There is so much controversy about the effect of soy foods on men’s fertility. With each study that is released, a news article follows declaring it to be either safe or harmful for men’s fertility. I would like to go through some of the studies in this area and review the significance of them before drawing a conclusion based on the current evidence. Soy is well-known for its health benefits for men, including the reduction of risk for both cardiovascular disease and prostate cancer. Due to these benefits, it has become a larger part of the North American diet than it ever has been in the past so it is important to fully understand its impact on hormones and fertility.
Effects of Soy on Infants and During Pregnancy on Adult Male Fertility
The following studies investigate reproductive effects on men who were given soy products either through infant formula, or while their mothers were pregnant. This is a very sensitive time of development for the reproductive organs, so much concern remains about the possibility of permanent negative effects.
Soy Formula in Infant Male Marmoset Monkeys causes no Adverse Effects
This study concluded that infant feeding with soy formula has no major adverse reproductive effects in male marmoset monkeys. Although it did not appear to affect fertility, soy infant formula did alter testis size and cell composition.
Again I would like to mention here that both rats and monkeys produce much higher levels of equol (an estrogen like substance which is much stronger than soy isoflavones) in their intestines than humans in general. The equol is produced through fermentation of isoflavones by bacteria which reside in the intestine. It’s hard to compare humans directly with rats or monkeys especially when it comes to estrogenic effects. Studies investigating the effects of phytoestrogens on the fertility of different animal species have been very inconsistent. This indicates that soy has very species specific effects on fertility and highlights the need for more studies on humans before we can draw definitive conclusions.
Study on Vegetarian Mothers and Hypospadias in Infants
This famous study investigated the difference between vegetarian and omnivorous women and the likelihood of a condition known as hypospadias in their newborns. Hypospadias is a condition (which is currently on the rise) where the urethral opening is in a lower position. This study found that significantly more of the vegetarian mothers had babies with hypospadias. As vegetarians have a greater exposure to phytoestrogens than do omnivores, the researchers concluded that phytoestrogens may have a negative effect on the developing male reproductive system. However, this study was not specific for soy, it only examined whether or not vegetarians tended to have more infants with hypospadias. Other factors cannot be excluded for example, vegetarians could be more likely to be deficient in other vitamins or nutrients such as B12, and could also be consuming a larger amount of estrogenic pesticide residue, and this study did not question participants about consumption of organic foods. It was also found that the vegetarian mothers who did not take iron supplements had more infants with hypospadias. In Japan, there are 1/10th the number of infants born with hypospadias as there are in North America and yet there is a much higher level of phytoestrogen consumption. Therefore, this study is not fully conclusive that phytoestrogens are the cause of this developmental condition since there are too many unaccounted for variables.
Study on Soy Formula in Infants and Reproductive Outcome In Young Adulthood
This study on 811 men and women, who were fed either soy or cow milk formula as infants were assessed in young adulthood for their pubertal maturation, menstrual and reproductive history, height/weight, and current health. It concluded that exposure to soy formula does not appear to lead to different general health or reproductive outcomes than exposure to cow milk formula in infancy. This study did not go into details asking about length of time to conceive. Also, no reproductive health markers were reported for male subjects with the exception of sexual maturation. Although men were questioned about pregnancy outcomes in partners the results were not reported.
In conclusion on the subject of male reproduction and feeding of infant soy formula, it appears that overall there may be a risk for some long-term reproductive developmental changes, however, the full effects of this are unknown and may not go so far as to cause fertility concerns. However, as we know from so much current data, breast milk is a far superior nutrition method for infants, and avoids any of the risks that soy formula may hold.
Studies on Male Adult Animals
Phytoestrogenic Plant given to Adult Male Mice – Some Effects on Reproduction
A phytoestrogenic plant(pueraria mirifica) was given in two doses, one high and one low, to a group of adult male mice. Neither treatment had effect on testicular weight, sperm count, LH, FSH or testosterone. However the high (100mg/kg) dose reduced the weight of epididymis, seminal vesicle and sperm motility. There were no effects on fertility. This effect was seen to be reversible after the phytoestrogen was stopped. However, this plant, though it does contain some of the same components as soy, is not identical to soy.
Acute Exposure of Adult Male Rats to Dietary Phytoestrogens Causes Temporary Reduction in Fertility
This study found that lipid peroxidation damage of sperm was increased in rats fed a high phytoestrogen diet for 3 days. No such changes were noted in low phytoestrogen group. As in the previous study, this effect was temporary, with fertility returning to control levels by day 12. Rats who were fed the phytoestrogens for longer than 6 days did not show this reduction in fertility and in fact showed no change in any reproductive parameters.
Phytoestrogens cause no Negative Effects on Fertility of Rhesus Monkeys
In this study, phytoestrogens were given to rhesus monkeys at the age of puberty. They had no adverse effects on the reproductive systems of male or females as evaluated by hormone concentrations. Cardiovascular benefits were observed in the monkeys receiving the phytoestrogens.
Studies on Adult Men
Soy Products Related with Slightly Lower Testosterone and Lower Estradiol in Japanese Men
This study on Japanese men investigated the relationship between soy product intake and serum testosterone and estrogen concentrations. The results found that blood levels of estradiol concentration were significantly lower with increased soy product intake, and blood estrone levels were not related to soy intake. Testosterone levels were also lower with increased soy intake but this effect was so slight it did not reach significance in the study. This study also concluded that this may be part of the reason soy reduces risk of prostate cancer in men.
Soymilk Given to Japanese Men Results in Lower Estrogen Concentrations
This second study on Japanese men investigates the effects of drinking 400 mL daily soymilk on serum estrogen and testosterone concentrations. In contrast to the previous study, the results of this study indicate that soymilk consumption is associated with lower levels of the estrone form of estrogen. In this study there was no effect of soymilk on any of the other hormones measured including testosterone, estradiol, and sex hormone binding globulin.
These two studies indicate that soy can affect serum estrogen levels. It is known from other research that estrogen is required for proper formation of sperm, but also, that elevated levels of estrogen can interfere with fertility (especially if testosterone to estrogen ratios are altered). So, what we can say is that a good level of balance of estrogen is required for optimal male fertility, and the real question is, does soy interfere with the balance of estrogen enough to impact actual fertility parameters in males. These two studies do not answer this question, so we need to look more to studies on soy consumption and the end result on adult male fertility.
Soy Food Intake Related to lower Sperm Concentration Among Men from an Infertility Clinic
This very well-known study took a group of men from a fertility clinic and evaluated the relationship between soy food intake and sperm quality and count. It found that there was a relationship between the intake of soy foods and the reduction of sperm concentration. 72% of men in this study were either overweight, or obese according to their BMI levels. The relationship was more pronounced in the men who had the highest sperm concentration and among overweight or obese men. Soy foods did not reduce sperm motility, sperm morphology, or ejaculate volume. This suggests that because androgens are converted into estrogen in fatty tissue, this may increase tissue sensitivity to phytoestrogens in those who have higher amounts of body fat. This study did not consider that those who eat more soy could be exposed to more estrogenic pesticides (it did not ask about consumption of organic versus non organic soy). It also did not account for the addition of soy in many foods that may not have been reported by the participants (such as soy based additives in baked goods, processed foods and so forth). Therefore, although this study is quite interesting, it not conclusive. This study does however, make an important association between elevated body mass index, and effects of soy on fertility in men.
Healthy Adult Men of Normal BMI: Soy Isoflavones have No Negative Effect on Sperm Parameters
This new study from the University of Guelph which involved healthy adult men with a healthy body mass index investigated the effects of isoflavones on sperm parameters. In this study, men were given a daily serving of soy isoflavones in low concentration, high concentration, and then isoflavone free milk protein isolate. The different substrates were given for 57 days each separated by a 28 day ‘break period’. The study showed no significant effect of soy isoflavones on sperm concentration, motility and morphology of the men. This study adds to the evidence that soy has a much lesser effect on semen parameters in men of healthy body mass index.
In summary, more research needs to be done before we can have any conclusive answers about the impact of soy on male fertility. There are many conflicting studies on this subject, which indicates we need to investigate further. There are a few points though that we can learn from the current research which can probably be protective to male fertility, and also allow men to have some of the health benefits that soy foods can provide.
1) Soy can have a temporary, acute effect on adult male reproductive parameters if taken in high quantities, especially if not normally included in the diet. Therefore, it would not be a good idea to consume large amounts of soy directly around the time when your partner is ovulating.
2) Soy appears to reduce sperm concentration in males who are overweight or obese, so if you are overweight, try to achieve a healthy BMI. In cases where BMI is high, soy foods might not be the best staple for the diet while trying to conceive.
3) It is probably likely that small amounts of organic soy have little negative effect on reproduction in males of healthy body mass index and can provide health benefits such as improvement of cardiovascular profiles, and reduction of risk for prostate cancer. More research still needs to be done in order to truly understand the impact of soy fertility of healthy adult males.
- Fiona McCulloch, ND
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