A study in Sweden examining the effects of zinc and the omega-3 fatty acid DHA on mortality in prostate cancer patients was presented at the American Association for Cancer Research annual meeting in April. Five-hundred twenty-five men with prostate cancer were followed for twenty years after being diagnosed with prostate cancer. Baseline dietary habits, stage of cancer at diagnosis, and deaths over the twenty years were recorded and analyzed.1
The authors chose to investigate these two nutrients because zinc and omega-3s share the common action of attenuating the inflammatory response, and chronic systemic inflammation may fuel prostate cancer progression. Importantly, zinc and DHA are both somewhat more difficult to obtain on a vegan diet.
Zinc is especially concentrated in the prostate, but zinc levels become depleted in cancerous cells. Addition of zinc to cultured prostate cancer cells leads to cell death, possibly by suppressing the activity of inflammatory molecules. A previous study found that long-term zinc supplementation was associated with reduced risk of advanced prostate cancer.2
In the current study, the researchers organized the study participants into quartiles according to their intakes of zinc and DHA. In men who were diagnosed at with early stage cancers, the highest quartile of zinc intake (15.7 mg zinc daily or more) was associated with a 74% reduction in risk of death from prostate cancer compared to the lowest quartile (12.8 mg zinc daily or less). Absorption of zinc tends to be low on a vegan diet - beans, whole grains, nuts, and seeds have high zinc content, however these foods also contain substances that inhibit the aborption of zinc.3 A 2009 study of vegetarians found a high prevalence of zinc deficiency.4 To correct for bioavailability, the zinc requirement for vegans may be as much as 50% higher than that of omnivores.5 I recommend zinc supplementation or a red blood cell zinc level to assure sufficient zinc status in vegans or those who minimize animal foods.
The connection between omega-3 intake and prostate cancer is somewhat complex. For example, flaxseed oil was found to increase prostate cancer risk, whereas whole flaxseed, EPA, and DHA were found to be protective.6,7,8 EPA and DHA are known to have anti-cancer and anti-inflammatory properties.9 In this study, the highest quartile of DHA intake was associated with 30% reduced risk of overall prostate cancer mortality, and a 45% risk reduction in men diagnosed at early stages, supporting the idea that DHA is protective against prostate cancer. Plant foods contain ALA, which can be elongated to DHA, but since the major food source of DHA is fish, and many vegans levels (especially older men) are deficient, I recommend a laboratory cultivated DHA supplement made from micro-algae, which is vegan and also more environmentally sustainable than fish or fish oil. My experience with drawing thousands of DHA levels on vegans is that severe deficiencies are more common in men compared to women, as their efficiency in converting the shorter chain ALA into DHA can be insufficient and the levels tend to go down with aging. It can be important to either take DHA or confirm adequacy with a blood test, especially in middle aged and older men.
1. Meyer MS, Kasperzyk JL, Andren O, et al. Anti-inflammatory nutrients and prostate cancer survival in the Örebro Prostate Cancer Survivors Cohort. [Abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; 2010. Abstract nr 5747
MedPageToday. AACR: Zinc Linked to Prostate Cancer Survival. http://www.medpagetoday.com/MeetingCoverage/AACR/19685
2. Gonzalez A, Peters U, Lampe JW, White E. Zinc intake from supplements and diet and prostate cancer. Nutr Cancer. 2009;61(2):206-15.
3. Hunt JR. Bioavailability of iron, zinc, and other trace minerals from vegetarian diets. Am J Clin Nutr 2003;78(suppl):633S-9S.
4. de Bortoli MC, Cozzolino SM. Zinc and selenium nutritional status in vegetarians. Biol Trace Elem Res. 2009 Mar;127(3):228-33.
5. Frassinetti S, Bronzetti G, Caltavuturo L, et al. The role of zinc in life: a review. J Environ Pathol Toxicol Oncol. 2006;25(3):597-610.
6. Brouwer IA, Katan MB, Zock PL. Dietary alpha-linolenic acid is associated with reduced risk of fatal coronary heart disease, but increased prostate cancer risk: a meta-analysis. J Nutr 2004 Apr;134(4):919-22
7. Demark-Wahnefried W, Polascik TJ, George SL, et al. Flaxseed supplementation (not dietary fat restriction) reduces prostate cancer proliferation rates in men presurgery. Cancer Epidemiol Biomarkers Prev. 2008 Dec;17(12):3577-87.
8. Leitzmann MF, Stampfer MJ, Michaud DS, et al. Dietary intake of n-3 and n-6 fatty acids and the risk of prostate cancer. Am J Clin Nutr. 2004 Jul;80(1):204-16.
9. Spencer L, Mann C, Metcalfe M, et al. The effect of omega-3 FAs on tumour angiogenesis and their therapeutic potential. Eur J Cancer. 2009 Aug;45(12):2077-86.