Is it possible to eat too much soy?

Hello again

 I had a S1 G1 T1 N0 small ER+ cancer removed in Feb last year . I had one SNB which was negative followed by a course of radiotherapy . My Consultant said he didn’t see any need to give me Aromatase Inhibitors and prescribed Tamoxifen . However , having experienced a central retinal vein occlusion in 2019 which resulted in me losing a lot of sight in my left eye I was concerned about the ocular side effects and then my Oncologist said that as central retinal vein occlusion is a thrombotic event that I should not take Tamoxifen anyway. She recommended Anastrozole which I stopped taking after 5 months due to the joint pains mainly and didn’t want to try Exemestane.

My treatment team are aware of this , my Consultant thought it was reasonable for me to stop it however I’m now having some concerns. I’m 58 my last period was at 55 and blood test showed that I was post- menopausal but I’ve started having some spotting .

I don’t take supplements containing soy but I’ve been eating it regularly with no problems , however I increased my soy intake recently as I’m trying to move towards a healthier plant based diet . When I was still having periods I noticed that I bled more frequently if I was eating / drinking a lot of soy but I didn’t think it could cause this to happen post - menopause. .

I’ve now decided to stop eating soy for a while . I’m wondering if I should ask for another blood test to get my hormone levels re- checked in a few weeks . I don’t know if it’s normal to get occasional spotting post - menopause ? I have a BCN appointment next month and will certainly tell them .

 I’m reluctant to restart Anastrozole but if it turns out I’m making too much oestrogen then I would consider it .Do you or anyone else have any advice ?


Hi JoanneN  

Thanks for posting and asking whether an increased soy intake can cause post-menopausal bleeding or spotting.  

Soy contains isoflavones and lignans, which are weak phytoestrogens. Phytoestrogens are naturally occurring plant substances that have a chemical structure similar to the female hormone oestrogen which is why there has been concern about its safety. But phytoestrogens are not the same thing as female hormones and don’t affect the body in the same way. 

Some studies have suggested that large doses of soy and other phytoestrogens may cause postmenopausal bleeding in some women, although these studies were looking at soy supplements and not at dietary intake.  

There isn’t any evidence to suggest a moderate amount of soy in a person’s diet is harmful, however, the safety of supplements containing soy is yet to be established so they are generally not recommended. 

You may be interested in our information on diet, lifestyle and breast cancer recurrence. Our booklet Diet and breast cancer has further information that you may find helpful. 

Postmenopausal bleeding can have a number of causes and it is important to talk to your GP about the symptoms you are experiencing. They can assess you and refer you for further investigation if necessary. 

If your previous blood tests have shown that you are post-menopausal and you have not had a period for 3 years, it is not usually necessary to recheck your hormone levels. However, it may be helpful to contact your breast care nurse before next month to discuss your concerns with them.  

We offer a range of free supportive services for anyone who has had a diagnosis of breast cancer which you may be interested in. They include face to face and online courses and events. 

You may be interested in our resources that are particularly for those who have come to the end of their main hospital treatment. These are known as our Moving Forward services and include our Moving Forward booklet and Moving Forward courses.  

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