Soy makes a woman's menstrual cycle longer, and decreases her hormone levels. Many doctors fear this can interfere with a woman hoping to become pregnant.
Studies have found that a woman ingesting only 60mg of soy a day gains an average of 2 1/2 days on her cycle. Consuming the soy also lowers the levels of both follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
While the Chinese and Japanese diets only include 2 Tbsp (10g) of soy a day as a condiment, the Western diet has used soy as a filler in just about every processed food. In addition, excess soy produced by farmers is marketed as a health food for even more intake.
In essence, soy has a component that mimics estrogen, although a weaker type. When a woman in her childbearing years takes in this estrogen, it interferes with her natural fertility cycle.
For men, research published in the Lancet found that eating estrogen-like foods such as soy caused lower sperm counts in males. Studies show that men who take in soy protein have up to 76% decrease in testosterone production.
Male fertility is strongly tied to Zinc, which is necessary for sperm production and testosterone metabolism. Soy contains phytates which are known to interfere with Zinc absorption.
In fact, a popular tradition in Japan is for wives to feed their husbands tofu when they want to prevent pregnancy! Buddhist monks traditionally eat tofu to lower their libido.
Speaking of low libido, this is also caused by low thyroid levels. The estrogens in the soy cause this to happen, interfering with the thyroid's ability to absorb iodine. So soy packs a double whammy.
Be sure to especially avoid soybean oil, which is full of polyunsaturated fatty acids. These acids as well as margarine have been linked to infertility in both sexes.
Soy Consumption in China and Japan
Soy Products - Safe or Harmful?
Soy, Alzheimers and Brain Aging
Lisa Shea's Library of Low Carb Books