Trace metals, salts and cancer

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Manganese, copper and zinc are dietary trace elements, classified as antioxidants because they are essential for the production of superoxide dismutase (SOD) and selenium is also essential for glutathione peroxidase. Together with catalase, these form enzymatic defence against carcinogenic free oxygen radical reduction metabolites. These three enzymes are responsible for the end stage of mopping up of the free radicals before they have time to damage the DNA.

It has been postulated that intensive farming food techniques and food processing may reduce these trace metals in our diet.  Research from animal studies has shown that there is an increased risk of cancer in the presence of cooper, manganese, seleniun or zinc deficiencies particularly under conditions of high carcinogenic attack where more SOD is needed. Zinc is abundant in many food sources and tends to accumulate more in the prostate, and one laboratory study suggested that this may offer some protection against prostate cancer cell growth.  

Zinc in excess, as you can image, is likely to be harmful. This was highlighted in the Health Professionals Follow-Up Study (HPFS). In this study, 50,000 health professionals were recruited between 1986 and 2002, from several medical fields. They were required to provide information on their eating, smoking and drinking habits, including the type of alcohol they preferred and supplements they took. During the study, 3,348 cases of prostate cancer were diagnosed. Further analysis of the diet and type of cancer showed that men who took normal amounts of zinc had the normal incidence of prostate cancer but those who took supplemental zinc at levels of more than 100mg/day or for long durations were more than twice as likely to develop advanced prostate cancer.

Selenium has been shown to slow the progression of cancer cells when it was added to their culture medium in the laboratory. This was affect was also independent of the enzymatic SOD pathway, indicating that selenium on its own may have a direct anticancer effect. In humans two large studies in 1980 and 1990 showed that a low selenium status was associated with an increased risk of developing cancer. They also showed that in patients with selenium deficiency, the cancer they developed were more likely to be aggressive and fatal. The Harvard Health professional survey, for example, linked low selenium status (measured on toenail clippings!) with higher rates of aggressive prostate cancer. Both Finish and Taiwanese studies have linked blood lower levels of selenium with higher rates of lung and a liver cancer called Hepatocellular carcinoma (HCC). In China, where the incidence of HCC is high, the inhabitants of one village were supplemented with sodium selenite whilst another 5 villages were given simple salt. After 6 years, involving over 130,000 people, there was a 35% reduction in the HCC rate in the selenium supplemented village but no change in the others.

These data prompted the designed and initiation of an excellent double-blind randomised trial in the USA called the Nutritional Prevention Study. It recruited 1312 individuals with a history of skin cancer and prescribed either placebo or 220 micrograms of selenium a day. The primary aim (end point), was to see if dietary selenium supplementation could reduce the risk of recurrent skin cancer. There was no difference in the number of skin cancers between the selenium or placebo group. However, when the data was analysised in more detail, a significantly lower level of lung, bowel and prostate cancer was seen in the selenium group and this lived up to robust statistical evaluation. Several large ongoing prostate prevention studies including the SELECT study are now underway across the world to try and confirm these findings and fine tune the optimal selenium dose required. Many suspect however that the people who would benefit from selenium supplements are those with a dietary deficiency in the first place for example those living in area with low soil levels.

Calcium. Four prospective cohort studies, relating to calcium and prostate cancer, have been published. Two recommended a mean calcium intake between 1330-1840mg/day and showed no benefit or risk or associated risk. Two others, one involving 86,404 men in the CP II Nutrition cohort, with mean intake of >2000mg/day from food and supplements, actually showed a significantly higher rate of prostate cancer. Five of nine further questionnaire surveys associated high intake of dairy food with an increased risk of prostate & breast cancer but interpretation of these was complicated the fact that high diary was associated with high fat intake. The detrimental effect of excessive calcium is thought to lay in the finding that high dietary calcium can reduce blood and cellular vitamin D levels. As will be described in more detail below Vitamin D has demonstrated anticancer effects via its anti-proliferative properties and these benefits are therefore lost with calcium excess.

Advice on healthy dietary mineral intake

Selenium - recommended 60-75mcg/day. (No more than 200mcg/day). Zinc (no more than 11mg zinc per day.:

 Calcium - recommended dietary calcium intakes 1000mg/day for women and 800mg / day for men


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