Over the years people I have known who were undergoing treatment for Cancer have shared with me that their doctors advised them to avoid flax seeds, soy milk and tofu because these contain large amounts of phytoestrogens,which their doctor told them could bind with estrogen receptors and stimulate the growth of cancer. When someone would tell me this, I would always then ask them this question: |
And I these people have always told me some version of this:
"No, my doctor said that those are fine for me to eat."
This never made any sense to me -- as mother's milk (and this is true for milk from all mammal mothers -- cows, people, dogs etc.) is actually a hormone-delivery system made by nature to enable a nursing mother to deliver dozens of hormones (not just estrogen, but many others too including cortisol, testosterone and IGF-1) to her baby that stimulate growth -- and the exact percentage of each each of these hormones, as well as the amounts of protein, and fat in mother's milk, is quite different for each species, depending upon how quickly the infants are intended to double their birth weight. Cow's milk for example is designed to put 200-300 lbs on a calf in just six months.
Well just recently someone else I know told me that they are avoiding flax seeds because of concern about cancer -- and again when I asked, I found out that they are still consuming animal products and specifically mentioned butter, which they are not at all concerned about the actual animal estrogen that is present in butter.
Thinking about this, I became curious about how the amounts of phytoestrogens in flax seeds compared to the amounts of actual animal estrogen in butter, and so I reached out Brenda Davis, a dietician who is known around the world, as the author of about a dozen books addressing plant-based nutrition. You may recognize some of them:

Food Item Phytoestrogen Content (mg/100g)
Flaxseed 380
Soybeans 104
Tofu 27
Tempeh 18
Miso 11
Note: Flaxseeds are particularly rich in lignans, a type of phytoestrogen. Healthline+1Wikipedia+1
Estrogen Content in Animal-Based Foods:
Food Item Estrogen Content (pg/g)
Butter 539
Gouda Cheese 170
Whole Milk 33.7
Note: These values represent picograms of estrogen per gram of food. ScienceDirect
- Phytoestrogens are plant-derived compounds that can mimic estrogen but generally have much weaker effects.
- Estrogens in animal products are actual hormones that can interact directly with human estrogen receptors.
- Flaxseed: Contains approximately 380 mg of phytoestrogens per 100g, primarily lignans.Mount Sinai Health System+4Healthline+4PurpleCarrot Nutrition+4
- Butter: Contains about 539 picograms (pg) of estrogen per gram.ScienceDirect+1PubMed+1
1 milligram (mg) = 1,000,000 picograms (pg)
Therefore, 380 mg (phytoestrogens in flaxseed) = 380,000,000 pg
Implications:
While flaxseed has a high phytoestrogen content, these compounds are structurally different from the estrogens found in animal products like butter and cheese. Phytoestrogens tend to have weaker effects on estrogen receptors and can sometimes act as anti-estrogens, potentially offering protective benefits against certain cancers. In contrast, the estrogens in animal products are bioidentical to human estrogens and may have more direct hormonal effects.
Understanding the differences in estrogenic compounds between plant-based and animal-based foods is crucial. While both contain compounds that can interact with estrogen receptors, their sources, structures, and potencies vary significantly, and plant estrogens tend to be protective.
1TBS of flax weighs 9 gms, and dividing the number you gave me (for 100 gms) by 100, I get 3,800 pg in each gram, and 1 TBS of butter weighs 14 grams, and at 539 pg/g that's 7,506 pg of estrogen:
1 TBS of flax seeds has 34,200 pg of phytoestrogen.
vs
1 TBS of butter has 7,506 pg of estrogen
The key factor here isn't just the quantity of estrogenic compounds, but rather their biological activity and how they interact with the body’s hormone system.
1. Potency Matters: Weak vs. Strong Estrogens
- Animal estrogens (like estradiol found in butter and dairy) are bioidentical to human estrogens, meaning they directly bind to estrogen receptors and exert a strong hormonal effect. Even at lower concentrations, they can significantly influence hormonal balance and have been linked to increased risks of hormone-sensitive cancers, such as breast and prostate cancer (Ganmaa & Sato, 2005).
- Phytoestrogens from flax and soy, on the other hand, are structurally different and have much weaker estrogenic activity—often 100–1,000 times weaker than human or animal estrogens. In many cases, they can act as estrogen blockers, reducing the impact of stronger estrogens in the body (Adlercreutz, 2002).
- Animal estrogens activate estrogen receptors strongly, directly contributing to hormonal activity.
- Phytoestrogens, especially lignans from flax, tend to bind preferentially to estrogen receptor beta (ER-β), which has an anti-proliferative effect—potentially reducing cancer risk rather than increasing it (Kuiper et al., 1998).
- Dairy estrogens are absorbed efficiently and enter circulation in their active form, exerting a direct hormonal impact.
- Phytoestrogens require metabolic conversion, and the extent of their effect depends on gut microbiota composition and individual metabolism (Setchell et al., 1984). Some individuals convert them into equol, a metabolite that has a beneficial, mild estrogen-modulating effect.
- High dairy consumption is associated with increased risks of hormone-sensitive cancers, particularly breast and prostate cancer, due to its estrogen content (Ganmaa et al., 2002).
- Flax and soy intake, despite their high phytoestrogen content, are linked to lower rates of these cancers, suggesting a protective rather than harmful effect (Messina, 2016).
- The potency difference (animal estrogens are much stronger)
- The receptor binding differences (phytoestrogens can block stronger estrogens)
- The epidemiological data (phytoestrogens tend to be protective, not harmful)
Instead of just comparing absolute amounts, a better question is:"Which source of estrogenic compounds is more likely to raise estrogen levels in the body and contribute to hormone-related conditions?"
Dairy estrogens: Strong activators, increase estrogen exposure → Potentially harmful
Flax phytoestrogens: Weak, can block stronger estrogens → Potentially protective
Thus, avoiding flax due to concerns about estrogen while continuing dairy consumption is biologically inconsistent with what we know about how these compounds function.