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Perhaps many researchers are unaware of the profound differences
between the two diets. Purified diets are made with refined
ingredients, such as casein, corn starch, sucrose, corn oil, and a
fixed amount of vitamins and minerals. A scientist has complete
control over the ingredients added or subtracted (1).
In contrast, grain-based chow diets are made with ground plants,
such as corn, wheat, oats, soybean, and alfalfa. There’s a problem –
plants vary nutritionally from harvest to harvest, and some,
including soy and alfalfa, contain significant amounts of
biologically active compounds called phytoestrogens, which also
vary between harvests (2).
Phytoestrogens in chow add potentially powerful
variables to experiments
Phytoestrogens are abundant in soy meal – the major protein source
in most commonly-used laboratory chows; and a subclass of
chemicals in the phytoestrogen family known as isoflavones are major
culprits (3). Isoflavones interact with estrogen receptors and elicit
estrogenic or anti-estrogenic effects (4). Therefore, they can interfere
with studies influenced by exogenous estrogens. Estrogens regulate a
variety of physiological pathways. They are involved in the growth
and function of reproductive tissues of both males and females, as
well as maintenance of the skeletal, central nervous and cardiovascular
systems. Hence, isoflavones can interfere with countless studies and
even render the interpretation of results meaningless.
To date, numerous studies in humans and animals leave little doubt
that isoflavones in soy affect a variety of experimental endpoints.
A brief mention of some examples follows, and extensive reviews on
this topic have been published (5, 6, 7) FIG. 1.
Cancer. The low incidence of many cancers in Asian countries,
where soy consumption is high, sparked an interest in soy’s potential
protective effects (8). As partial estrogen antagonists, isoflavones in soy
were thought to play a role in preventing cancer. Indeed, laboratory
research has confirmed the epidemiological findings. For example,
rodents consuming isoflavones have fewer tumors and/or a delay
in tumor development in mammary, liver, colon, and prostate
cancer (9, 10, 11, 12). Experimental results sometimes conflict and
the outcome likely depends on a variety of factors, including the
isoflavone amount, duration of feeding, and hormonal state of the
animal. Still, the evidence for isoflavones’ role in cancer is mounting
and must not be dismissed as insignificant in cancer research.
Metabolic Syndrome. Dietary soy has been shown to attenuate
the development of hypertension in spontaneously hypertensive
rats (13). Soy isoflavones have also been shown to reduce serum
cholesterol and triglycerides (14, 15), and prevent the development
of fatty liver (16). Rats fed high fat diets with soy as the protein
source gained less weight and had less body fat compared with
those fed high-fat diets with casein (17). Insulin-sensitivity was also
improved in soy-fed mice (18).
Osteoporosis. Studies in both humans (post-menopausal women)
and animal models of osteoporosis (ovariectomized rodents) show
bone sparing effects of soy isoflavones (19, 20), and the results in
animals are remarkably consistent.
Reproductive Development. Certain levels of isoflavones are
capable of accelerating puberty and stimulating uterine growth in
female rodents (21). In male rodents, a decrease in the weight of the
prostate gland and sexual organs, as well as a decrease in testosterone,
has been reported (22, 23). A literature review reveals inconsistent
findings, and, as with other endpoints, likely depends on the dose and
other factors. Yet, scientists seem to agree that dietary phytoestrogen
content must be considered in studies directly or indirectly involving
sexual development.
Behavior. Since estrogen receptors are expressed in the brain, it’s
no surprise that dietary isoflavones affect behavioral parameters.
Effects on learning and memory, social interaction, anxiety-related
behaviors, locomotor activity, and pain sensitivity have been
reported (6, 24).
A cautionary tale
Mounting empirical evidence to date has implicated isoflavones in
a wide range of experimental endpoints, leaving little doubt that,
when choosing a diet, careful consideration must be given to the
role of these compounds in mammalian physiology. After all, can
meaningful conclusions really be drawn if the control diet itself has
protective or adverse effects on the phenotype one is studying?
Nutrition scientists who, surprisingly, use grain-based diets as a
control for purified diets must be made aware of these differences.
In addition, the knowledge must be imparted to scientists studying
the efficacy of drugs on a variety of diseases that may be affected
by compounds in soy. These include osteoporosis, cancer, heart
disease, hypertension, pain sensitivity, and psychiatric disorders.
Understandably, many researchers are reluctant to switch from
grain-based chows to purified diets. They dare not change the diet
in ongoing experiments, since these rely heavily on comparisons to
historical data. Yet, a strong argument can be made for a break
from the past: Was the “same diet” actually used over the years?
The answer is no, since chows vary from batch to batch. Indeed,
dietary phytoestrogen variability has likely caused a frustrating
variability in data, or a misinterpretation of results, in many studies.
Does removing the phytoestrogen source from
grain-based chow fix the problem?
In recognition of the large body of evidence of phytoestrogens’
effects, some chow manufacturers now sell chow without soy meal
or alfalfa. Instead, the ingredients include ground corn, wheat
middlings, and ground wheat. The phytoestrogens in these diets are
minimized but not eliminated. Importantly, these ingredients are
not refined. Therefore, it is impossible to know which yet-to-be-discovered
biologically active compounds are in the diet. Indeed, in
1940, an effect of phytoestrogens was noticed for the first time –
when it was observed that sheep grazing on red clover, a
phytoestrogen-rich plant, were less fertile (25). It can certainly be argued that there’s no such thing as a perfect diet
in laboratory research, but at the very least, for experiments to be
meaningful, we should strive to eliminate dietary variables, especially
those known to elicit biological responses – namely, phytoestrogens. |
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Literature References
1. Ricci MR and Ulman EA. Laboratory Animal Diets: a critical part of your in vivo research.
Animal Lab News 4(6): 1-5, 2005.
2. Thigpen JE, Setchell KDR, Ahlmark KB, Locklear J, Spahr T, Caviness GF, Goelz MF,
Haseman JK, Newbold RR, and Forsythe DB. Phytoestrogen content of purified, open- and
closed-formula laboratory animal diets. Laboratory Animal Sciences 49(5): 530-536, 1999.
3. Brown NM and Setchell KDR. Animal models impacted by phytoestrogens in commercial chow:
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2001.
4. Ososki AL, Kennelly EJ. Phytoestrogens: a review of the present state of research. Phytotherapy
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5. Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment (COT).
www.food.gov.uk/multimedia/pdfs/phytoreport0503, 2003.
6. Jensen MN and Ritskes-Hoitinga M. How isoflavone levels in common rodent diets can interfere
with the value of animal models and with experimental results. Laboratory Animals 41: 1-18,
2007.
7. Thigpen JE, Setchell KD, Saunders HE, Haseman JK, Grant MG, Forsy DB. Selecting the
appropriate rodent diet for endocrine disruptor research and testing studies. ILAR Journal 45(4): 401-416, 2004.
8. Yildiz F. Phytoestrogens in Functional Foods. CRC Press, Inc., 2006.
9. Barnes S. Effects of genistein on in vitro and in vivo models of cancer. Journal of Nutrition 125:
777S-783S, 1995.
10. Barnes S, Grubbs C, Setchell KDR, and Carlson J. Soybeans inhibit mammary tumors in models of breast cancer. Prog. Clinical Biology Research 347: 239-253, 1990.
11. Barnes S, Peterson TG, Grubbs C, and Setchell KDR. Potential role of dietary isoflavones in the prevention of cancer. Advances in Experimental Medicine and Biology 354: 135-147, 1994.
12. Fitzsimmons JTR, Orson NV, and El-Aaaser. Effects of soybeans and ascorbic acid on experimental carcinogenesis. Comparative Biochemistry and Physiology 93A: 285-290, 1989.
13. Cho TM, Peng, N, Clark JT, Novak L, Roysommuti S, Prasain J, and Wyss JM. Genistein attenuates the hypertensive effects of dietary NaCL in hypertensive male rats. Endrocrinology 148(11): 5396-5402, 2007.
14. Bakhit RM, Klein BP, Essex-Sorlie D, Ham JO, Erdman JW, and Potter SM. Intake of 25 g of soybean protein with or without soybean fiber alters plasma lipids in men with elevated cholesterol concentrations. Journal of Nutrition 124: 213-222, 1994.
15. Carroll KK and Kurowska EM. Soy consumption and cholesterol reduction: review of animal and human studies. Journal of Nutrition 125: 594S-597S, 1995.
16. Ascencio C, Torres N, Isoard-Acosta F, Gomez-Perez FJ, Hernandez-Pando R, and Tovar AR. Soy affects serum insulin and hepatic SREBP-1 mRNA and reduces fatty liver in rats. Journal of Nutrition 134:522-529, 2004.
17. Torre-Villalvazo I, Tovar AR, Ramos-Barragan VE, Cerbon-Cervantes MA, and Torres N. Soy protein ameliorates metabolic abnormalities in liver and adipose tissue of rats fed a high fat diet. Journal of Nutrition 138: 462-468, 2008.
18. Cederroth CR, Vinciguerra M, Gjinovci A, Kuhne F, Klein M, Cederroth M, Caille D, Suter M, Newmann D, James RW, Doerge DR, Wallimann T, Meda P, Foti M, Rohner-Jeanrenaud F, Vassalli J, and Nef S. Dietary phytoestrogens activate AMP-activated protein kinase with improvement in lipid and glucose metabolism. Diabetes 57: 1176-1185, 2008.
19. Messina M, Ho S, Alekel DL. Skeletal benefits of soy isoflavones: A review of the clinical trial and epidemiological data. Current Opinion in Clinical Nutrition and Metabolic Care 7: 649-658, 2004.
20. Picherit C, Chanteranne B, Bennetau-Pelissero C, Davicco MJ, Lebecque P, Barlet JP, and Coxam V. Dose-dependent bone-sparing effects of dietary isoflavones in the ovariectomised rat. British Journal of Nutrition 85(3): 307-316, 2001.
21. Casanova M. You L, Gaido KW, Archibeque-Engle, Janszen DB, and Heck, H. Developmental effects of dietary phytoestrogens in Sprague-Dawley rats and interactions of genistein and daidzein with rat estrogen receptors alpha and beta in vitro. Toxicological Sciences 51: 236-244, 1999.
22. Weber KS, Setchell KDR, Stocco DM, and Lephart ED. Dietary soy-phytoestrogens decrease testosterone levels and prostate weight without altering LH, prostate 5 alpha-reductase or testicular steroidogenic acute regulatory peptide levels in adult male Sprague-Dawley rats. Journal of Endocrinology 170: 591-599, 2001.
23. Wisniewski AB, Klein SL, Lakshmanan Y, and Gearhart JP. Exposure to genistein during gestation and lactation demasculinizes the reproductive system in rats. Journal of Urology 169: 1582-1586, 2003.
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25. Johnston I and Williamson G. Phytochemical Functional Foods. CRC Press, Inc., 2003. |
