Soy Diet and Prostate Cancer

Nagi B. Kumar, R.D., explains the value of incorporating soy into your diet to prevent and/or lessen the effects of prostate cancer.

What effect has soy shown to have in cancers?

Kumar: Soy is a very interesting compound. The first group of studies was done in Japan and other Asian countries that showed intake of soy was highly correlated with the lowest incidence of prostate cancer and other hormonal cancers like breast cancer. There were some comparison studies done earlier looking at men in Japan compared to men in Finland. It showed that men in Japan had the lowest instance of prostate cancer compared to men in Finland. Those findings gave way to other studies, especially in animals, to detect what was in soy that was making it protective against prostate cancer. These showed a couple of things: Soy may stimulate production of a substance called sex hormone binding globulin, which is a protein secreted by the liver. It binds the testosterone and doesn’t make it available to promote prostate cancer. So, this is one of the protective mechanisms hypothesized at that time. Then researchers started looking at prostate cells in vitro in petri dishes to see whether concentrates of soy changed prostate cell progression or regression. They certainly found some indication that prostate cells were reduced by these concentrates of soy. These are the three studies that led the way to clinical trials to potentially see if we can intervene with soy and soy products to decrease prostate cancer risk.

In your first study, what did you discover as the effects of soy on prostate cancer?

Kumar: This was one of the first clinical trials where we mimicked the Japanese diet in the Western population in men with early stage prostate cancer. These are men who have no treatment options, and they are called the watchful waiting group. They have to check their prostate specific antigens, or PSAs, for an increase. If there is an increase in PSA, you will also see an increase in disease progression. So, we took these early stage prostate cancer patients and gave them 16 milligrams of whole soy protein with the isoflavones for a three-month period and observed hormonal changes. We observed the increase in sex hormone binding globulin, the decreases in testosterone levels, and their prostate specific antigen. We were trying to detect what the mechanism is. Was the hormone stimulating less proliferation or more proliferation? It was a randomized double-blinded trial where we gave these men soy for three months. Lo and behold, we did not see a dramatic change in hormonal levels, although we saw a reduction in testosterone levels in the soy group compared to the control group. The control group took protein shakes, very similar to the breakfast drinks available in the supermarket. In the soy group, we noticed that over 70 percent of them reduced their PSA, and about 20 percent of that group showed over a three-point reduction in PSA. This is dramatic for a three-month intervention, especially for a disease that progresses very slowly. They say that the period for prostate cancer development is anywhere between 15 to 20 years. In a three-month period, we can make a change.

How significant is a three-point drop in PSA within three months?

Kumar: If you ask any prostate cancer patient, he will tell you it is a significant change. PSA is not an indicator of disease. PSA can be elevated for any reason. For example, you can have an elevated PSA if you have prostatitis or an infection of the prostate, but for a prostate cancer patient, it is an excellent surrogate marker of disease progression. If it increases, it shows there is something proliferating there. If the PSA is reduced, it is also very indicative of disease digression. That is the only quantifiable marker we have other than to do a biopsy and look at it. To do a biopsy in an early stage prostate cancer patient, you really have to take biopsy from over a dozen sights in the prostate. PSA is the easiest non-invasive surrogate marker of proliferation and probably one of the best that is currently available.

A lot of people are skeptical and wonder how much of a difference soy can really make?

Kumar: We really do not know definitely what soy can do. However, in our study, we totally mimicked what the Japanese population was doing. We used the ratio of one milligram of isoflavones per kilogram of body weight. We know that a Japanese male usually takes four ounces to six ounces of soy a day, so we mimicked that quantity in this first clinical trial, and with that, we saw a change in the PSA. It is very encouraging that maybe more amounts of soy can even better dilate hormone levels of PSA. We used an amount that can in a very short period demonstrate the change.

To get the recommended doses of soy, what types of foods do you recommend to include in your diet?

Kumar: It is well documented that consuming whole soy products is probably the best way to get your soy and isoflavones, which are the potent chemicals in soy that reduce prostate cancer risk. In the first study, we used whole-soy products. In the second study we just initiated, we reduced the content to the purified form of isoflavones because we know now that it is the isoflavones that are reducing the cancer risk. To avoid the side effects of taking a bulky milk shake twice a day, we reduced it to a purified isoflavone tablet. The same amount of isoflavone can be obtained by consuming two ounces to three ounces of soy a day in the daily diet. The other observation researchers have made is the fermented form of soy is better absorbed in the gastrointestinal track than whole-soy products. For example, the miso and tofu form of soy isoflavones is better absorbed because it is fermented. We recommend that if you are not consuming soy to take two ounces to three ounces of soy. It is harmless and potentially beneficial.

What is your hope for the study you have just initiated based on what you already know about soy and prostate cancer?

Kumar: We have taken the research one step further. In the first study, we are looking at the same group of men, that watchful waiting group where the Gleason score is below six, who are not on any other treatment. That is why they are called the early stage watchful waiting patients. We give them 80 milligrams of purified isoflavones. It is, again, a randomized double-blinded clinical trial. We also give them a multi-vitamin and multi-mineral supplement, so that they do not take anything else; a lot of men and women are taking numerous nutritional supplements these days. Again, it is a three-month clinical trial, and we are observing the same markers including the sex hormone binding globulin that I was talking about in the blood and of course PSA. These are the two markers we are looking at. However, in another study, we have taken prostate cancer patients to look at what is actually happening at the cellular level in the prostate cancer. The only way to look, we know in a petri dish, at it in a live, human being is to look at the tissue change in the prostate. We cannot go about doing biopsies of prostates in men to look at the tissue change. We have taken the opportunity to detect this from prostate cancer patients who are already having a prostatectomy. We take the tissue from the biopsy that determines they have prostate cancer. Once they are diagnosed with prostate cancer and they know they are going to have a prostatectomy, they wait about four to six weeks before they have the surgery. We intervene in that four- to six-week period and give them three doses of isoflavones — a 40-milligram, a 16-milligram and an 80-milligram tablet of isoflavones. We intervene during this short period and take the biopsy from their prostatectomy to examine whether we have stopped the proliferation rate of the prostate cancer tissue. We also want to see how many cells we can kill or how many prostate cancer cells undergo the natural cell death.

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