurate consumption values, and a relation between intake and retention.
Examples of these problems are provided by our studies to determine tile retention of
certain radionuclides by infants.
The analytical problem lies ill measuring radionuclides in extremely small concentra-tions. Sr’#{176}levels in infant foods, for exam-ple, ranged from 0.1 to 10 pCi/gm ash dur-ing the past 5 years. At the lowest concen-tration, the acceptable reproducibility of
duplicate samples was plus or minus 8%.
The counting error was 5% for
radiochemi-cally separated strontium’#{176} from 10 gm ash, counted overnight in an anti-coincidence beta counter. Other significant sources of error contributing to the plus or minus 8 value include background irregularities in
overnight counting and corrections for
chemical recovery.
The success and failure of sampling are both illustrated in Table I. The national av-erages of the Pasteurized Milk Network are remarkably representative of the Sr90/Ca ratios in homogenized, skimmed, and evap-orated milk consumed by the infants in our study. Values for premodified milk and soy-bean-milk formulae, however, lie outside these ranges during some of the periods.
Should such formulae be sampled? The
ex-tensiveness of sampling is obviously deter-mined by balancing the need for inclusive-ness against the costs and difficulties.
