The Role of Soy Foods in Prostate Cancer Prevention & Treatment

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Soy is put to the test for the treatment of prostate cancer.

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Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

A compilation of 13 “observational studies” on soy food consumption and the risk of prostate cancer found that soy foods appear to be “protective.” Observational studies, as opposed to interventional studies; they observed what people were eating, but didn’t intervene and try to change their diets. So, they observed that men who ate more soy foods had lower rates of prostate cancer.

But, the problem with observation studies is that there could be confounding factors. Maybe people who choose to eat soy also make other healthy lifestyle decisions, like eating more fruits and vegetables, and maybe that’s actually why they have less cancer. Most of the studies tried to control for these other lifestyle factors, but you can’t control for everything.

Most of the studies were done in Asia; so, maybe tofu consumption is just a sign of eating more of a traditional diet. Maybe the reason non-tofu consumers got more cancer is that they had abandoned their traditional diet. If only there was a Western population that ate a lot of soy we could look at it. There is: Seventh Day Adventists.

In the 70s, more than 12,000 Adventist men were asked about their use of soy milk, and then, they were followed for up to 16 years to see who got cancer, and who did not. So: “Does high soy milk intake reduce prostate cancer incidence?” “Frequent consumption…of soy milk was associated with [a whopping] 70 per cent reduction of the risk of prostate cancer.” Similarly, in a “Multiethnic…Study” that involved a number of groups, soy intake appeared protective in Latinos, too.

Prostate cells carry beta type estrogen receptors, which appear to act as tumor suppressors, kind of a “‘gatekeeper’…inhibiting invasion [and] proliferation,” and preventing the prostate cells from turning cancerous in the first place. And, those are the receptors targeted by the phytoestrogens in soy, like genistein, which inhibits prostate cancer cell invasion and spread in a petri dish at the kinds of levels one might get consuming soy foods. The prevention of metastases is critical, as death from prostate cancer isn’t caused by the original tumor, but its spread throughout the body, which explains recommendations for men with prostate cancer to consume soy foods as well.

Wait a second. Do you think the amazing results Dean Ornish and colleagues got—apparently reversing the progression of prostate cancer with a plant-based diet and lifestyle program—was because of the soy? It wasn’t just a vegan diet, but “a vegan diet supplemented with…[a] daily serving of tofu [and] a…soy protein [isolate powder].” There have been studies showing men given soy protein powders develop less prostate cancer than the control group, but what was the control group getting? Milk protein powder.

Those randomized to the milk group got six times more prostate cancer. But is that from the beneficial effects of soy, or the deleterious effects of the dairy? Dairy products are not just associated with getting prostate cancer, but also dying from prostate cancer. Men diagnosed with prostate cancer who then ate more dairy tended to die sooner. And, “[b]oth low-fat and high-fat dairy consumption were…associated with an increased risk of [a] fatal outcome.”

The best study we have on soy protein powder supplementation for prostate cancer patients found no significant benefit, and neither did a series of soy phytoestrogen dietary supplements. Maybe that’s because they just used isolated soy components rather than a whole soy food. “Taking the whole food approach may be more efficacious.” It’s hard to do controlled studies with whole foods, though; I mean, you can make fake pills, but how do you give people placebo tofu?

This group of Australian researchers got creative, coming up with “a specially manufactured bread…containing…soy grits,” compared to just placebo regular bread, and then gave slices to men diagnosed with prostate cancer awaiting surgery. And, in just about three weeks time, did see a remarkable difference—the first study to show a diet incorporating a whole soy food could “favorably affect” prostate cancer markers. But, it’s not like you can go out and buy soy grit bread.

This study was a little more practical. Twenty men with prostate cancer, who were treated with radiation or surgery, but seemed to be relapsing, were asked to drink three cups of regular soy milk a day. Here’s what happened to the PSA levels in each of the 20 patients before they started the soy milk. They were all rising, suggesting they had relapsing or metastatic cancer growing inside of them. Here’s what happened during the year on soy milk. The blue lines are the folks where the soy milk appeared to have a positive effect, slowing or reversing the rise. The red lines represent those that got even worse during that period, and the black lines are those in which there appeared to be no significant change. So, of the 20, six got better, two got worse, and the remaining 12 remained unchanged. So, they conclude soy food may help in “a subset of patients.”

Based on all these studies, the results Ornish got were probably more than just the soy. Similarly, the low prostate cancer rates in Asia are probably more than just the soy, since the lowest rates are also found in parts of Africa, and I don’t think they’re eating a lot of tofu in Africa. Indeed, in the multiethnic study, other types of beans besides soy also appeared protective for Latinos, and all the groups put together, when looking at the most aggressive forms of prostate cancer. And so, the protection associated with plant-based diets may be due to eating a variety of healthy foods. 

Please consider volunteering to help out on the site.

Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

A compilation of 13 “observational studies” on soy food consumption and the risk of prostate cancer found that soy foods appear to be “protective.” Observational studies, as opposed to interventional studies; they observed what people were eating, but didn’t intervene and try to change their diets. So, they observed that men who ate more soy foods had lower rates of prostate cancer.

But, the problem with observation studies is that there could be confounding factors. Maybe people who choose to eat soy also make other healthy lifestyle decisions, like eating more fruits and vegetables, and maybe that’s actually why they have less cancer. Most of the studies tried to control for these other lifestyle factors, but you can’t control for everything.

Most of the studies were done in Asia; so, maybe tofu consumption is just a sign of eating more of a traditional diet. Maybe the reason non-tofu consumers got more cancer is that they had abandoned their traditional diet. If only there was a Western population that ate a lot of soy we could look at it. There is: Seventh Day Adventists.

In the 70s, more than 12,000 Adventist men were asked about their use of soy milk, and then, they were followed for up to 16 years to see who got cancer, and who did not. So: “Does high soy milk intake reduce prostate cancer incidence?” “Frequent consumption…of soy milk was associated with [a whopping] 70 per cent reduction of the risk of prostate cancer.” Similarly, in a “Multiethnic…Study” that involved a number of groups, soy intake appeared protective in Latinos, too.

Prostate cells carry beta type estrogen receptors, which appear to act as tumor suppressors, kind of a “‘gatekeeper’…inhibiting invasion [and] proliferation,” and preventing the prostate cells from turning cancerous in the first place. And, those are the receptors targeted by the phytoestrogens in soy, like genistein, which inhibits prostate cancer cell invasion and spread in a petri dish at the kinds of levels one might get consuming soy foods. The prevention of metastases is critical, as death from prostate cancer isn’t caused by the original tumor, but its spread throughout the body, which explains recommendations for men with prostate cancer to consume soy foods as well.

Wait a second. Do you think the amazing results Dean Ornish and colleagues got—apparently reversing the progression of prostate cancer with a plant-based diet and lifestyle program—was because of the soy? It wasn’t just a vegan diet, but “a vegan diet supplemented with…[a] daily serving of tofu [and] a…soy protein [isolate powder].” There have been studies showing men given soy protein powders develop less prostate cancer than the control group, but what was the control group getting? Milk protein powder.

Those randomized to the milk group got six times more prostate cancer. But is that from the beneficial effects of soy, or the deleterious effects of the dairy? Dairy products are not just associated with getting prostate cancer, but also dying from prostate cancer. Men diagnosed with prostate cancer who then ate more dairy tended to die sooner. And, “[b]oth low-fat and high-fat dairy consumption were…associated with an increased risk of [a] fatal outcome.”

The best study we have on soy protein powder supplementation for prostate cancer patients found no significant benefit, and neither did a series of soy phytoestrogen dietary supplements. Maybe that’s because they just used isolated soy components rather than a whole soy food. “Taking the whole food approach may be more efficacious.” It’s hard to do controlled studies with whole foods, though; I mean, you can make fake pills, but how do you give people placebo tofu?

This group of Australian researchers got creative, coming up with “a specially manufactured bread…containing…soy grits,” compared to just placebo regular bread, and then gave slices to men diagnosed with prostate cancer awaiting surgery. And, in just about three weeks time, did see a remarkable difference—the first study to show a diet incorporating a whole soy food could “favorably affect” prostate cancer markers. But, it’s not like you can go out and buy soy grit bread.

This study was a little more practical. Twenty men with prostate cancer, who were treated with radiation or surgery, but seemed to be relapsing, were asked to drink three cups of regular soy milk a day. Here’s what happened to the PSA levels in each of the 20 patients before they started the soy milk. They were all rising, suggesting they had relapsing or metastatic cancer growing inside of them. Here’s what happened during the year on soy milk. The blue lines are the folks where the soy milk appeared to have a positive effect, slowing or reversing the rise. The red lines represent those that got even worse during that period, and the black lines are those in which there appeared to be no significant change. So, of the 20, six got better, two got worse, and the remaining 12 remained unchanged. So, they conclude soy food may help in “a subset of patients.”

Based on all these studies, the results Ornish got were probably more than just the soy. Similarly, the low prostate cancer rates in Asia are probably more than just the soy, since the lowest rates are also found in parts of Africa, and I don’t think they’re eating a lot of tofu in Africa. Indeed, in the multiethnic study, other types of beans besides soy also appeared protective for Latinos, and all the groups put together, when looking at the most aggressive forms of prostate cancer. And so, the protection associated with plant-based diets may be due to eating a variety of healthy foods. 

Please consider volunteering to help out on the site.

Doctor's Note

That soy milk stat from the Adventist study is astounding. What about fermented soy foods, though? That was the subject of Fermented or Unfermented Soy Foods for Prostate Cancer Prevention?.

Reversing the progression of cancer? Check out How Not to Die from Cancer.

Given the power of diet, it’s amazing to me how difficult Changing a Man’s Diet After a Prostate Cancer Diagnosis can be. It’s not all or nothing, though. Check out Prostate Cancer Survival: The A/V Ratio.

For soy and breast cancer survival, see Is Soy Healthy for Breast Cancer Survivors?.

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