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Reduction in the Absorption of Dietary Strontium in Man by Nutritional Factors
V. Höllriegl, M. Röhmuß, U. Oeh, E. Werner, P. Roth
GSF-National Research Center for Environment and Health, Institute of Radiation Protection,
Ingolstädter Landstrasse 1, D-85764 Neuherberg, Germany
email: vera.hoellriegl@gsf.de
Abstract. Radioactive isotopes of strontium, mainly 90Sr, may considerably contribute to the total activity of
long-lived fission products released into the environment in nuclear accidents. Consequently, the ingestion of
strontium radionuclides with contaminated foodstuffs may result in a significant internal radiation exposure of
members of the public. The committed radiation dose dependents significantly on the fraction of the ingested
activity that crossed the gut wall (f1 value). In controlled tracer kinetic investigations, the influence of black and
green tea on the gastrointestinal uptake of strontium was investigated, since tannin, a constituent of tea, is well
known for being an inhibitor of gut uptake of some trace metals. In further experiments, the effect of sodium
alginate and pectin, which are naturally occurring non-absorbable polyelectrolytes, on the gastrointestinal
absorption of strontium was studied in human subjects. The data obtained show that the uptake of ingested
strontium from green tea was reduced by a factor of 1.4 in comparison to aqueous solution, whereas
administration of black tea had a less pronounced effect on strontium absorption. Addition of alginates in milk
resulted in a large reduction of the f1 value by a factor of about 20. Investigations with pectin indicate that this is
also a possible inhibitor of strontium uptake. Alginates and pectin therefore seem to be effective nutritional
factors to reduce the strontium uptake into the systemic part of the body, which in turn results in a corresponding
reduction of the effective dose after ingestion of radionuclides of strontium.
1. Introduction
Strontium-90 from the fall-out of atomic explosions and from accidental release by industrial plants,
presents a risk to health. Strontium is absorbed into the human body and deposited in the skeletal
tissue in much the same way as calcium, although some biological discrimination against strontium
seems to exist, which takes place mainly in the absorptive phase. In view of the possibility of dietary
contamination with Sr-90, it is desirable to be able to inhibit the absorption by binding the ingested
radionuclide before absorption can occur. It is important, however, that calcium absorption should not
be reduced considerably.
Nutritional factors may influence the fractional absorption of strontium from the gastrointestinal tract
into the systemic part of the body. A number of factors have been found both in animals and humans
to increase absorption, including fasting and low dietary levels of calcium, magnesium and
phosphorus [1, 2, 3]. On the other hand, high calcium diets and administration of alginates have been
shown to decrease the absorption of ingested strontium [3, 4, 5, 6]. Furthermore, the selective
inhibition of radiostrontium uptake by rats following the administration of pectic acid or pectin-like
compounds has been reported [7, 8, 9]. Up to now, there exist no similar investigations for pectin in
humans. Alginates are structurally carbohydrates consisting of polymerised mannuronic and guluronic
acids, which are extracted from brown seaweed. Like alginate, pectin is known as non-toxic and non-
absorbable polyelectrolyte and is found in fruit and vegetables and mainly prepared from waste citrus
peel and apple pomace. Pectin consists of homopolymeric partially methylated poly-galacturonic acid
residues. Both substances are used commercially in the food and drug industries for many purposes,
such as gelling agents, emulsifiers and thickeners. Additionally, pectin has been shown to be effective
in preventing the gastrointestinal uptake of lead [10].
It is well known that tea, which contains polyphenolic compounds as ingredients, e.g. tanning agents
(tannin), have a wide range of effects in vivo and in vitro, including antioxidant and metal-chelating
activities. Tea is reported to have the potential to reduce the gut uptake of several trace metals, mainly
of iron [11, 12] or other elements, e.g. molybdenum [13]. Black and green teas both contain similar
amounts of flavonoids, they differ however in their chemical structure.
In 1965, Hayashi et al. reported about the inhibitory effect of green tea on the gastrointestinal
absorption of Sr-90 in rats [14].
This paper deals with the effect of some dietary factors and foodstuffs such as green and black tea as
well as the effect of pectin in comparison to alginates on the uptake of ingested strontium in humans.
2
For 90Sr, the committed radiation dose after ingestion of contaminated foodstuffs depends significantly
on its fractional intestinal absorption, i.e. the f1-value to be used in the dose calculations. Information
on the determination of the fractional intestinal absorption of strontium in humans can be obtained by
controlled tracer kinetic investigations. The application of radionuclides of strontium to healthy
volunteers in such investigations is ethically hardly justifiable, even if the resulting committed
radiation doses are small. Stable isotopes used as tracers represent an ethically acceptable alternative
for biokinetic investigations, as they are free from any radiation risk for the volunteer subjects [15,
16]. Stable tracers are artificially produced by modification of the natural isotopic composition of the
element of interest. Biokinetic investigations can thus be conducted by the administration of one or
more tracers in suitable form to volunteer subjects. It is then possible to quantify the tracers in
biological samples, e.g. blood or urine, from the measurement of their concentrations, of their isotopic
abundances or isotopic ratios, provided that the isotopic composition of the natural element and of the
tracer(s) is known (see Table I).
Table I: Natural [17] and enriched stable isotope abundances (as certified by Chemotrade GmbH,
Düsseldorf, Germany)
________________________________________________
Mass number Srnat 84Sr 86Sr
______________________________
Percentage of atom
________________________________________________
84 0.56 76.40 0.01
86 9.86 4.64 95.70
87 7.00 1.96 1.17
88 82.58 17.00 3.12
________________________________________________
2. Materials and methods
Two solutions of strontium, one enriched with the stable isotope 84Sr, the other enriched with 86Sr,
were purchased as strontium carbonates from Chemotrade GmbH, Düsseldorf, Germany. The metal
isotopes 84Sr or 86Sr were brought into solution as follows: An aliquot of 30 mg of 84Sr or 86Sr was
dissolved in 0.30 ml of hydrochloric acid (15%) and the solution was filled up to the desired
concentration with deionised water (MilliQ). For oral applications, a stock solution of 86Sr with
concentration of 1g Sr/l was prepared. For intravenous application, the 84Sr solution contained
approximately 42 mg Sr/l and was made isotonic by adding adequate amounts of sodium chloride and
sodium citrate, whereby the pH value was adjusted between 6 and 7 by adding sodium hydroxide
(30%). Aliquots of 10 ml were filled into glass ampoules, melted tight and sterilized at 120oC within
20 min. Sterility of samples was checked according to German European Pharmacopoeia.
The isotope ratios of strontium were determined by means of a thermal ionisation mass spectrometry
(TI-MS). For that purpose, a thermal ionisation source with dual filament technique and mass
separation by a sector field magnet (Finnigan MAT262; Bremen, Germany) was used.
Kinetic data were attained by means of a double tracer technique [18, 19]. Applying this method, a
defined amount of 86Sr isotope is administered orally, while a solution of 84Sr is simultaneously
injected intravenously. The double-isotope method provides an accurate estimate of fractional
absorption and is a convenient technique in that only a single sample of blood (or urine) needs to be
collected.
Human investigations were conducted on healthy volunteers in accordance with the Helsinki
Declaration and according to the protocol approved by the Ethics Committee of the Medical Faculty,
Technical University Munich, Germany. The participants were given oral and written information
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about the study and written consent was obtained before their participation in the study. A total of 35
tracer kinetic investigations were carried out on ten human volunteers (six males/four females, see
Table II), aged between 24 and 61 years (mean ± SD: 41.1 ± 12.4).
Table II: Characteristics of volunteer subjects
__________________________________________
Volunteer Age Sex Weight (kg)
1 T.H. 38 m 75
2 M.R. 39 f 62
3 E.P. 50 f 60
4 F.R. 33 m 70
5 E.W. 59 m 85
6 U.L. 61 m 75
7 A.L. 35 m 82
8 J.B. 24 f 65
9 B.M. 28 f 78
10 J.B. 44 m 77
_______________________________________
The investigations started in the morning after an overnight fast of the volunteers. While receiving the
tracers, the persons were under permanent medical control. A 24-hour urine sample and one blood
sample were collected prior to the tracer application (blank values). The volunteers received an
intravenous injection of a sterile isotonic solution of about 0.4 mg 84Sr. Simultaneously 1 mg of 86Sr
given as strontium chloride in 100 ml of aqueous solution, of black tea (Assam), or of green tea
(Sencha) was administered orally. For the preparation of tea, a portion (1.5 g) of black tea was infused
with 250 ml of boiling water for 5 min, whereas 2.2 g of Sencha tea (green tea) was infused for 2 min
with 100 ml water of 70o C. In another series of experiments, 1 mg of 86Sr mixed in an aqueous
solution with pectin (3 g/100 ml, apple pectin from Naturwerk, Hannover, Germany) or in milk with
alginate was administered to the volunteers. Pharmaceutically prepared tablets of alginates (Gaviscon,
Novartis Consumer Health GmbH, Germany) according to an amount of 3.5-5.0 g of alginic acid were
pre-dissolved in 50 ml of water and then mixed with 100 ml of milk [6]. The prepared solutions of
pectin and of alginate were rather viscous but drinkable.
After the tracer administration, blood samples were withdrawn via an in-dwelling catheter at given
intervals until 24 hours, as well as urinary excretion was collected. Blood plasma was separated from
whole blood by centrifugation. Aliquots of 100 ml of urine were mixed with 5 ml of concentrated
nitric acid, and thereafter all biological samples were kept frozen until analysis.
TIMS requires thorough sample purification from the biological matrix constituents which could
reduce the ionisation efficiency of the trace element and interfere with the signal analysis. In order to
get reliable results for the isotope ratios, a number of other elements and molecules have to be
separated by an appropriate preparation of the sample for the mass spectrometric analysis, which is
described in detail elsewhere[20]. Briefly, an aliquot of 0.25 ml of plasma (or 1.0 ml of urine) was
mixed with 1.0 ml nitric acid (65%, subboiling distilled) and wet-ashed in a microwave oven.
Strontium was separated selectively from the samples by means of an ion-exchange-chromatography
applying Sr.Spec columns (particle size 100-150 µm), which were obtained in a ready-to-use form
from EIChrom Industries Inc. (Paris, France) [21]. All procedures related to mineral separation and
purification were done in a clean room.
The f1 value was determined from the ratio of the oral 86Sr to the intravenously injected 84Sr in a
plasma sample taken 24 h after simultaneous administration, according to the following equation [20]:
4
f1 =
po
iv
iv
po
m
m
z
z
86
84
84
86 ⋅ (1)
where
z86po, z84iv the 86Sr and 84Sr tracer concentrations in blood (or urine);
m84iv, m86po the amounts of the isotopes 84Sr and 86 Sr as administered tracers.
po, iv per oral, intravenous
The results are expressed as arithmetic mean values with their standard deviation (mean ± SD) and
were analysed with the unpaired Student’s t-test [22]. For the null hypothesis, the mean values of f1 for
aqueous solution and the other administered solutions were tested for difference.
3. Results and discussion
Table III and fig. 1 present the mean absorption data of uptake of 1 mg of strontium from black or
green tea in comparison to aqueous solution, as well as of the data of fractional uptake from the
prepared pectin and alginate solutions respectively. The data refer to plasma samples taken 24 hours
after tracer administration. The uptake of Sr from aqueous solution was used as reference. In
comparison to the data from aqueous solutions, drinking of green tea decreases the mean absorption of
Sr from 61% to 44%. This represents a reduction of about 30%. The mean f1 value for green tea is
statistically significant lower on the level of 1% (p < 0.01). A similar inhibitory effect of green tea
(also Sencha tea) against Sr-90 absorption through the intestine was found by using rats [14], when the
green tea was administered 30 minutes before the administration of 90Sr, while the simultaneous
administration of the tea and 90Sr caused a reduction in absorption of about 18%, which is statistically
not significant. In contrary to this single dose study, studies with tannin on the effectiveness in
reducing the retention of chronically ingested radiostrontium in rats showed no beneficial effect in
reducing the body burden of radiostrontium [23].
Table III: Fractional intestinal absorption of strontium (f1 value) from aqueous solution, green tea,
black tea, and pectin and alginate in milk (100-150 ml). Oral 86Sr amount per experiment: 1 mg.
Values obtained from plasma taken 24 hours after tracer administration.
__________________________________________________________________________________
Experiment with Number of f1 value Range
Experiments (Mean ± SD)
Aqueous solution 10 0.61 ± 0.12 0.40 - 0.78
Green tea 7 0.43 ± 0.10 0.36 - 0.64
Black tea 8 0.50 ± 0.21 0.22 - 0.84
Pectin solution 6 0.23 ± 0.12 0.10 - 0.40
Alginate/milk solution 4 0.03 ± 0.02 0.02 - 0.06
__________________________________________________________________________________
In the present investigation, drinking of black tea showed a small reduction in Sr absorption from 61%
to 50% in comparison to aqueous solution, which was determined to be not significant. However, the
number of experimental data is still limited and the data showed a considerable variation of f1 values
(range. 0.22 – 0.84). Probably, the tanning agents in green tea could bind more strontium and therefore
reduce the intestinal uptake significantly. Both green tea and black tea come from the leaves of the
plant Camellia sinensis, the processing that the leaves undergo however make the final tea different.
For black tea (not for green tea), the leaves are fermented (fully oxidized) before drying, which could
alter the tanning agents so that they could lose or change their activity.
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As expected, the absorption of Sr was inhibited by the polysaccharide solutions. More than 60% less
Sr is absorbed from the pectin solution (f1 = 0.23 ± 0.12) than from aqueous solution of equivalent Sr
content. The most striking effect was obtained after administration of the alginate solution (inhibition
by about 95%). Both differences in mean values were significant at the 0.1% level (p < 0.001).
f1 value of strontium
0,0
0,2
0,4
0,6
0,8
Aqueous Black Green Pectin Alginate
solution tea tea in aqueous in milk
solution
**
***
***
FIG. 1. Comparison of fractional intestinal strontium absorption (f1 value) from aqueous solution,
black and green tea, pectin and alginate solutions. Oral 86Sr amount of administrated solution: 1 mg.
f1 values were derived plasma samples taken 24 hours after tracer administration. The results are
expressed as mean values with their standard deviation (mean ± SD). ** p < 0.01 and *** p < 0.001
(statistical significance between mean f1 for aqueous solution and green tea, pectin, and alginate).
The data obtained in these controlled human investigations confirm that the uptake of ingested
strontium is reduced with high efficiency from 61% to 3% (20-fold) when administrated together with
alginates; the polyelectrolyte pectin inhibited Sr uptake by a factor of 2.7. The difference in their
effectiveness as strontium binders may depend on their stereochemical configuration, that is, their
particular position and reactivity within the sugar units themselves. Administration of green tea
resulted in a decrease of the f1 value for Sr by a factor of 1.4. Drinking green tea seems to be of lower
effect to reduce the uptake of ingested strontium in humans in comparison to alginate and pectin.
Black tea had no significant effect on the uptake of strontium into the human body.
It is concluded that alginates as well as pectin seem to be effective in binding strontium before its
uptake into the systemic part of the human body, which in turn results in a corresponding reduction of
the effective dose after ingestion of radionuclides of strontium. However, pectin has a higher affinity
to Ca ions in comparison to Sr ions as compared with alginates [24]. Therefore, intake of pectin
preparations for a long time could disturb the normal calcium uptake into the human body [25].
Alginates, in contrast, do not appear to affect calcium absorption in humans [26].
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Acknowledgements
The authors would like to thank Mr. F. Wagner for his valuable technical assistance, Prof. Dr. med T.
Zilker and Dr. med. N. Felgenhauer (Department of Toxicology of the II. Medical Clinic of Technical
University Munich, Germany) for their medical assistance, and Prof. Dr. med. Wagner (Institute of
Microbiology, Immunology and Hygiene, Technical University of Munich) for sterility check of the
tracer ampoules. This study was supported partly by the Commission of the European Communities
(Grant No FIGD-CT-2000-0053).
