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There are a few studies that suggest that high levels of soy protein may decrease fertility.

There are not many reports done, but generally studies show that in countries where soy products are consumed in large amounts, the birth rate is no lower than in countries where soy is not a part of consumed diet. On the other hand, there are studies that suggest that soy might have negative impact on reproductive health.

Generally speaking, soy is a great source of proteins and actually contains phytoestrogens, plant-derived estrogen (female reproductive hormone estrogen). Isoflavones are the type of phytoestrogen and are known to come in different forms and different effects: some might act positive and have similar effect to estrogens in the body, producing a very weak effect compared to the real hormone, others may have decreasing effect on fertility: they might function as antiestrogens and reduce the activity of estrogen.

However, there are a few studies that suggest that high levels of soy protein may decrease fertility. In a Report in The Journal of Clinical Nutrition a small number of studies have shown that high levels of soy can:

  • increase menstrual cycle length,
  • decrease follicle-stimulating hormone
  • decrease leutinizing hormone.

It is important to bring out that the participants in the study were drinking three 12-ounce glasses of soy milk (60 g soy protein equivalent to 45 mg of isoflavones) for a month. These are very high levels and therefore generate such results. The results cannot be applied to a typical soy consumer who does not consume this much soy food.

Soy and sperm count

Soy reduces sperm count, it is not a myth. Several studies have demonstrated that a nutrition rich in soy food could reduce the sperm count by as much as half. When scientists conducted such soy and fertility study, they looked at 99 men who were currently suffering from fertility problems. Those men who were on a daily basis consuming the most soy-based foods had the most significant impact on their sperm count. The leading belief behind this effect is due to the chemicals known as isoflavones that are found in soy products and mimic the estrogen hormone.

In another study, researchers compared the effect of two scoops of pure soy protein powder (56g/day) on the testosterone levels of 12 healthy young men. They were consuming the soy protein powder for 4 weeks, and the average testosterone levels of the subjects had decreased by almost 20 %. After about two weeks the men stopped taking the soy supplement, and their testosterone levels returned to normal.

An article from the journal Natural Toxins concluded that evidence from studies of different animal species has proved that eating high levels of phytoestrogens can have negative effects on fertility. But also that there is no current data to suggest that consumption of phytoestrogens at the levels normally encountered in nutrition are likely to be harmful. If you have been wondering how to increase sperm count, it is wise not to overconsume soy products, as with any other food: eating soy in moderation allows you to avoid any potential harm, as well as leaving room in your diet for variety. The more variety in your diet, the more likely you are to get all the important nutrients that your body needs.

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