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Studying D, Soy and Prostate Cancer

David Feldman, MDEarly research suggests that vitamin D and soy, alone or in combination, might be useful in preventing and treating prostate cancer. An AICR grantee is investigating their potential.

Prostate cancer is the second most common type of cancer diagnosed in men, following skin cancer. A renowned expert on vitamin D and prostate cancer, AICR grantee David Feldman, MD of Stanford University, has been looking into how vitamin D and soy might slow or stop prostate cancer growth.

Vitamin D is not really a vitamin, Dr. Feldman explains. "Vitamin D is a precursor to a potent hormone called calcitriol that regulates hundreds of genes in most tissues of the body. Taking vitamin D is more like taking a substance that the body then converts to an active drug."

Calcitriol helps the body absorb calcium. It also has potent anti-cancer effects on certain pathways involved in the growth of cancer cells. This is why vitamin D is being studied for its potential to reduce the risk for breast, prostate and colorectal cancers.

Reinforcing Protection with Soy

Over time, the body breaks calcitriol down to an inactive form. To boost its potency, Dr. Feldman and his co-workers used vitamin D or calcitriol in combination with the phytochemical in soy called genistein, which, in Dr. Feldman's earlier lab studies, inhibited the action of the enzyme that inactivates calcitriol.

Rats fed a high soy diet (20 percent of calories as soy) showed a substantially greater inhibition of prostate tumor growth when they also received injections of calcitriol.

What's more, studies of prostate tissue from prostate cancer patients who ate diets with a controlled amount of soy showed a significant influence on some of these same genes. These early studies hold promise for future clinical studies of vitamin D and soy supplements in prostate cancer patients.

Use Caution with D and Soy

Although the combination of vitamin D and soy is being studied to see if it protects against the development or progression of prostate cancer, Dr. Feldman cautions men against trying it on their own because significant side effects occur and the combination can be toxic. High concentrations of calcium in the blood (a toxic and potentially fatal condition known as hypercalcemia) occurred in mice fed a high-soy diet and treated with calcitriol.

Since vitamin D is absorbed at different rates in individuals – depending on whether a person consumes food sources like milk and whether one's skin is exposed to sunlight – it is not possible to give a general recommended dose for it. Dr. Feldman suggests getting a blood test to find out how much vitamin D is in your body, followed by supplementation if advised by your healthcare provider.

Dr. Feldman and his colleagues are extending their studies through a new AICR-funded study, Diet-Induced Obesity and Breast Cancer: Protective Role of Vitamin D.

Published on May 16, 2013

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