Selenium and Cancer Prevention

Introduction

Selenium is a trace element widely and irregularly distributed in the environment. Its availability and consumption in food may vary significantly depending on the region. The daily requirement for selenium is between 200 and 250 micrograms (or a minimum of 1 microgram per kilo body weight and per day). Selenium occurs in animal and vegetal foods. The essentiality of selenium in human nutrition was derived from its anti oxidative action, being a part of the glutathione-peroxidase system (GPx). A deficit in selenium will result in a decrease of the GPx activity, and therefore, in an increase of cell damage which cannot be counter-balanced by other anti oxidative systems. Selenium is the main defense against oxidants. What is mostly ignored, is that a selenium deficiency may also result in a thyroid hormone utilization defect. Selenium deficiency symptoms appear in whole population groups when the selenium content in the diet is inadequate. Selenium toxicity states are rare.

Studies

Numerous studies have been published documenting the preventive effect of supplementation with selenium on the occurrence of cancer.

In the Linxian study selenium supplementation resulted in a 13 percent reduction in cancer mortality. <>Between 1986 and 1991, 29,584 persons took part in a randomized nutritional intervention trial in Linxian, China, an area whose residents had chronically low intakes of several nutrients and high rates of esophageal and gastric cardia cancer as well as stroke. Using a one-half replicate of a 2(4) factorial design, we randomized individuals to one of eight groups which received combinations of four supplements: retinol and zinc (factor A), riboflavin and niacin (factor B), vitamin C and molybdenum (factor C), and beta-carotene, alpha-tocopherol (vitamin E), and selenium (factor D). Deaths that occurred during 5 years of supplementation were ascertained and classified according to cause. At the end of the supplementation period, we measured blood pressure readings and determined the prevalence of hypertension. Participants who received factor D had reductions in total mortality (9%) and total cancer mortality (13%).

(International Epidemiology Institute, Rockville, MD. Vitamin/mineral supplementation and cancer risk: international chemoprevention trials. Proceedings of the Society for Experimental Biology an Medicine, 1997; Nov, 216(2):291-6) In another study, selenium supplementation resulted in 35.1 percent less precancerous lesions An intervention trial was undertaken among the general population of 130,471. Individuals in five townships were involved for observation of the preventive effect of Selenium. The 8-years follow- up data showed reduced liver precancerous lesion incidence by 35.1% in selenized table salt supplemented vs. the non supplemented population. On withdrawal of Se from the treated group, PLC incidence rate began to increase.

(Yu SY; Zhu YJ; Li WG. Cancer Institute, Chinese Academy of Medical Sciences, Peking Union Medical College, Beijing, China. Protective role of selenium against hepatitis B virus and primary liver cancer in Qidong. Biological Trace Element Research, 1997 Jan, 56(1):117-24.)

In a recent double-blind trial, selenium supplementation reduced prostate cancer incidence by 63 percent A total of 974 men with a history of either a basal cell or squamous cell carcinoma were randomized to either a daily supplement of 200 microgram of selenium or a placebo. Patients were treated for a mean of 4.5 years and followed for a mean of 6.5 years. The selenium treatment was associated with a significant (63%) reduction in the secondary endpoint of prostate cancer incidence

(Clark LC; Dalkin B; Krongrad A; Combs GF Jr; Turnbull BW; Slate EH; Witherington R; Herlong JH; Janosko E; Carpenter D; et al. Decreased incidence of prostate cancer with selenium supplementation: results of a double- blind cancer prevention trial. British Journal of Urology, 1998 May, 81(5):730-4.) There was no effect of selenium on the existing skin cancers in this trial. Detractors use this fact to pretend that Nutraceuticals in general and selenium in particular are useless for cancer prevention.

National Cancer Institute

The National Cancer Institute recognizes the power of selenium to prevent lung, colorectal, and prostate cancer.

Selenium Monitoring: The selenium status of a person can be assessed by monitoring the selenium plasma and urine level and the selenium content of an hair sample. Human plasma should not contain less than 100-150 and more than 2,800 nanograms of selenium per milliliter. Urine may contain from 50 to 100 nanogram per milliliter. Hair selenium should 27 micrograms/g. Erythrocyte and plasma selenium and glutathione peroxidase specific activities, hair and fecal selenium, and urinary selenium excretion were increased by and were linearly related to L- selenomethionine dose. Hair selenium was most sensitive to L-selenomethionine dose, with an 84- fold increase in the 300 micrograms selenium/(kg-d) group relative to controls (r = 0.917). Daily urinary selenium excretion (80-fold, r = 0.958), plasma selenium (22-fold, r = 0.885), erythrocyte selenium (24-fold, r = 0.920), and fecal selenium (18-fold, r = 0.911) also responded strongly to L-selenomethionine. Erythrocyte and plasma glutathione peroxidase specific activities increased 154% and 69% over controls, respectively. Toxicity was associated with erythrocyte selenium > 2.3 micrograms/mL, plasma selenium > 2.8 micrograms/mL, and hair selenium > 27 micrograms/g. Plasma, erythrocyte, and hair selenium concentrations may be useful for monitoring and preventing the toxicity of L-selenomethionine administered to humans in cancer chemoprevention trials.