Nutrition focused medical practitioners have advised men with prostate cancer to take supplemental zinc for so long that few can still remember where this practice originated. Several papers now suggest zinc may actually be ill advised for these patients to take.
Zinc levels in the prostate gland are higher than in any other soft tissue of the body so it was easy to assume that zinc is important for prostate function and health. On top of this, zinc levels in healthy prostate tissue are almost 7 times as high as in prostate cancer tissue.  Several cell culture studies reported that the addition of supplemental zinc inhibited prostate cancer cell growth.
Iguchi reported in a 1998 that addition of zinc triggers necrosis in prostate cancer cells. 
Liang reported in 1999, “… that zinc inhibits human prostatic carcinoma cell growth, possibly due to induction of cell cycle arrest and apoptosis. There now exists strong evidence that the loss of a unique capability to retain high levels of zinc is an important factor in the development and progression of malignant prostate cells.”  Zinc was reported to inhibit prostate cancer invasion.  It was not unreasonable to think that taking zinc might have similar actions on prostate cancer in men as it does on prostate cancer cells in Petri dishes and so men started to take zinc supplements, lots of zinc supplements.
It was only a few years until signs started to appear that this assumption might not prove true. In July 2003 a paper published in the Journal of the National Cancer Institute reported that taking up to 100 mg of zinc per day did not change risk of prostate cancer but that men who took more than 100 mg/day had more than double the risk of advanced prostate cancer compared to men who did not take zinc. Men who took any supplemental zinc for more than ten years also had double the risk of prostate cancer (n= 46 974). 
These cautionary findings were confused by a 2011 Swedish study that assessed dietary zinc levels in 525 men when they were first diagnosed with prostate cancer. Those with high dietary zinc intake had a 36% lower risk of dying from prostate cancer than those with low intake. These results strongly suggested “… that high dietary intake of zinc is associated with lower prostate cancer-specific mortality after diagnosis, particularly in men with localized disease.”  Keep in mind this study compared zinc intake from food, not from supplements.
A 2014 cell study explained that long-term zinc exposure helps prostate cancer cells become resistant to treatment by a range of chemotherapy drugs including cisplatin. Zinc lead to chemo-resistance in a range of different tumor cell lines via KRAS NF-κB.  Then in early 2017 these mechanisms were more clearly defined by a paper by Kratochvilova et al who showed that long term treatment with zinc “… significantly enhanced cisplatin resistance, invasiveness, cellular antioxidant capacity, synthesis of glutathione, and expression of treatment resistance- … genes” in prostate cancer cells.  This is not what we would want to happen.
This has gotten far more complicated. Two decades ago we were still approaching cancer as if it were a deficiency disease, kind of like scurvy and vitamin C. We assumed that it would boil down to identifying specific nutrients that would promptly restore health, or at a minimum prevent cancer. Over the years it has slowly dawned on the researchers and on practitioners alike, that treating cancer is not going to be as simple as we once thought. The relationship between nutrients and cancer is anything but simple, the effects varying with timing, dose and other factors as yet beyond our grasp. While low doses of zinc from the diet may lower risk of this disease, high doses in the form of supplements may make prostate cancer more aggressive and harder to treat.
Two decades ago, it made sense for men to take zinc. These days, taking zinc supplements to prevent or treat prostate cancer may just not be a very good idea.
1. Zaichick V, Sviridova TV, Zaichick SV. Zinc in the human prostate gland: normal, hyperplastic and cancerous. Int Urol Nephrol 1997;29:565–74.
2. Iguchi K, Hamatake M, Ishida R, Usami Y, Adachi T, Yamamoto H, Koshida K, et al. Induction of necrosis by zinc in prostate carcinoma cells and identification of proteins increased in association with this induction. Eur J Biochem. 1998 May 1;253(3):766-70.
3. Liang JY, Liu YY, Zou J, Franklin RB, Costello LC, Feng P. Inhibitory effect of zinc on human prostatic carcinoma cell growth. Prostate. 1999 Aug 1;40(3):200-7.
4. Ishii K, Usui S, Sugimura Y, Yoshida S, Hioki T, Tatematsu M, Yamamoto H, et al. Aminopeptidase N regulated by zinc in human prostate participates in tumor cell invasion. Int J Cancer. 2001 Apr 1;92(1):49-54.
5. Leitzmann MF, Stampfer MJ, Wu K, Colditz GA, Willett WC, Giovannucci EL. Zinc supplement use and risk of prostate cancer. J Natl Cancer Inst. 2003 Jul 2;95(13):1004-7.
6. Epstein MM, Kasperzyk JL, Andrén O, Giovannucci EL, Wolk A, Håkansson N, Andersson SO, et al. Dietary zinc and prostate cancer survival in a Swedish cohort.
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7. Holubova M, Axmanova M, Gumulec J, Raudenska M, Sztalmachova M, Babula P, Adam V, et al. KRAS NF-κB is involved in the development of zinc resistance and reduced curability in prostate cancer. Metallomics. 2014 Jul;6(7):1240-53.
8. Kratochvilova M, Raudenska M, Heger Z, Richtera L, Cernei N, Adam V, Babula P, et al. Amino Acid Profiling of Zinc Resistant Prostate Cancer Cell Lines: Associations With Cancer Progression. Prostate. 2017 Jan 19.