Toxicological evaluation of some food
additives including anticaking agents,
antimicrobials, antioxidants, emulsifiers
and thickening agents
WHO FOOD ADDITIVES SERIES NO. 5
The evaluations contained in this publication
were prepared by the Joint FAO/WHO Expert
Committee on Food Additives which met in Geneva,
25 June - 4 July 19731
World Health Organization
Geneva
1974
1 Seventeenth Report of the Joint FAO/WHO Expert Committee on
Food Additives, Wld Hlth Org. techn. Rep. Ser., 1974, No. 539;
FAO Nutrition Meetings Report Series, 1974, No. 53.
SILICON DIOXIDE AND CERTAIN SILICATES
Explanation
These compounds have been evaluated for acceptable daily intake
by the Joint FAO/WHO Expert Committee on Food Additives (see Annex 1,
Ref. No. 20) in 1969.
Since the previous evaluation, additional data have become
available and are summarized and discussed in the following monograph.
The previously published monograph has been expanded and is reproduced
in its entirety below.
BIOLOGICAL DATA
BIOCHEMICAL ASPECTS
Silica, silicic acid and the calcium, magnesium and aluminium
salts occur ubiquitously in the environment and some have been used
for many years medically. Food contains various amounts of SiO2, for
example: potatoes 10.1, milk 2.1, drinking-water 7.1, mineral-water
22.5, beer 131 gammaSiO2 per g or cm3 (Baumann, 1960).
Very small amounts of silica are normally present in all body
tissues but there is no evidence that they play any physiological
role.
Silicic acid is a normal constituent of the urine, where it is
found as early as a few days after birth. The amount excreted in the
urine, which varies considerably according to the diet, is in the
order of 10 to 30 mg per day (Thomas, 1965). The silica content of
human tissue varies from 10 to 200 mg/100 g dry weight (spleen 15 mg,
lung 140 mg) (Anon., 1964). The normal level of silicic acid in human
blood is below 1 µg SiO2/cm3; the concentration in the corpuscles is
practically the same as that in the plasma. Silicic acid is present in
plasma in a molybdate reactive form and is not bound to protein or any
other substance of high molecular weight. Ingested monomeric silicic
acid rapidly penetrates the intestinal wall and becomes distributed
throughout the whole extra-cellular fluid. It enters the blood
corpuscles at a slower rate (Baumann, 1960).
Silica dust was administered intragastrically to rabbits and dogs
leading to a rise in urinary silica output without significant
variation in blood silica levels. Considerable absorption took place
with peak excretion in dogs occurring between four to eight hours
after administration. There appears to be little retention in any
organ of the body even if animals ingest large amounts of silicates in
their food. Intragastric 5% silicic acid administered to dogs leads to
considerable absorption and urinary excretion, peak excretion
occurring between three to eight hours after dosing. I.v. infusion of
neutralized sodium silicate (1 mg/ml) in dogs leads to rapid urinary
elimination of about 50% of the dose (King et al., 1933).
Rats receiving silica flour, powdered sand or magnesium
trisilicate orally in large amounts were shown to have crystals of
these substances in uninflammated myocardium. Entry was via the
intestinal epithelium (Reimann et al., 1965, 1966). Using
histochemical techniques lysosomal damage was demonstrated in
macrophages which had ingested silica particles (Nadler & Goldfischer,
1970).
Administration of 5 g of the siliceous materials listed below in
20 ml of milk by stomach tube to cats showed the following urinary
excretion within 120 hours; silicic acid (fresh) 43.3, calcium
silicate 37.2, magnesium trisilicate 34.1, silicic acid (moist) 29.0,
SiO2 (quarz) (air sediment very fine) 20.8, magnesium silicates
(talc) 9.2, diatomaceous earth 8.8 and calcium silico aluminate
hydrate (kaolin) 7.6 mg SiO2. From this it can be observed that free
silica is attacked to a variable extent depending on its physical and
chemical condition. Several silicates are apparently unattacked, as
nontreated animals excreted an average of 8.6 mg in 120 hours. Some
complex silicates appear to suffer partial decomposition by the
hydrochloric acid of the stomach, with partial solution of some of the
products in the intestine (King & McGeorge, 1938).
"Quarz water" (water in which silicic acid had been dissolved as
a result of contact with quarz powder) given to rats over a prolonged
period as the only source of liquid intake did not show any SiO2
storage in tissues (Klösterkotter, 1956).
In vitro investigations on slices of tissue showed that
monomeric silicic acid penetrated liver and kidney cells easily,
whereas spleen and muscle cells were more or less impenetrable
(Kirsch, 1960).
Ten gamma monomeric SiO2/cm3 was shown in vitro to damage the
enzymes of isolated mitochondria in rat liver (Hanger et al., 1963).
Very small amounts of silica are normally present in all body
tissues. Recently it was found that silicon meets the criteria for an
essential trace element in the chick (see short-term studies).
TOXICOLOGICAL STUDIES
Special studies on carcinogenicity
See under long-term studies.
Special studies on reproduction
Rat
A two-generation reproduction study with the oral administration
of 100 mg/kg bw per day amorphous silica to rats was conducted
simultaneously. The parent generation (one male and five females)
produced five litters with a total of 25 rats. Half a year later one
male and five females of the first generation were mated; the number
of animals in the second generation was 21. Neither malformation nor
any other adverse effects were noted (Mosinger, 1969).
Acute toxicity
LD50 Reference
Animal Route (mg/kg bw)
Rat Oral 3.16 Elsea, 1958a
The oral LD5 in mice of finely ground silicic acid is >5 g/kg
bw (Kimmerle, 1968).
Rabbits receiving 3 mg of silicon in their conjunctival sac
showed mild irritation for 48 hours (Elsea, 1958a).
The probable lethal dose of silica (oral) for man is over 15 g/kg
bw. The probable oral lethal dose of magnesium trisilicate for man is
over 15 g/kg bw. There is some doubt as to whether large doses cause
laxation. The probable oral dose for man of sodium silicate lies lower
at between 0.5 and 5 g/kg and may well be due to its alkalinity
(Anon., 1964). Data on inhalation toxicity of silica and silicates are
not relevant to consideration of toxic hazard by the oral route.
Short-term studies
Rat
Oral administration of 50 mg amorphous silica to rats for three
months did not cause any toxic effects but no experimental details are
available (Malten & Zielhuis, 1964). Micronized silica gel was fed to
four groups of 10 male rats each in their diet at 0%, 0.2%, 1.0% and
2.5% levels for 28 days. No adverse effects on mortality or abnormal
gross autopsy findings were discovered. Body weight gain was
significantly reduced at the 2.5% level and nearly significantly at
the 1.0% level. No other parameters were examined (Keller, 1958).
Fifteen male and 15 female rats received daily 50 mg of amorphous
polymeric silicone dioxide (99.8 SiO2 content of water-free compound)
by stomach tube for three months. There was no adverse effect on body
weight gain and mortality. Pathology of organs not enumerated showed
no abnormalities in comparison with the controls (Kuschinsky, 1955).
Groups of 15 male and 15 female rats were fed diets containing
silica at concentrations of 0.0, 1.0, 3.0 and 5.0% for 90 days.
A fifth positive control group received a diet containing 3.0%
cosmetic talc. No evidence of systemic toxicity caused by silica was
found in terms of survival, body weights and food consumption. No
appreciable deposition of silicon dioxide was seen in the kidney,
livers, spleen, blood and urine in the animals fed at the 5% level. No
gross or microscopic pathology attributable to silicon were seen
(Elsea, 1958b).
The same compound was fed to 20 male and 20 female rats at a
concentration of 500 mg/kg bw per day for six months. The same number
of animals served as the control. After four-and-a-half months five
females were mated. No adverse effects were noted on mortality, body
weight gain, haematology (haemoglobin, erythroctye- and leucocyte-
count) and reproductive performance. Histopathology of stomach,
intestines, pancreas, liver and kidney of the test group showed no
significant difference to that of the control group. Litter size,
birth weight and morphological development of the offspring as well as
weight gain were normal (Leuschner, 1963).
In a similar experiment (see Leuschner, 1963) the same results
were obtained when feeding a hydrophobic preparation of amorphous
polymeric SiO2 in which some of the silanol groups on the surface
reacted with dimethyl-dichlorosilane (98.5% SiO2 content of water-
free compound) (Leuschner, 1965).
The compound (see Leuschner, 1965) was also fed to groups of five
male and five female rats at levels of 0, 500, 1000, 2000 mg/kg bw for
five weeks. The highest level was raised after 14 days to 4000, after
28 days to 8000 and after 42 days to 16 000 mg/kg bw. When the level
was raised to 16 g/kg, all animals lost weight and four animals died.
Seven days after the level had been raised to 8000 mg/kg, the animals
did not gain weight normally. At 1000 mg/kg two rats out of 10 showed
slight changes in the liver epithelia. At higher levels atrophy of the
liver epithelia, regression of the basophilic structure and glycogen
content were observed. Histopathology of other organs (not enumerated)
including the kidney of the animals at all levels showed no
significant changes compared with the controls (Leuschner, 1964).
Fifteen rats of each sex were fed one of the four silicon
compounds (silicon dioxide, aluminum silicate, sodium silicate and
magnesium trisilicate) for four weeks at the same levels used in the
dog experiment (see below). Polydipsia, polyuria, and soft stools,
seen intermittently in a few animals fed magnesium trisilicate or
sodium silicate, were the only clinical symptoms observed. No
compound-related lesions were seen in any of the rats (Newberne &
Wilson, 1970).
Rabbit
The dermal effects of silica was tested on groups consisting of
two male and two female rabbits at levels of five and 10 g/kg/day. A
negative control group received methyl cellulose solution (0.5% w/w)
and a positive control group received 10 g/kg/day of cosmetic talc.
Applications were made five days/week for three weeks. No evidence of
systemic toxicity caused by silica was found in terms of body weight,
behaviour, silicon content of blood, urine, spleen, liver and kidney.
No gross or microscopic pathology was seen in the major organs
examined or in the skin (Elsea, 1958c).
Dog
Pure-bred beagles of both sexes about six months of age were
fed either silicon dioxide, aluminum silicate, sodium silicate or
magnesium trisilicate for four weeks. The doses used provided
approximately equivalent amounts of silicon dioxide as the end product
(0.8 g/kg/day). Group sizes ranged from six to nine dogs of each sex.
Polydipsia and polyuria were observed in a few animals fed sodium
silicate and magnesium trisilicate. All clinical tests on blood and
urine were within normal limits. However, histopathologic studies
revealed characteristic renal lesions in all dogs fed sodium silicate
or magnesium trisilicate but none in the other groups. The lesions
were visible grossly in all but one animal (Newberne & Wilson, 1970).
Chicken
Day-old deutectomized cockerels were kept in a trace element
controlled environment and fed a synthetic low silicon diet. The diet
of the test groups was supplemented with sodium metasilicate
(Na2SiO3œ9H2O) at a level of 100 mg/kg. 114 chickens were in the
control groups and 114 chickens in the test groups. Growth rates and
the appearance of the animals were evaluated at two- to three- day
intervals. The animals were killed at the end of a 25- to 35- day
period. Gross pathology and histological examinations were carried out
on the organs of each chick. Differences between the chicks on the
basal and silicon-supplemented diets were noted after one to two
weeks. At the twenty-third day of the study the average weight for the
low silicon group was 76 g compared to a weight of 116 g for the
supplemented group (p <0.02). The average daily weight gain for the
control groups was 2.57 g and that of the test groups reached 3.85 g
(p < 0.01). The animals on the basal diet were smaller and all their
organs appeared relatively atrophied as compared to the test chickens.
The leg bones of the deficient birds were shorter, of smaller
circumference and thinner cortex. The metatarsal bones were relatively
flexible and the femur and tibia fractured more easily under pressure
than those of the supplemented group. Thus the effect of silicon on
skeletal development indicates that it plays an important role in an
early stage of bone formation (Carlisle, 1972).
Long-term studies
Rat
Twenty male and 20 female Wistar rats, with starting weights of
70 g, received daily one feed pellet containing amorphous silica
(>98.3% SiO2) prior to feeding for two years. The silica content of
pellets was regularly adjusted in order to ensure a steady consumption
of 100 mg/kg bw per day. The animals were fed a synthetic diet. At the
end of two years the survival rates of both male and female rats were
100%. No adverse effects on behaviour, clinical signs and weight gain
were noted. The pathologic results of test groups were comparable with
those of the controls. No evidence of carcinogenic effects was
obtained.
OBSERVATIONS IN MAN
A single dose of 50 mg of monomeric silicic acid in 50 cm3
liquid was tolerated by two volunteers. Higher doses should be taken
either with more liquid or at intervals of about 20 minutes in order
to avoid polymerization of silicic acid in the urine (Baumann, 1960).
A single dose of 2.5 g of amorphous polymeric silicon dioxide to
volunteers did not significantly raise the SiO2 excretion in the
urine thus suggesting poor absorption of the compound (Langendorf,
1966).
The mean 24-hour excretion of SiO2 in five male subjects on
regular diet was 16.2 mg. The value varied widely and was related to
the amount of SiO2 in the diet. Urinary silica excretion was
increased in healthy subjects when Mg2Si3O8 n H2O was taken by
mouth (Page et al., 1941).
Sixty to 100 g daily for three to four weeks of 12% amorphous
silicic acid administered orally to patients suffering from gastritis
or enteritis were tolerated without adverse effects. Only one-
thousandth of the substance administered was excreted in the urine
(Sarre, 1953).
In experiments with two volunteers, it was shown that, after
ingestion of 50 mg of monomeric silicic acid in 50 cm3 liquid, the
renal excretion of SiO2 per time unit was not related to the quantity
of urine excreted in the same time unit. Maximum excretion appeared
after one to two hours. Even at high concentrations up to more
than 700 µg SiO2/cm3 urine, the silicic acid was still present
in a molybdate reactive form. Silicic acid polymerizes above
100-150 gammaSiO2/cm3. The speed of polymerization is dependent on
pH and concentration. The experiment was designed so as to exclude
damage to the urinary tract through precipitation of proteins by
polymeric silicic acid formed by polymerization of monomeric silicic
acid at high concentrations. If the urine at concentrations in the
order of 700 gammaSiO2/cm3 was taken at longer intervals, such as
two hours, the concentration of monomeric silicic acid was below the
total SiO2 concentration, thus suggesting that some polymerization
had taken place. There was indication of storage breakdown of
reabsorption (Baumann, 1960).
Oral administration of a single dose of 2.5 g of amorphous
polymeric silicon dioxide (99.8% SiO2 content of the water-free
compound) to 12 volunteers caused a slight but statistically
insignificant increase in the silicon dioxide level of the urine
(Langendorf et al., 1966).
Observations in humans indicated that various conditions such as
lung diseases, chronic diseases and especially growth retardation in
children were associated with silicon deficiency. Therefore he
recommended silicon therapy for conditions characterized by under-
developed and/or damaged mesenchymal tissues (Monceaux, 1973).
Comments:
The available data on orally administered silica and silicates,
including flumed silicon dioxide, appear to substantiate the
biological inertness of these compounds. Any silicate absorbed is
excreted by the kidneys without evidence of toxic cumulation in the
body, except for the reported damage to dog kidney by magnesium
trisilicate and sodium silicate. Methods for estimating silica in body
tissues have been greatly improved in recent years making some of the
earlier data somewhat less valuable. A number of short-term studies in
two species are available.
Talc and magnesium silicate are specified free from asbestos-like
particles. This stipulation is made while acknowledging the fact that
existing methods for estimating asbestos-like particles in talc and
magnesium silicate are not yet fully adequate.
EVALUATION
Estimate of acceptable daily intake for man
(a) Silicon dioxide and certain silicates except magnesium silicate
and talc:
Not limited.*
(b) Magnesium silicate and talc:
Temporarily not limited.*
FURTHER WORK OR INFORMATION
Required by June 1976.
(1) For magnesium silicate studies to elucidate the reported kidney
damage in dogs. Long-term feeding studies on talc demonstrated to be
free from asbestos-like particles.
(2) A satisfactory method for estimating asbestos-like particles in
talc and magnesium silicate.
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Elsea, J. R. (1958a) Unpublished report, January 8, from Hazleton
Laboratories, Inc.
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Laboratories, Inc.
Elsea, J. R. (1958c) Unpublished report, May 6, from Hazleton
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Hanger, R., Kirsch, K. & Standinger, Hj. (1963) Beitr. Silikose
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