Updated: Nov 23, 2022
If you look online you can find a lot about the role of phytoestrogens, peas, flax, and legumes in human fertility, but what about their impact on canine fertility?
I put on my science hat and did a very deep dive into this question and found that while legumes and other plant-based foods high in phytoestrogens CAN affect canine fertility, it's not what most of us thought.
Not even close.
Yes, there are some phytoestrogens that can absolutely have a negative impact on canine fertility, but there’s only one really bad culprit, and most of the others are neutral or positively affect health and fertility in dogs. So if you're wondering whether to include these foods in your dog's diet, the answer is probably yes (with one notable exception)!
This is a long blog post, but I needed to explain the biochemistry and history of both the evidence and some myths to explain what you need to know as a responsible breeder about phytoestrogens, legumes, flax, peas and fertility.
There’s a list of take-home messages at the end, but please try to read everything. You’ll not only want to understand it for yourself, but you’ll want to explain it to others.
If you’re not a reader and prefer a video explanation or want to explore this and other breeding nutrition topics in detail, check out the full course on Breeding Nutrition.
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Estrogens and Phytoestrogens
Do you know the difference between estrogen and phytoestrogen?
Most people don’t, but it’s an important distinction to understand.
Estrogen is a hormone produced by all vertebrates (and some insects) while phytoestrogens are produced by plants and have estrogen-like effects in vertebrate animals (animals with backbones).
Estrogens are a class of hormones present in both males and females and are actually a type of steroid. Estrogens are responsible for many important functions, such as the development of sex characteristics and the regulating of heat cycles. In bitches, estrogen levels can vary significantly during different times, such as before and during pregnancy. While estrogen is often associated with female health, it’s also important for male health too. Low or high estrogen levels can cause problems in both sexes.
Both males and females produce estrogens, but males produce them in smaller quantities. They are produced in the ovaries and placenta in females and the testes in males.
Phytoestrogens are produced in plants and are a wide class of compounds—there are lots and lots of different kinds with lots and lots of different effects on the body. [Cvejić 2012, Mazur 2000]
Most phytoestrogens are part of the flavonoid class of compounds. Flavanoids are GOOD for canine health, with a lot of health benefits. [Cvejić 2012]
Some phytoestrogens can have estrogen-like effects in animals. Different plants produce different types of phytoestrogens and in different amounts. Remember this, because this is key to understanding how they can affect fertility in our dogs. [Cvejić 2012]
Some phytoestrogens can have an estrogenic effect, meaning they act like estrogens and bind to estrogen receptors in animals. These phytoestrogens are sometimes referred to as "endocrine disruptors." [Magnusson 2015, Hedelin 2008]
Other phytoestrogens have anti-estrogenic effects, which means they block estrogens from working. [Hedelin 2008]
Phytoestrogens down regulate the alpha-type estrogen receptors, which hypothesized to be the reason they have anti-cancer effects. [Adlercreutz 2002]
Phytoestrogens have also been shown to “balance” estrogen metabolites. Estrogen metabolites are what are left over after the estrogen has been used by the body. There are both “good” and “bad” metabolites (do you see a pattern here?). Phytoestrogens favorably balance the ratio of these good and bad actors to help slow cancer growth and other effects. [Domínguez-López 2020, Saljoughian 2007]
I want to note some research limitations as you read through this. Research is expensive, so we have the most research for human health, since we as a society invest so heavily in that. Beyond human research, the cattle industry has the best funded animal research. After that, we may see research in other species, like mice and rats, which are inexpensive. Unfortunately, there’s not a whole lot of formal research in dogs. So when available, I’ll of course use canine studies, but we may need to look to other species for some of our information. That’s not ideal, but sometimes it’s all we have.
Are Phytoestrogens Bad for Dogs and Their Fertility?
I see blanket statements online all the time that phytoestrogens are bad for dogs and canine fertility.
THIS IS NOT AN ACCURATE STATEMENT.
If someone tells you this, they do not understand the biochemistry of phytoestrogens. Phytoestrogens are too broad a class of molecules for the broad statement "phytoestrogens are bad" to be true. [Cvejić 2012]
Most phytoestrogens have a positive effect on canine health and either a positive or neutral effect on fertility. [Cvejić 2012]
Some phytoestrogens can be harmful to health and/or fertility. [Cvejić 2012]
And then for some it depends on the situation of the individual and the dose.
The two types of phytoestrogens we want to be concerned with for canine fertility are coumestrols and genistein. Both coumestrols and genistein fit into all three categories above, depending on situation, life stage, and individual circumstances. [Hedelin 2008]
Genistein. Genistein is an estrogenic isoflavone found mostly in soy and red clover. [Setchell 2001, USDA 2007]. One study of dog foods found genistein and coumestrol in dogs foods that contained soy, but none in foods that didn't have soy as an ingredient. [Cerundolo 2004] Genistein is also implicated in impaired thyroid function in dogs. [Cerundolo 2009]
Coumestrol. Coumestrol has even more of a pronounced estrogenic effect than genistein and can affect reproductive health. It’s found in significant amounts in red clover and soy. Note that studies only show an effect in female reproduction, not male reproduction. [Kumar 2018, USDA 2007]
Some large epidemiological studies show a strong connection between high phytoestrogen intake and prevention of chronic disease and lower rates of heart problems and certain cancers. [Thompson 2006]
This table shows that phytoestrogens are associated with a number of both positive and negative health outcomes in humans [Mazur 2000]
As you'll see in this post, with very limited exceptions, phytoestrogens are healthful for dogs. [Tanprasertsuk 2022]
The Dose Makes the Poison
There’s a saying in toxicology that “the dose makes the poison.” [Grandjean 2016] That means that ANYTHING can be toxic at certain quantities. And some things can be beneficial in one dose, neutral in another, and toxic in another.
Fat soluble vitamins like vitamins A, D, and E are needed and healthy but can be toxic if the dose is too high. [Saljoughian 2021]
Even water can be toxic if you drink too much. [Peechakara 2022]
So this is another important concept to remember when looking at phytoestrogens, legumes, peas, and flax and this whole nutrition discussion.
Example in Humans
Here are examples of how some phytoestrogens are good sometimes for some individuals while bad for another, and vice versa. [MacMahon 1973, Samavat 2015, Brahmachari 2021, Mazurakova 2022]
Women with one type of breast cancer may be told to avoid a particular type of phytoestrogen, while women with another type of breast cancer may be told to increase that SAME type of phytoestrogen. [Hedelin, 2008, Hsieh 1998, Kapinova 2018, Battacharya 2021, Mazurakova 2022]
Another example is that soy has been shown to be protective against breast cancer in humans when eaten at a young age, but more of a risk factor for older women. [Adelcrutz 2002]
Phytoestrogens and Males
There isn’t a whole lot of research out there on phytoestrogens and male dogs, but in humans there’s no clear link between phytoestrogens and male fertility.
Supplementation with isoflavones, a type of phytoestrogens, was shown in humans to have no effect on sperm count, motility, or volume.
I found a couple of studies on coumestrols and male fertility in dogs those concluded that at the doses fed, there was no impact on male fertility (read a little further for discussion of what coumestrols are). [Kumar 2017, Kumar 2018]
Main Phytoestrogens in Food and Supplements
If there are many types of phytoestrogens out there, what are they? Let’s take a look at the main types found in food and supplements.
There are four classes of phytoestrogens that relate to this discussion. [Mazur 1998, Mazur 2000]
Isoflavones. Found in high concentration in soy products. [Doerge 2002, Mazur 2000]
Prenylflavonoids. A sub-class of flavonoids also known as “adaptogens” to herbalists; many show potential anti-cancer effects. Commonly found in hops, beer, and cannabis. Also called prenylated flavinoids. [Birt 2001, Dufall 2003, Miranda 1999, Santos 2020, Lingrong 2022, Mazur 2000]
Coumestans. Commonly found in legumes and show a wide range of pharmacological activities including estrogenic, anti-cancer, anti-inflammatory, anti-osteoporotic, organ protective, neuroprotective, anti-diabetic and anti-obesity, antimicrobial, immunosuppressive, antioxidant and skin-protective activities ;many may have anti-cancer effects. [Tu 2021, Mazur 2000]
Lignans. A large sub-group of polyphenols found in seeds, vegetables, and cereal grains; they are precursors to phytoestrogens. Found in highest amounts in flax seeds and sesame seeds, but also in cereal grains, soy, cruciferous vegetables, and some fruits. May help prevent certain cancers and heart disease. [Peterson 2010, Rodriguez-Garcia 2019, Yang 2021, Mazur 2000]
As you can see here, most phytoestrogens in food and supplements have positive health impacts. [Tanprasertsuk 2022]
Phytoestrogens with Estrogenic Effects
Most phytoestrogens either are neutral or have positive health benefits for both humans and dogs. [Tanprasertsuk 2022]
There are a few, however, we want to be concerned about with estrogenic effects. These estrogenic effects can affect both overall health and fertility in our dogs (and us, too). [Norman 1995, Kaldus 1989]
So which ones should we be concerned about and why?
This table shows the four main phytoestrogen classes and those with reproductive or a potential negative health impact are highlighted in red. [Knight 1996, Mazur 2000]
If you look closely at this table, a few things stand out.
The phytoestrogens we want to avoid giving our dogs (genistein and coumestrol) are found in significant amounts in soy and red clover ONLY. [USDA 2007]
While peas are in the coumestan category, they do not contain significant amounts of coumestrol, which is the phytoestrogen we want to avoid. [USDA 2007]
Peas contain repensol and trifoliol, which although they are coumestans, are not associated with any negative reproductive effect, and are NOT estrogenic. [Tu 2021, Mazur 2000]
Lignans, a main phytoestrogen found in flax seeds, are NOT estrogenic. Lignans are, however, well associated with positive health benefits. [Imran 2013]
Let’s remember what’s good and what’s bad: We want to avoid genistein and coumestrol, as they are problematic for reproduction.
The rest are either neutral or associated with positive effects.
Here are some tables to help you see which phytoestrogens are healthful and which are harmful.
Select coumestrol levels from the USDA database. [USDA 2007]
What we see in these tables are:
Soy is high in the isoflavone genistein. High levels of genistein have been shown to impact fertility in animals and humans [Setchell 2001]
Chickpeas have negligible amounts of genistein and most other legumes we feed to dogs have even less (other than soy)
Peas have no phytoestrogens with any level of consequence including not having genistein or coumestrol
Flaxseed has no genistein or coumestrol, but is high in the healthful lignan secoisolariciresinol, which is well-associated with positive health effects.
I want to talk briefly about lignans, a type of polyphenol, as they have some potentially important positive effects you may want to be aware of. [Peterson 2010, Rodriguez-Garcia 2019, Thompson 2006, Yang 2021, Mazur 2000]
Lignans were shown in one study with a shorter time to pregnancy in humans. That means the women who added lignans to their diet got pregnant faster than those who didn’t. [Mumford 2014]
Secoisolariciresinol (Seco) has several positive health benefits including protective for blood pressure, cardiac health, bone health, and more in humans and some animals [Kezimana 2018, Adolphe 2009, Imran 2015]
There are a few studies that showed deletrious effects when flax is fed in excessive quantities (common sense tip: don't feed flax in excessive quantities).
– In one study HIGH quantities (10% of the diet) can lower birth rate in rats [Tou 1998]
– In HIGH quantities (5% of the diet) can cause reduced ovary size and delay puberty in rats [Tou 1998]
– For comparison, 5% is about 2.5 teaspoons per cup, which is higher than in any commercial dog food I'm aware of or in any sensible balanced raw or homemade diet.
A Little More About Flax
Remember, we don’t always have studies on dogs to look at and while dogs are not humans or cows or rats, we can look to those studies to help us fill the gaps until we have better canine studies one day (hopefully).
There have been several studies on flax and fertility in dairy cows, goats, and sheep with varied results, although most showed an increase in reproductive performance. Here are a few of those.
Flax increased reproductive performance in sheep [Didarkah 2020]
One study showed no effect on reproduction. [Bork 2010]
Another study in dairy cows showed that compared to sunflower seeds, pregnancy losses were lower in cows fed flax (9.8%) than sunflower (27.3%) In this study, the flax diet increased the size of the follicle and reduced pregnancy losses. [Ambrose 2006]
Feeding flax increased uterine health and embryo survival in cows. [Petit 2007]
Conception rates in cows fed flax were 37.5 percent higher than those not fed flax [Petit 2001]
Dairy cows fed flax returned to estrus earlier, although pregnancy rate did not change. [Jahani-Moghadam 2015]
Embryo number and quality increased in goats fed flax [Dutra 2018]
Similar results have been found in rodent studies, but for the sake of brevity I don't include them here.
Flax, Omega-3s, and Dogs
While flax seeds are high in omega 3 fatty acids, they are not, however, a source we should rely on for dogs since they are high in alpha-linolenic acid (ALA). Dogs are not very efficient in converting ALA to DHA, which is the omega 3 we want to ensure our breeding dogs have adequate amounts of.
DHA is a critical nutrient for breeding dogs and puppies. I cover Omega 3s and DHA in greater detail in the Breeding Nutrition course.
The Slandering of the Innocent Pea
Let’s look closely at peas and why so many breeders avoid them.
I was both amused and saddened when I took a good look at how the reputation of the pea was destroyed in the dog breeding community.
Peas have no isoflavone levels of any significance.
Peas have coumestans, which are good antioxidants and provide protection against reproductive cancers [Dahl 2012]
Peas do not have coumestrols, the type of coumestans that are estrogenic (note the -estrol part of the word) [Dahl 2012]
So Why Do Breeders Avoid Peas?
It all started in India in the 1930s…. (cue dramatic music)
In 1935, a researcher named NC Nag injected mice with purified oil extracted from peas and saw reduced pregnancy. He hypothesized that it was the meta-xylohydroquinone (M-X) in the peas that caused this effect. The study isn't readily available since it's so old, but reports of it are found in a few places in the literature. [Thiersch 1956]
The research continued and in 1949 an M-X pill was created and tested on women at a hospital in India. It was a 2-year trial with 727 women in it. The pill was abandoned because of its low rate of efficacy. In that study it took doses of 350mg twice a month to see a decrease in fertility.
I want to repeat that: the research was abandoned because the pea extract didn’t work well as a contraceptive.
Just to be sure, there were several research projects in the 1950s that looked at M-X, this time in rats. In 1956 Sanyal had a small study in rats that showed in large enough doses, M-X can interfere with implantation. [Sanyal 1956, Bannerjee 1956]
In another study they gave M-X to male and female breeding rats. When the rats were given 1mg of M-X, there was an increase in the loss of pregnancy. No evidence was shown that M-X prevents implantation of the embryo.
Also, lower doses had NO effect. [Kumari 1956]
Here’s an important note: 1mg of M-X for a rat is a HUGE dose. Female lab rats weigh a little over half a pound on average.
Most interestingly, Sanyal discontinued his research on M-X when he learned that peas were a staple of the diet in Tibet for some a couple hundred years and that there was no reduction in population or pregnancy rates when peas were the major source of protein.
The last studies on M-X as a contraceptive were published in 1963.
Prahlad et al gave rats M-X for 30 days. No change in fertility was noted either in the rats given M-X or the control group that weren’t given M-X. The authors also noted problems with earlier studies. [Prahlad 1963]
Malkani et al gave M-X to 23 humans looking for any anti-progesterone effect and noted none. [Malkani 1963]
While you may say, well those are mostly rat studies, the important advantage of looking at rat studies for something like this is that rats have a much shorter lifespan and we see reproductive changes much more quickly. So what would take 15 years to observe in humans can be seen in rats in just a month.
Very important information: it was noted in these studies it required LARGE DOSES of M-X to see any effect. Let’s relate this back to the pea because this is very important, and if you miss this you will miss the point.
Remember the adage “the dose makes the poison?” This is what we must look at here: the dose of M-X and the relation of the pea to the dose.
First, M-X is derived from pea oil. Have you ever seen pea oil on the shelf at your grocery store? I sincerely doubt it, and that’s because there’s 0.6 grams of fat per 145 grams of peas. That means that peas have around 1% fat (oil).
I wasn’t able to discover how much M-X is in pea oil, but I found that pea oil had fatty acids in these amounts: palmitic, 12–21.0% (10.5% average), stearic 2.2–4.2% (3.2% average), oleic 14.2–33.3% (20.15% average), linoleic 41.3–60.9% (51.1% average), linolenic 2.6–13.4% (8% average). [Griffiths 1984] This gives us an average of 89.75% of pea oil comprising fatty acids.
If we use the average values of these ranges, this means that on average 89.75 is NOT M-X, since it’s something else (these fatty acids). So at most, we can assume about 10% of pea oil can possibly be M-X, and it’s likely a whole, whole lot less than that since there are many other compounds in the remaining 10% of pea oil, such as polyphenols, peroxides, pigments, polycyclic aromatic hydrocarbons, glycerol, phosphatides, sterols, triglycerides, diglycerides, monoglycerides, tocopherols, volatile compounds, peroxides, and many, many other compounds.
So how many peas would we have to feed our dogs to see any effect, even assuming there’s a possible modest affect from M-X on fertility (which has not been proven)?
Remember, in rats it took a dose of 1mg of M-X to have abortive effects in a half pound rat (or 2 mg per pound) and 350 mg to have minor effects seen in humans in some studies (or 3.5mg per pound; the average weight of women in India was about 100lbs at the time of the study). And remember, other studies showed no effects in either rats or humans despite dose.
Remember, the dose makes the poison
Peas have right around 1% fat. Assuming that M-X comprises 1% of the fat content of peas is generous. Using that generous assumption, for every cup of peas you could possibly have 1.45mg of M-X.
Let’s play devil’s advocate and assume the rat and human studies that showed contraceptive effects were correct.
Remember, those studies showed a minimum dose required to see effects. That was 1mg for ½ lb rats and 350mg for 100 lb humans.
How many cups of peas would you have to feed your dog to reach the quantities that showed effects in humans or rats?
That’s about 0.69 cups for the 1mg dose required to see an effect in that one rat study. You'd have to feed a rat over ⅔ cup of peas to possibly see a contraceptive effect.
For a 10lb dog that would be 20x, that, or 13.8 cups of peas to reach the dose that showed an effect in a single rat study. If you used the human study for dosing it would be even more (over 24 cups of peas).
Even if you assumed M-X was the ENTIRETY of the non-fatty acid content of pea oil, which is not possible, you’re looking at needing to feed a 10lb dog over 1⅓ cups of peas per day to see an effect, according to the rat study dosing, and over 2¼ cups according to the human study dosing.
And if you’re thinking there could be M-X in pea protein, I haven’t seen any evidence there is. And even if there were (which there’s no evidence of), peas are about 30% protein, so extrapolating the math you would still need to feed a crazy amount of pea protein.
Isn’t it kind of ridiculous to think that, even if M-X has a contraceptive effect (which has NOT been proven), anyone could even get their dog to eat the quantity of peas required to affect reproduction?
A Myth Is Born
So if there’s no evidence that peas affect fertility, how did it come to pass that so, so many dog breeders think peas affect fertility.
It started with some well-meaning mommy bloggers who looked at the M-X studies from the mid-20th century and not understanding biochemistry jumped to the incorrect conclusion that peas cause infertility in humans. Some of those blogs are still out there and I just shake my head when I see the huge and very incorrect leaps of logic they take to "prove" their conclusions. I've provided references for everything else in this post, but I don't want to appear to be shaming any particular individuals because that's absolutely nor my intention so I'm not providing a reference or a link for this. f you don't want to take my word for it you can do a search, there are plenty of examples out there.
Sometime, someplace, a well-meaning dog breeder or two saw those mommy blogs and extrapolated that to dogs and the myth started to spread in the dog world, with no evidence-based foundation for it. (Again, I don't want to point fingers so I'm not providing links.)
And at some point, the myth lost the fact that the M-X was extracted from the teeny tiny bit of pea oil in peas and pea protein (which is an entirely dfferent fraction of the pea than oil) became the demon.
Remember the game of telephone a lot of us played as a kid? We'd line up and whisper a sentence in one person's ear and we would pass the message down the line. By the time it passed through a few people it was an entirely different message than the original one. Myths work kind of like that. They may start off with a kernel of truth but as they get passed down the line they morph into something completely different and often wildly inaccurate.
I know this was long. And I know in some places it was probably confusing. But I wanted to put everything I could in here to make sure anyone interested could look at this themselves and see the facts to do your own myth busting.
Here are take-home messages to sum things up and that you can share if you want to discuss this with other breeders, which I strongly encourage.
We need to bring this to light for the sake of our dogs and for the sake of our programs. Myths are harmful. We need to stop them where and when we can.
Take-Home Message for Phytoestrogens in General
There are two main phytoestrogens we need to be concerned with regarding reproduction and health: genistein and coumestrol.
They are ONLY found in significant amounts in soy and red clover (which I've never seen fed to dogs, although it can be in lawns and pastures).
So avoid foods and supplements for your breeding dogs that have soy or red clover and their derivatives and stop worrying about any of the other phytoestrogens.
Take-Home Message for Phytoestrogens and Flax
The take home for me here is that flax seeds are beneficial to health and possibly even to reproduction.
They should just not be relied on as the sole source of omega 3 fatty acids for your breeding dogs.
Take-Home Message for Phytoestrogens and Legumes
Other than soy, most legumes, including peas, have negligible to no amounts of estrogenic phytoestrogens.
Take-Home Message for Peas and Fertility
Peas are not M-X and have very, very teeny tiny amounts of M-X, so feeding peas differs from administering M-X, a highly purified compound.
There’s no evidence in any species studied that whole peas or pea protein negatively affect fertility.
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