Beynen AC, 2019. Sorbate and sorbitol in petfoods

Abstract

Sorbate and sorbitol in petfoods

Full text

Petfood Magazine 2019; Nr 2: 24-25.
Anton C. Beynen
Sorbate and sorbitol in petfoods*
*Based on article in Dutch (1)
Main points
Sorbate and sorbitol are six-carbon chains that serve as petfood preservatives. Sorbate is the acid
residue of sorbic acid (hexa-2,4-dienoic acid), a polyunsaturated, medium-chain fatty acid. Potassium
sorbate is frequently found on labels of dry petfoods. Sorbitol is a polyalcohol with a hydroxyl group
attached to each of its carbon atoms. It is incorporated into some semi-moist petfoods and treats.
Common petfood applications of potassium sorbate are 0.01 to 0.2% in the finished products; for
sorbitol the inclusion range is 3 to 12% (2). In Europe, potassium sorbate is legally classified as
technological additive for feedstuffs (3), while sorbitol is listed as feed material (4).
Potassium sorbate and sorbitol, as declared by petfood labels, are chemically synthesized
compounds. Such chemicals are label unfriendly: many dog and cat owners have an aversion to
synthetic petfood components. An American pet store chain has announced that by February 2020
all food products with potassium sorbate will be removed from the shelves (5). Sorbitol not only
serves as petfood preservative, but also as sugar substitute and oral/rectal laxative. Those
applications may evoke or amplify negative associations in the minds of pet owners.
There is little information on the metabolism of sorbate and sorbitol by dogs and cats, but combining
data from various sources gives a fair view. Sorbate likely is efficiently absorbed and then enters the
hepatic pathway of fatty acid oxidation, yielding ATP, carbon dioxide and water as end products.
Alternatively, sorbate may be converted into ketone bodies. Sorbitol is slowly and inefficiently taken
in. Absorbed sorbitol probably is partly converted into glucose and partly excreted with urine.
Data on toxicity of sorbate (cf. 6) and sorbitol in dogs are limited, while two major research reports
are available in abstract form only. As to cats, the information is even scarcer. When integrating the
accessible archives on toxicological and metabolic characteristics of sorbate and sorbitol, it follows
that their maximum contents in commercial, complete dog and cat foods are safe. This stand is
supported by lack of signals from the market about suspected associations between the
preservatives and negative health effects in dogs and cats.
Sources
In bound form, sorbic acid occurs as a natural compound. Potassium sorbate can be isolated from
berries of the rowan tree (Sorbus aucuparia), after their treatment with potassium hydroxide (7).
Fresh berries contain the equivalent of about 0.1% sorbic acid. There are cosmetics and skin care
products with potassium sorbate derived from rowan berries (8). Sorbic acid is factory synthesized
by a reaction between propanedioic acid (malonic acid) and 2-butenal.

Sorbitol occurs naturally, amongst others, in apples, pears, prunes and berries (9). Dried prunes
contain about 10% sorbitol. Industrial sorbitol is usually synthesized with corn starch as starting
material. Glucose is liberated by hydrolysis of the starch, followed by reduction of its aldehyde
moiety into a hydroxyl group, thus yielding sorbitol.
Preservative action
Sorbic acid and its potassium, sodium and calcium salts are allowed as feed preservatives (3). For
petfood, potassium sorbate is typically used, possibly due to its greater solubility in water. In
practice, the dose of potassium sorbate is 0.01 to 0.2% of the finished product (2). In an aqueous
medium, potassium sorbate dissociates after which sorbic acid also arises. The acid can pass the
cellular membrane of fungi and bacteria, accumulate in the cellular liquid and disturb metabolism,
resulting in growth retardation of the microorganisms.
Sorbitol is a humectant: the hydroxyl groups bind free water. Water with dissolved nutrients acts as
growth medium for fungi and bacteria, whereas a water-poor medium leads to dehydration of the
microorganisms. The addition of sorbitol to semi-moist petfood lowers the amount of free water,
thereby blocking growth of fungi and bacteria. Applications of sorbitol range from 3 to 12% in semi-
moist food (2).
Metabolism
In dogs, the digestibility of medium-chain fatty acids in dietary triacylglycerols is higher than that of
longer-chain fatty acids (10). Thus, ingested sorbate likely is efficiently absorbed by dogs and cats.
The metabolic, step-wise processes of fatty acid oxidation and coupled ATP formation probably are
comparable for dogs, cats and rats. The latter species completely degrades orally administered
sorbic acid to carbon dioxide and water (11), following hepatic beta-oxidation and/or ketogenesis.
Furthermore, the intermediary metabolism of sorbic acid was found to be identical with that of
caproic acid (hexanoic acid), pointing to similar caloric values of the two medium-chain fatty acids
(11).
In rats, radioactivity in blood and expired air was measured after oral administration of 14C-labeled
sorbitol or glucose (12). Compared with glucose, radioactivity from sorbitol appeared in blood and
expired air at lower rates. This outcome corresponds with the concepts of slow, passive absorption
of sorbitol and hepatic conversion of sorbitol into glucose, via fructose.
In 25-kg dogs, a test meal consisting of a mixture of 276 g wet food and 25 g sorbitol slightly raised
blood glucose and serum insulin concentrations (13). Sorbitol intake was equivalent to feeding dogs
a dry food with 6.7% sorbitol once a day. Under that condition, some of the absorbed sorbitol
probably is excreted in urine (14). The caloric value of sorbitol in dogs may be substantially lower
than that of glucose.
Sorbate: toxicity

There are two communications on the toxicity of potassium sorbate in dogs. A brief notice (15) reads
as follows: “.... eight dogs received 1 percent and eight dogs 2 percent potassium sorbate in the diet
for 3 months and gained the same weight as four control dogs. At autopsy, gross examination
revealed no deleterious effects attributable to potassium sorbate. “
Puppies received a semi-purified control food (n = 3) or food with 5% caproic acid (n = 3) or 5%
sorbic acid (n = 2) in the dietary dry matter (16). For formulation of the test foods, the acids were
added to the control food at the expense of 2.5% margarine fat and 2.5% sucrose. Food intake,
growth and blood hemoglobin were comparable for the three groups. The dogs’ remained in
excellent condition. Examination of organs and tissues did not reveal visible aberrances.
For four weeks, groups of two young male cats with comparable body weight received a commercial
canned food with 0, 0.1, 0.2, 0.5 or 1% sorbic acid (17). The authors gave the following description of
the results: “Sorbic acid was readily acceptable at all dose rates, and the clinical behavior and
appearance of all the cats remained normal throughout the course of the trial.” Two additional cats
were fed for one week at a level of 2%, this dosage not producing any toxic effects.
Sorbitol: toxicity
Dogs fed a dry, extruded kibble diet with 7.2% sorbitol instead of beet pulp had unchanged, good
feces quality. When the sorbitol was level was doubled, the extra addition being at the expense of
brewers’ rice, the dogs did not experience diarrhea, but looser stool was observed (13). The animals
were allowed to eat the diet at their leisure and were not given a large dose at a single time.
Perhaps, some dogs do develop diarrhea when fed a semi-moist diet with 12% sorbitol once daily.
Dietary sorbitol concentrations higher than those studied (13) may not only induce diarrhea, but also
polyuria. Due to the low absorption efficiency, significant amounts of sorbitol will reach the large
intestine, resulting in excessive bacterial fermentation and osmotic diarrhea. In dogs, a considerable
fraction of intravenously administered sorbitol reached the urine (14). This may be associated with a
diuretic effect (18), due to water dragging by the excreted sorbitol. The dietary level at which
sorbitol causes diarrhea and polyuria is unknown.
In excerpt form, a two-year study has been described in which 8 female and 8 male Beagle dogs
consumed a food without or with 20% sorbitol (19). Sorbitol feeding was found to increase weight
gain, while relative organ weights were unchanged or slightly lowered. There were no sorbitol-
related, visible tissue changes. Feces consistency and urine production are not mentioned.
For three days, three times daily, dogs were administered sorbitol by stomach tube (20). A dose of
12 ml distilled water with 25% sorbitol had no visible, irritating effect on the gastrointestinal mucosa
in 9 out of 10 dogs. Three ml with 90% sorbitol caused irritation in 5 of 10 dogs, possibly because the
compound in high concentration is dehydrating to exposed tissue. The observations make clear that
a semi-moist food with 12% sorbitol does not have a gastrointestinal irritant effect.
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