Selenium: On the trail of the tumor killer?
Selenium is part of important enzymes that protect the body from aggressive compounds. The trace element therefore not only works to prevent cancer, but also reduces the growth of existing tumors. In what quantities and against which cancers selenium helps, however, is still contentious.
Selenium against cancer
The idea that selenium could protect against cancer is not new. Already in 1969, scientists had noticed a lower cancer mortality in areas with selenium-rich soils compared to areas with low-selenium soils. In addition, they observed that the level of dietary selenium intake or selenium levels in the blood is related to the incidence of tumor diseases. Many years ago, a need for selenium was derived, which is well above the average food intake.
Selenium: building block of important radical scavengers
Selenium occurs in the human body in so-called selenoproteins whose biological effects are very diverse. Among other things, the trace element is thus involved in inactivating cancer-causing (carcinogenic) or mutagenic substances, influencing malignant changes in cells and acting on the immune system. In artificially produced tumors, selenium was particularly effective in preventing the development and early stages of abnormally altered cells. The effect of selenium depends on its chemical form, the dose and the corresponding cancer trigger. Oxygen radicals can damage the genetic material and thus trigger mutations. They are also able to activate chemical carcinogens. Scientists assume In addition, oxygen radicals can change harmless viruses to become pathogenic or carcinogenic. The selenium-dependent enzymes glutathione peroxidase and thioredoxin reductase repel the harmful oxygen radicals. Presumably, other selenium-containing proteins act as radical scavengers.
Selenium blocks carcinogenic metals
Metals such as cadmium, lead, zinc, arsenic and chromium can also cause cancer. Some of these metals stimulate the formation of oxygen radicals, alter the genome or inhibit repair enzymes. In addition, they can develop co-carcinogenic activities, that is, support other cancer triggers, or stimulate the growth of tumor cells. Selenium can prevent cancer by binding these metals to metal selenides or protein complexes, rendering them harmless.
Animal experiments have shown that cells that have sufficient selenium are slower to grow than cells deficient in selenium. With slower growth, the body will probably have more time to repair the damaged cells. This reduces the risk of mutations. Also in experiments with human tumor cells, cancer researchers found that selenium inhibits cell growth. In addition, they stimulated in experiments the production of the tumor suppressing protein p53, triggering the programmed cell death (apoptosis).
Selenium against cancer: trace element interferes with cancer growth
Selenium probably influences the tumor cells by inhibiting the arrangement of the microtubules during cell division. This prevents the growth of the cancer cell. In studies with human liver cells it could be observed that the tumor cells partially regressed after selenium administration. This resulted in the inhibition of the cancer-causing substances. In addition, genes have been activated that regulate the growth and division of normal cells.
Furthermore, scientists suspect that the thioredoxin reductase and probably other selenoproteins regulate certain functions of the immune system. On the one hand, they stimulate the cell proliferation of white blood cells, the lymphocytes, and stimulate the formation of receptors for specific signaling substances of the immune system (cytokines). On the other hand, selenium-containing enzymes prevent an inhibition of the tumor necrosis factor TNF, which hinders nuclear division or proliferation in a number of tumor cells.
How does selenium work against cancer – studies sometimes contradictory
Selenium thus acts on the carcinogenesis in the body through various mechanisms. However, some biological effects have so far only been demonstrated in animal models. It is therefore questionable to what extent they are transferable to humans. To demonstrate the efficacy of selenium for human cancer prevention, numerous epidemiological observational studies have been conducted since the early 1980s. For example, US studies have shown that a higher selenium concentration in the blood and toenails is associated with a lower incidence of prostate cancer. A Japanese study came to similar conclusions: at a high selenium concentration, the risk for prostate cancer was halved. Also for lung, large intestine, Gastric and breast cancer scientists found an anti-carcinogenic effect of selenium. However, other studies failed to confirm a protective effect of the trace element.
A series of intervention studies should therefore clarify whether targeted selenium administration can reduce the incidence of cancer. For example, residents of Linxian, a region in China, were given various mineral and vitamin supplements. In the group, which received a mixture of selenium, beta-carotene and vitamin E daily for over five years, the cancer mortality rate dropped by 13 percent. In the same study, the researchers observed that higher blood levels of selenium were associated with a lower incidence of esophageal cancer and malignant gastric tumors (cardia), but not with cancer of other gastric regions.
Cancer incidence decreased with selenium
Another intervention study, the “Nutritional Prevention of Cancer” study, caused a great stir when discussing the effect of selenium on cancer risk. Former skin cancer patients were investigated to see whether the administration of 200 micrograms of selenium daily affects the recurrence of carcinoma of the skin. Although the supply showed no effect on the skin cancer risk. However, the scientists observed that overall mortality, deaths from cancer and cancer incidence of lung, prostate and colorectal cancer could be reduced by the use of selenium. However, there are also conflicting results: another study, which examined the growth and recurrence of ulcers of the colon under a selenium treatment, could find no overall protective effect.
Selenium and cancer
Since the results are so contradictory, well-known nutritional institutions, such as the German Nutrition Society (DGE) or the US Institute of Medicine, do not yet recommend higher levels of selenium. The suspected positive effects of increased selenium intake are also countered by possible unwanted side effects. Long-term exposures above the upper limit of 400 micrograms per day recommended by the Institute of Medicine are considered hazardous to health, as chronic intoxication can not be ruled out. This can be manifested by brittleness and loss of hair and nails, indigestion, skin rashes, garlic-like odor of breath, fatigue and irritability.
Too much selenium is harmful
But even lower dosages can cause side effects. Additional intake of selenium may lead to increased activation of selenium-containing enzymes of the thyroid hormone metabolism in persons who are undersupplied with iodine, as is often the case in the German population. As a result, fewer thyroid hormones would be formed and thus increase the already high number of diseases of the thyroid gland. In addition, it is currently unclear whether the system of selenium storage in the organism is harmless. This already takes place at an amount which only slightly exceeds the saturation of the selenium-dependent proteins. This could lead to the storage of Selenschwermetallkomplexen in internal organs.
The insufficient data and the complex mechanisms of action require further intensive research. Only then can the benefits of higher selenium intake be clearly assessed and new feed recommendations derived.