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Department of Nutritional Sciences Oklahoma State University Stillwater, OK 74078
Dear Editor:
We thank Dr. Oakenfull for his comments on our recent publication
(1
) regarding the possible cholesterol-lowering role
of saponins in soy. As we mentioned in this publication, the health
claim of the U.S. Food and Drug Administration pertaining to soy
protein (2
) does not specify whether the
cholesterol-lowering benefits are derived from the protein itself,
its nonprotein constituents or their combination. Our findings suggest
that soy protein, when depleted of its nonprotein components, e.g.,
isoflavones and saponins, does not prevent the ovarian hormone
deficiency-associated rise in blood cholesterol in hamsters.
However, the question remains as to what compounds, when extracted from
soy protein, render it ineffective in lowering cholesterol. During the
extraction process of isoflavones, saponins are also removed. Hence,
the failure of ethanol-extracted soy protein to prevent the rise in
cholesterol due to ovariectomy may in part be credited to its saponin
content. In a recent study by our laboratory (3
), the
addition of isoflavone-rich extract to the diets of 6-mo-old
ovariectomized female Golden Syrian hamsters for 120 d clearly
produced a dose-dependent response capable of preventing
hypercholesterolemia. Unfortunately, the saponin content of this
isoflavone-rich extract was not known; thus, the question remains
unanswered. The cholesterol-lowering effect of isoflavones in this
study (3
) was further complicated by the fact that the
isoflavone-rich extract was added to a casein-based diet.
Saponins have been reported to be effective in reducing serum
cholesterol concentrations in the context of casein-based diets
(4
), and, hence, it is difficult to distinguish between
the isoflavones and saponins in preventing hypercholesterolemia.
However, the independent hypocholesterolemic effect of isoflavones
added to the diet can be confirmed when isoflavones in pure forms are
used. Uesugi and colleagues (5
) recently found that
daidzin and glycitin, two of the major isoflavones in soy, prevented
the unfavorable changes in lipid metabolism in ovariectomized rats.
Similarly, we have observed that genistin, another major isoflavone in
soy, and the synthetic isoflavone, ipriflavone, prevented the rise in
serum total cholesterol when added to casein-based diets of
ovariectomized hamsters (6
). Considering the evidence that
both these groups of compounds present in soy, i.e., isoflavones and
saponins, may independently lower cholesterol, dose-response
studies are needed in which pure saponins, isoflavones, and their
combination are evaluated for their independent, additive or
synergistic effects on lipid metabolism using the same animal model
because the response across species may differ.
Manuscript received 29 June 2001. Revision accepted 9 August 2001.
LITERATURE CITED
1. Lucas, E. A., Khalil, D. A., Daggy, B. P. & Arjmandi, B. H. (2001) Effect of ethanol extracted soy protein isolate in a hamster model of postmenopausal hypercholesterolemia. J. Nutr. 131:211-214.
2. U.S. Food and Drug Administration (1999) Food Labeling: Health Claims; Soy Protein and Coronary Heart Disease; Final Rule. Federal Register 64 FR 57699 October 26. .
3. Devareddy, L., Lucas, E. A., Khalil, D. A., Juma, S., Hammond, L., Hodges, B. S., Sinichi, N., Daggy, B. P. & Arjmandi, B. H. (2001) Soy isoflavones dose-dependently lower plasma cholesterol in ovarectomized hamster. FASEB J 15:A991.
4. Oakenfull, D. G., Topping, D. L., Illman, R. J. & Fenwick, D. E. (1984) Prevention of dietary hypercholesterolemia in the rat by soya and quillaja saponins. Nutr. Rep. Int. 29:1039-1046.
5. Uesugi, T., Toda, T., Tsuji, K. & Ishida, H. (2001) Comparative study on reduction of bone loss and lipid metabolism abnormality in ovariectomized rats by soy isoflavones, daidzin, genistin, and glycitin. Biol. Pharm. Bull. 24:368-372.[Medline]
6. Lucas, E. A., Devareddy, L., Khalil, D. A., Juma, S., Hammond, L., Hodges, B. S., Sinichi, N., Daggy, B. P. & Arjmandi, B. H. (2001) Ipriflavone prevents the ovariectomy-induced hypercholesterolemia in hamsters. FASEB J 15:A991.
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