Soy and Fertility: To Eat or Not to Eat?

Will taking soy products make it difficult for me to conceive?

Soya milk and tofu were the staple food for G, a 34-year-old teacher and a vegetarian. She was getting married soon and came for a preconception check. She was keen to get pregnant after marriage because of her age. But she was confused and in a dilemma as her friend advised her to cut off soy completely in her diet as it might affect her chances of conception. She was not totally convinced until she read some negative reports of the effects of soy on fertility in some female magazines. “Will taking soy products make it difficult for me to conceive?” she asked.

G appeared healthy on examination. Her BMI was normal at 22. Her menstrual cycles were regular, occurring once every 30 days and lasting 4-5 days. There was no menstrual pain or abnormal vaginal discharge. Examination of the reproductive organs was normal. This was confirmed by an ultrasound scan.

Her husband, 36, was an accountant. He did not smoke or drink and exercised regularly with G at the gym about two to three times a week. His semen analysis was normal with good sperm count, normal sperm motility and morphology.

What is soy?

Soy is a popular food in Asia. It belongs to the pea family and is rich in protein, polyunsaturated fats, fibre, vitamins, and minerals. Soy products such as tofu and soya milk have been consumed by vegetarians as a main source of protein all over the world

Soy and female fertility

There is a controversy regarding the effect of soy on the fertility of both men and women. This arises because soy contains compounds called isoflavones which are plant-derived female hormones. They can exert a weak effect on human body and have been shown to reduce fertility in experimental animal studies. In humans, a few clinical studies have shown that taking isoflavones can reduce the levels of pituitary hormones which may then affect the growth of the egg and ovulation. But the quantity of soy taken in those studies is very large and many times more than the normal dietary consumption. Other studies, however, suggest the exact opposite, that soy consumption may have a positive effect on fertility. A large study of more than 116,000 female nurses aged 25-42 years old, called the Nurses Health Study II, has shown that excessive animal protein intake was associated with decreased fertility. Addition of tofu improved the fertility outcome, thus supporting the benefits of soy.

Another recent study (2015) of 315 women presenting in an infertility clinic in Massachusetts also found that intake of soy foods was positively related to the probability of having a live birth during in vitro fertilization treatment.

General consensus among the clinicians is that eating a moderate amount of soya does not affect female fertility and is safe for women trying to conceive.

Soy and male fertility

The worry that soya products reduce male fertility arose in 2008 when researchers from the Harvard School of Public Health in Boston found that regular consumption of these foods may lower the sperm count. They studied 99 men from infertile couples. They found that the men who ate soy foods had, on average, 41 million fewer sperm per ml than men who didn’t eat these foods. But there were no changes observed in other parameters. The conclusions from this study are not definitive.

On the other hand, a number of studies, in which controlled amounts of isoflavones from soy were fed to human subjects, have found that the compounds have no effect on the quantity, quality or motility of sperm.

All in all, there is not enough evidence to suggest that soy influences sperm count and reduces male fertility. G and her husband listened attentively and appeared satisfied with my explanation. She continued to eat her normal diet and came back to see me with a positive pregnancy test 3 months after her marriage. She is currently awaiting the arrival of her baby in a few weeks’ time.

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