Objective—To identify and determine the concentrations
of phytoestrogens in commercial dog foods.
Sample Population—24 commercial dog foods,
including 12 moist or dry extruded commercial dog
foods that contained soybeans or soybean fractions
and 12 foods without any soybean–related ingredients
listed on the label.
Procedure—Foods were analyzed for phytoestrogen
content, including 4 isoflavones (genistein, glycitein,
daidzein, and biochanin A), 1 coumestan (coumestrol),
and 2 lignans (secoisolariciresinol and matairesinol) by
use of acid-methanol hydrolysis and high-pressure liquid
chromatography with UV-absorbance detection.
Phytoestrogens were identified and quantified by reference
to authentic standards.
Results—Isoflavones, coumestans, and lignans were
undetectable in diets that did not list soybean–related
ingredients on the label. Only 1 of the 12 diets that
included soybean or soybean fractions had undetectable
concentrations of phytoestrogens and that
product contained soy fiber. The major phytoestrogens
were the isoflavones daidzein (24 to 615 µg/g of
dry matter) and genistein (4 to 238 µg/g of dry matter).
Conclusions and Clinical Relevance—Soybean and
soybean fractions are commonly used ingredients in
commercial dog foods. Dietary intake of phytoestrogens
may have both beneficial and deleterious health
effects. Our results indicated that certain commercial
dog foods contain phytoestrogens in amounts that
could have biological effects when ingested longterm.
( Am J Vet Res 2004;65:592–596)
Objective—To compare effects of short-term administration
of a soy diet with those of a soy-free diet on
serum thyroid hormone concentrations in healthy
Animals—18 healthy adult cats.
Procedure—Cats were randomly assigned to receive
either a soy or soy-free diet for 3 months each in a
crossover design. Assays included CBC, serum biochemical
profile, thyroid hormone analysis, and measurement
of urinary isoflavone concentrations.
Results—Genistein, a major soy isoflavone, was
identified in the urine of 10 of 18 cats prior to dietary
intervention. Compared with the soy-free diet, cats
that received the soy diet had significantly higher total
thyroxine (T4) and free T4 (fT4) concentrations, but
unchanged total triiodothyronine (T3) concentrations.
The T3/fT4 ratio was also significantly lower in cats
that received the soy diet. Although the magnitudes
of the increases were small (8% for T4 and 14% for
fT4), these changes resulted in an increased proportion
of cats (from 1/18 to 4/18) that had fT4 values
greater than the upper limit of the laboratory reference
range. There was no significant effect of diet on
any other measured parameter.
Conclusions and Clinical Relevance—Short-term
administration of dietary soy has a measurable
although modest effect on thyroid hormone homeostasis
in cats. Increase in T4 concentration relative to
T3 concentration may result from inhibition of 5'-iodothyronine deiodinase or enhanced T3 clearance.
Soy is a common dietary component that increases
serum T4 concentration in cats. ( Am J Vet Res 2004;