Monday, 25 August 2008

Soy, Sperm and other Stuff

While I
was away on holidays in July, a study was published that made the news headlines. The study, conducted by researchers from the Harvard School of Public Health (funded by National Institute of Environmental Health Sciences and the National Institute of Diabetes and Digestive and Kidney Diseases), found an inverse relationship between consumption of soy foods and sperm concentration.

That is, the men that ate the most soy products had a lower sperm concentration.

First, some background.

A Surplus of Soy

Soybeans were once grown almost exclusively for animal feed but Americans started eating it in ~1915 and have been consuming more and more of it, in higher and higher quantities, ever since.

Corn and soybean crops are subsidized by the government but up until the 1970s, policies and programs were put in place to prevent overproduction on the farms, to help farmers get fair prices for their crops. However, in the 1970s, Nixon's administration successfully dismantled these programs in their efforts to drive down costs of these crops, resulting in an overproduction of corn and soybeans. The more corn and soybeans the farmers produced the more money they could make… but this backfired on them since production was so high that it drove the cost down. As a result, farmers were getting paid less than it cost them to grow their crops! This, of course, led to unhappy farmers as well as a surplus of soybeans (and corn) available for human consumption (after exporting some, turning some into animal feed and converting some into oil used in margarines, cooking and salad oils).

Enter USDA-sponsored 'research and education' programs- basically government money to market soybeans to human consumers. Someone had to eat this surplus…

Asians eat soy: A marketing strategy

The reductive logic below has been used many times to explain the benefits of soy:

Asians have fewer heart attacks and less breast and prostate cancer than Americans and Asian women report fewer hot flashes and other symptoms of menopause.

Asians eat soybeans.

Soybeans must be responsible for all these health benefits.

Of course, this ignores the fact that over 1 billion people are considered 'Asian' and they don't all have the same lifestyles of diets. In fact, soy isn't even a staple in all Asian people's diets. Asians also have other things in common (apart from eating soybeans) that may contribute to their overall better health: genetics, weight- Asians generally have lower BMIs than Americans and less sedentary lifestyles, to name a few.

Soy & Cholesterol

One soy research found that eating 50 grams of soy protein versus animal protein a day reduced total cholesterol levels by 9.3%, the lousy LDL-cholesterol by 12.9% and triglycerides by 10.5%. If sustained over time, you could potentially reduce your risk of having a heart attack by 20%.

However, remember that soy is just one food and it's very hard to figure out the effect of one food from everything else you eat and do. If you eat soy because you think it's healthy, maybe you're also following other healthy habits.

Without adopting a healthier lifestyle (less saturated fat, more physical activity, quit smoking), soy will do nothing. Moreover, other studies have shown no link between soy protein and cholesterol levels.

Nonetheless, The FDA was under a lot of pressure from the government and food companies and relied heavily on that one study mentioned above to come out with their health claim that:
"Diets low in saturated fat and cholesterol that include 25 grams of soy protein a day may reduce the risk of heart disease".
Foods that contain 6.25 grams of soy per serving can carry this claim on their label (in the States, not in Cana

Isoflavones, aka Phytoestrogens

Complicating issues a bit more is that independent research suggests that it's the combination of isoflavones and the soy protein, not just the soy protein, that is responsible for the health benefits. The isoflavones in soy are phytoestrogens, weak estrogen-like substances made by plants.

25 grams of soy protein contains about 45mg isoflavones.

A Japanese person who eats a traditional soy-based diet consumes an average of 25-50mg soy isoflavones a day.

According to a Penn State College of Agricultural Sciences publication, the safe range of intake is 35–55mg isoflavones a day and it should not exceed 100 milligrams per day.

Average isoflavone content of various soy foods
per 100 grams (3.3 ounces) serving (unless indicated)

Boiled soybeans- 54 mg
Tempeh- 53mg
Tofu- 28 mg
Soy hot dogs- 15 mg
Soy milk- 10 mg
Soy mozzarella- 8 mg
Soy sausage- 4 mg
Soy nuts- 9 mg or 79 mg per 28 ounces
Gardenburger Lifeburger- 50 mg in 6 ounces
Soy Sensations Nutrition Bar- 60 mg in 2 ounces
Soy flour- 0.1–0.4 mg/g or 20 mg per half cup (50 grams)
Soy protein concentrate- 0.01– 0.2 mg/g
Soy oil- none
Soy sauce- none

Phytoestrogens and Breast Cancer

Again, using the same reductive logic as above, Japanese women have lower breast cancer rates and Japanese women eat more soy.
Therefore eating soy can help reduce breast cancer rates. Right?

Well, this makes sense because phytoestrogens act like estrogen but don't have the same effect. So these phytoestrogens can bind to our estrogen receptors, thus blocking the real estrogen, high levels of which have been known to increase breast cancer risk.

However, the reductive logic above is wrong; mainly because, as I mentioned previously, soy is actually not a staple in many Asian countries. The lower breast cancer rates are most probably due to other factors such as genetics, physical activity, weight, other dietary factors, childbirth patterns etc.

The research on soy and breast cancer is contradictory. A large study in China found that women with breast cancer ate the same amount of soy as those without cancer. Another Japanese study found the same thing in a 10-year follow-up study. However, another large Japanese study found that those that ate more soy had a lower risk of breast cancer.

There is also the possibility that soy can actually cause more harm: Although the estrogen-blocking activity of phytoestrogens may possibly be beneficial to younger women, in later years when estrogen levels are lower, these same phytoestrogens may actually mimic real estrogen and cause breast cancer cells to grow. The most abundant isoflavone in soy is called genistein and it has been found to possibly stimulate growth of estrogen-receptor positive breast cancer cells. This isoflavone can also interfere with the cancer-killing drug tamoxifen.

Hot Flashes

Again, the results are contradictory. Some studies showed that women who ate more soy had fewer hot flashes whereas others showed no differences.

Hot flashes are more likely to occur when you're stressed and less likely to occur if you exercise… perhaps the women who ate more soy were also more active and less stressed?

Soy and sperm

Now for the most recently published study:

99 men that presented at an infertility clinic for evaluation (these were the male partners in a subfertile couple) provided a semen sample and completed a food frequency questionnaire that assessed the frequency and quantity of soy foods they ate. These men were categorized into 4 groups: men who never ate soy products, ate soy products <2 times a month, 2 times/month to 2 times a week and ≥2 times a week.

After correcting for factors possibly affecting sperm concentration- smoking, caffeine intake, alcohol intake, BMI and age- it was found that men who ate the most soy had 41 million sperm/ml less than those that ate no soy. No association was found with soy food intake and sperm count, sperm movement or shape or ejaculation volume.
It's important to note that a reduced sperm concentration does not result necessarily in infertility.

Included in this group were men with potential infertility problems (10% had a sperm concentration below 20 million/mL; normal concentration is 80-120 million/mL) but the inverse association between soy and sperm concentration was greater in the men with higher sperm concentration. The association was also higher in the men that were overweight and obese (BMIs greater than 25) and 72% of the men in this study were in fact overweight or obese.

According to Jorge Chavarro, lead researcher of this study, the hypothesis is that because isoflavone found in soy products are phytoestrogens (weak estrogen-like substances) they may be harmful to male fertility. We know that the more overweight a man is, the higher his estrogen level is. As a result, it's possible that additional estrogen-like compounds in the diet can become an issue for these overweight and obese men.

The European Natural Soyfoods Manufacturers Association have disputed this study's findings stating the familiar: "Generations of Asians have regularly consumed soya without fertility disorders, and Asian countries have prodigiously produced very healthy, highly functioning children for centuries".

As we now know, soy isn't a staple in all Asian people's diets and other lifestyle factors may better explain the differences between the ethnic groups, ie. perhaps the fact that overweight and obesity rates are much lower in the Asian compared to the American population has something to do with it. 90% of the subjects in Chavarro's study were Caucasian.

Another small human study has looked at the association between soy and sperm count and found no change in semen quality with soy intake whereas another actually found a positive effect on sperm count with isoflavone intake.

Evidently, more research needs to be done. Chavarro himself says that it's too soon to draw conclusions about whether soy consumption affects male fertility but experts caution that if your sperm count is low, and especially if you're overweight, it may be a good idea to limit your soy intake.

Bottom Line:

Even healthy things should be eaten in moderation.

If you're a woman with breast cancer, taking tamoxifen or who has had ER+ breast cancer, stay away from soy.

If you're a man with fertility issues, especially if you're overweight, limit your soy intake.

Stay away from soy pills and powders since they may contain very high levels of isoflavones and the effects of taking these are unknown.

Soy foods are a great substitute for animal protein and dairy foods and a great source of protein, but vary your protein. If most of your protein comes from plants, make sure you get a good variety of beans and legumes, nuts and seeds as well as whole grains and vegetables.

Soybeans and the less processed foods made from them are good to have in moderation. Some sources say 2-3 servings of soy foods a day is fine whereas others suggest 2-4 servings a week. Use your judgment.

A product that contains soy is not necessarily healthy, You can now find a huge variety of processed soy foods- anything from dairy substitutes, cereals, power bars, drinks, snack foods- that can probably be categorized as junk food.


Chavarro JE, Toth TL, Sadio SM, Hauser R. Soy foods and isoflavone intake in relation to semen quality parameters among men from an infertility clinic. Oxford University Press: Human Reproduction. July 23 2008.

Reinberg, S. Soy linked to low sperm count. July 24 2008.

On Soya, Sperm and Men. July 24 2008.

Agrell, S. Tofu a day, sperm goes away: study.

US soy producers dispute low sperm count study. Jul 30 2008.

Willett, WC. Eat, Drink, and Be Healthy: The Harvard Medical School Guide to Healthy Eating. NY: Free Press. 2001.

Nestle, Marion. What To Eat. NY: North Point Press, 2006.

Pollan, Michael. The Omnivore's Dilemma: A Natural History of Four Meals. Penguin Books. 2006.

Brown, J Lynne, PhD, RD. Functional Ingredients: Soy Protein and Soy Isoflavones. Penn State College of Agricultural Sciences.

Liebman, Bonnie. The Soy Story. Center for Science in the Public Interest: Nutrition Action Healthletter. Sept 1998.

Henkel, J. Soy: Health Claims for Soy Protein, Questions About Other Components. USDA Consumer Magazine. May-June 2000.

Schardt, David. Phytoestrogens for Menopause. Center for Science in the Public Interest: Nutrition Action Healthletter. Jan/Feb 2000.

What is Tempeh?

1 comment:

Jme said...

Great research and advice. Thanks SH!