paper advising caution w soy (genistein) intake while on letrozole (AI)
 

Carcinogenesis. 2008 Jul 16. [Epub ahead of print]

Dietary Genistein Negates the Inhibitory Effect of Letrozole On The Growth Of Aromatase-expressing Estrogen-Dependent Human Breast Cancer Cells (MCF-7Ca) In Vivo.

Ju YH, Doerge DR, Woodling KA, Hartman JA, Kwak J, Helferich WG.
Department of Food Science and Human Nutrition, University of Illinois, 905 S Goodwin, Room 580 Bevier Hall, Urbana, IL.
Genistein (GEN), a soy isoflavone, stimulates growth of estrogen-dependent human tumor cells (MCF-7) in a preclinical mouse model for post-menopausal breast cancer. Anti-estrogens and aromatase inhibitors are front-line therapies for estrogen-dependent breast cancer. We have demonstrated that dietary GEN can negate the inhibitory effect of tamoxifen. In this study we evaluated the interaction of dietary GEN (at 250 - 1000 ppm in the AIN93G diet) and an aromatase inhibitor, Letrozole (LET), on the growth of tumors in an aromatase-expressing breast cancer xenograft model (MCF-7Ca) in the presence and absence of the substrate androstenedione (AD). Dietary GEN (250 and 500 ppm) or implanted AD stimulated MCF-7Ca tumor growth. Implanted LET inhibited AD-stimulated MCF-7Ca tumor growth. In the presence of AD and LET, dietary GEN (250, 500, and 1000 ppm) reversed the inhibitory effect of LET in a dose-dependent manner. Uterine wet weight, plasma estradiol (E(2)) levels (ELISA), and total plasma GEN and LET levels (LC-ES-MS/MS) were measured. Ki-67 (cellular proliferation), aromatase, and pS2 protein expression in tumors was evaluated using immunohistochemical (IHC) analysis. In conclusion, dietary GEN increased the growth of MCF-7Ca tumors implanted in ovariectomized mice and could also negate the inhibitory effect of LET on MCF-7Ca tumor growth. These findings are significant because tumors, which express aromatase and synthesize estrogen, are good candidates for aromatase therapy, dietary? GEN can reverse the inhibitory effect of LET on tumor growth and adversely impact breast cancer therapy. Caution is warranted for consumption of dietary GEN by postmenopausal women with estrogen-dependent breast cancer taking LET treatment.
PMID: 18632754

Thanks for posting this thread. You are always so good in spreading latest info for all of us.
Femara is my only defense against recurrence. I am 63, therefore, and fall into the class of patients included in this mouse study.
Do you or anybody know whether Tofu and soy derived products are equivalent with the soy genistein? I eat a lot of these products since I am not a meat eater. I got my protein from these sources.
If I must stay away from soy products, are eggs good replacement as a source of protein? Milk products are another group of substitutes, I suppose.
Thanks for the article as well as your answer.

as I understand it genistein is a protein found within the soy bean
 

I think it is found in tofu, but unsure if it is found in miso or soy sauce, but there are several compounds with soy that are worrisome for er+ breast cancer patients as well as some compounds which might turn out to be preventative (it is unclear as to what is the best form, amount, timing ie, consumption prior to puberty vs later vs exposure in utero). I doubt the research to answer these questions will happen soon in the present research environment as no drug company will pay for clinical trials to look into this (and they would be exceedingly hard to do as our diets/environmental exposures involve too many variables)

Eggs and milk are said to be good sources of all the essential amino acids.

Hope this helps

Other sources of protein besides dairy is the correct combinations of rice and legumes/beans. This route of protein is great because it is low in fat and high in fiber. Beans and legumes also contain bioflavoids and B vitamins.

Soy in the form of edamame, tofu, soy milk would all be included in the study.

Occassional use of these products should pose no problem (like once a week having tofu or once a week having some soy milk - but not once a week for every form. Having something once a week). I do not avoid soy but I never really ate it before so if I have chinese food and it has tofu, I eat it. I do have flax once a week though because I feel it has value although it may fall into the same phytoestrogen category as soy.