12.6: Selenium and Other Antioxidant Minerals
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- 36355
- Describe how selenium is involved in protection of the human body from environmental damage.
While the human body has acquired multiple defenses against free radicals, we also use free radicals to support the functions of some cells. For example, the immune system uses the cell-damaging properties of free radicals to kill pathogens. First, immune cells engulf an invader (such as a bacterium), then they expose it to free radicals such as hydrogen peroxide, which destroys its membrane. The invader is thus neutralized. Scientific studies also suggest hydrogen peroxide acts as a signaling molecule that calls immune cells to injury sites, meaning free radicals may aid with tissue repair when you get cut.
Free radicals are necessary for many other bodily functions as well. The thyroid gland synthesizes its own hydrogen peroxide, which is required for the production of thyroid hormone. Reactive oxygen species and reactive nitrogen species, which are free radicals containing nitrogen, have been found to interact with proteins in cells to produce signaling molecules. The free radical nitric oxide has been found to help dilate blood vessels and act as a chemical messenger in the brain. By acting as signaling molecules, free radicals are involved in the control of their own synthesis, stress responses, regulation of cell growth and death, and metabolism.
Sources of Free Radicals in the Environment
Substances and energy sources from the environment can add to or accelerate the production of free radicals within the body. Exposure to excessive sunlight, ozone, smoke, heavy metals, ionizing radiation, asbestos, and other toxic chemicals increases the number of free radicals in the body. They do so by being free radicals themselves or by adding energy that provokes electrons to move between atoms. Excessive exposure to environmental sources of free radicals can contribute to disease by overwhelming the free radical detoxifying systems and those processes involved in repairing oxidative damage.
Selenium
Selenium is an essential trace mineral. It is part of the structure of at least 25 proteins in the body, with functions in reproduction, thyroid hormone metabolism, DNA synthesis, and antioxidant and immune protection.6 As part of antioxidant enzymes, selenium helps to regenerate other antioxidants, including vitamin C. These enzymes also protect lipids from free radicals, and, in doing so, spare vitamin E. This illustrates how antioxidants work together to protect the body against free radical-induced damage.
Food Sources of Selenium
Organ meats, muscle meats, and seafood have the highest selenium content. Grains and some nuts contain selenium when grown in selenium-containing soils. The selenium content of the soils used to grow animal feed can also affect the selenium content of animal products.
Figure 8.18. Dietary sources of selenium. Source: NIH Office of Dietary Supplements
Selenium Deficiency and Toxicity
Chronic exposure to foods grown in soils containing high levels of selenium (above the UL of 400 micrograms per day) can cause brittle hair and nails, gastrointestinal discomfort, skin rashes, halitosis, fatigue, and irritability. Brazil nuts contain very high levels of selenium, so if eaten regularly could cause selenium toxicity. Selenium at doses several thousand times the RDA can cause acute toxicity, and when ingested in gram quantities, can be fatal.
The Protective Effects of Selenium
Selenium is a cofactor of enzymes that release active thyroid hormone in cells and therefore low levels can cause similar signs and symptoms as iodine deficiency. The other important function of selenium is as an antioxidant.
Around 25 known proteins require selenium to function. Some are enzymes involved in detoxifying free radicals and include glutathione peroxidases and thioredoxin reductase. As an integral functioning part of these enzymes, selenium aids in the regeneration of glutathione and oxidized vitamin C. Selenium, as part of glutathione peroxidase, also protects lipids from free radicals, and, in doing so, spares vitamin E. This is just one example of how antioxidants work together to protect the body against free-radical induced damage. Other functions of selenium-containing proteins include protecting endothelial cells that line tissues, converting the inactive thyroid hormone to the active form in cells, and mediating inflammatory and immune system responses.
Dietary Reference Intakes for Selenium
| Age Group | R.D.A. Males and Females (micrograms per day) | U.L. |
|---|---|---|
| Infants (0–6 months) | 15* | 45 |
| Infants (7–12 months) | 20* | 65 |
| Children (1–3 years) | 20 | 90 |
| Children (4–8 years) | 30 | 150 |
| Children (9–13 years) | 40 | 280 |
| Adolescents (14–18 years) | 55 | 400 |
| Adults (> 19 years) | 55 | 400 |
Selenium at doses several thousand times the RDA can cause acute toxicity, and when ingested in gram quantities can be fatal. Chronic exposure to foods grown in soils containing high levels of selenium (significantly above the UL) can cause brittle hair and nails, gastrointestinal discomfort, skin rashes, halitosis, fatigue, and irritability. The IOM has set the UL for selenium for adults at 400 micrograms per day.
Table 11.10.2: Selenium Content of Various Foods
| Food | Serving | Selenium (micrograms) | Percent Daily Value |
|---|---|---|---|
| Brazil nuts | 1 oz. | 544 | 777 |
| Shrimp | 3 oz. | 34 | 49 |
| Crabmeat | 3 oz. | 41 | 59 |
| Ricotta cheese | 1 c. | 41 | 59 |
| Salmon | 3 oz. | 40 | 57 |
| Pork | 3 oz. | 35 | 50 |
| Ground beef | 3 oz. | 18 | 26 |
| Round steak | 3 oz. | 28.5 | 41 |
| Beef liver | 3 oz. | 28 | 40 |
| Chicken | 3 oz. | 13 | 19 |
| Whole-wheat bread | 2 slices | 23 | 33 |
| Couscous | 1 c. | 43 | 61 |
| Barley, cooked | 1 c. | 13.5 | 19 |
| Milk, low-fat | 1 c. | 8 | 11 |
Oxidative Stress
Oxidative stress refers to an imbalance in any cell, tissue, or organ between the number of free radicals and the capabilities of the detoxifying and repair systems. Sustained oxidative damage results only under conditions of oxidative stress—when the detoxifying and repair systems are insufficient. Free radical-induced damage, when left unrepaired, destroys lipids, proteins, RNA, and DNA, and can contribute to disease. Oxidative stress has been implicated as a contributing factor to cancer, atherosclerosis (hardening of arteries), arthritis, diabetes, kidney disease, Alzheimer’s disease, Parkinson’s disease, schizophrenia, bipolar disorder, emphysema, and cataracts.
Aging is a process that is genetically determined but modulated by factors in the environment. In the process of aging, tissue function declines. The idea that oxidative stress is the primary contributor to age-related tissue decline has been around for decades, and it is true that tissues accumulate free radical-induced damage as we age. Recent scientific evidence slightly modifies this theory by suggesting oxidative stress is not the initial trigger for the age-related decline of tissues; it is suggested that the true culprit is progressive dysfunction of metabolic processes, which leads to increases in free radical production, thus influencing the stress response of tissues as they age.
Antioxidant Minerals
In addition to the antioxidant vitamins and phytochemicals, several minerals have antioxidant function, including selenium, manganese, iron, copper, and zinc.
Selenium Functions and Health Benefits
Around twenty-five known proteins require selenium to function. Some are enzymes involved in detoxifying free radicals and include glutathione peroxidases and thioredoxin reductase. As an integral functioning part of these enzymes, selenium aids in the regeneration of glutathione and oxidized vitamin C. Selenium as part of glutathione peroxidase also protects lipids from free radicals, and, in doing so, spares vitamin E. This is just one example of how antioxidants work together to protect the body against free radical-induced damage.
Other functions of selenium-containing proteins include protecting endothelial cells that line tissues, converting the inactive thyroid hormone to the active form in cells, and mediating inflammatory and immune system responses.
Observational studies have demonstrated that selenium deficiency is linked to an increased risk of cancer. A review of forty-nine observational studies published in the May 2011 issue of the Cochrane Database of Systematic Reviews concludes that higher selenium exposure reduces overall cancer incidence by about 34 percent in men and 10 percent in women, but notes these studies had several limitations, including data quality, bias, and large differences among different studies.Dennert, G. et al. “Selenium for Preventing Cancer.” Cochrane Database of Systematic Reviews 5 (2011): CD005195. http://www.ncbi.nlm.nih.gov/pubmed/21563143. Additionally, this review states that there is no convincing evidence from six clinical trials that selenium supplements reduce cancer risk.
Because of its role as a lipid protector, selenium has been suspected to prevent cardiovascular disease. In some observational studies, low levels of selenium are associated with a decreased risk of cardiovascular disease. However, other studies have not always confirmed this association and clinical trials are lacking.
Dietary Reference Intakes for Selenium
The IOM has set the RDAs for selenium based on the amount required to maximize the activity of glutathione peroxidases found in blood plasma. The RDAs for different age groups are listed in Table \(\PageIndex{8}\).
| Age Group | RDA Males and Females mcg/day | UL |
|---|---|---|
| Infants (0–6 months) | 15* | 45 |
| Infants (7–12 months) | 20* | 65 |
| Children (1–3 years) | 20 | 90 |
| Children (4–8 years) | 30 | 150 |
| Children (9–13 years) | 40 | 280 |
| Adolescents (14–18 years) | 55 | 400 |
| Adults (> 19 years) | 55 | 400 |
| *denotes Adequate Intake | ||
Selenium at doses several thousand times the RDA can cause acute toxicity, and when ingested in gram quantities can be fatal. Chronic exposure to foods grown in soils containing high levels of selenium (significantly above the UL) can cause brittle hair and nails, gastrointestinal discomfort, skin rashes, halitosis, fatigue, and irritability. The IOM has set the UL for selenium for adults at 400 micrograms per day.
Dietary Sources of Selenium
Organ meats, muscle meats, and seafood have the highest selenium content. Plants do not require selenium, so the selenium content in fruits and vegetables is usually low. Animals fed grains from selenium-rich soils do contain some selenium. Grains and some nuts contain selenium when grown in selenium-containing soils. See Table \(\PageIndex{9}\) for the selenium content of various foods.
| Food | Serving | mcg |
|---|---|---|
| Brazil nuts | 1 oz. | 544.0 |
| Shrimp | 3 oz. | 34.0 |
| Crab meat | 3 oz. | 41.0 |
| Ricotta cheese | 1 c. | 41.0 |
| Salmon | 3 oz. | 40.0 |
| Pork | 3 oz. | 35.0 |
| Ground beef | 3 oz. | 18.0 |
| Round steak | 3 oz. | 28.5 |
| Beef liver | 3 oz. | 28.0 |
| Chicken | 3 oz. | 13.0 |
| Whole-wheat bread | 2 slices | 23.0 |
| Couscous | 1 c. | 43.0 |
| Barley, cooked | 1 c. | 13.5 |
| Milk, low-fat | 1 c. | 8.0 |
| Walnuts, black | 1 oz. | 5.0 |
Source: US Department of Agriculture, Agricultural Research Service. 2010. USDA National Nutrient Database for Standard Reference, Release 23. http://www.ars.usda.gov/ba/bhnrc/ndl.
Manganese, Iron, Copper, and Zinc: Functions and Health Benefits
As with selenium, manganese, iron, copper, and zinc are essential cofactors for enzymes involved in detoxifying free radicals. In the proper doses they allow for optimal detoxification of free radicals. In excess and when not bound to proteins, manganese, iron, and copper actually accelerate the production of free radicals. This is an attribute of all antioxidants in general, although the effect is greater for certain antioxidants.
Antioxidants can become pro-oxidants when the conditions are altered. Recall that oxidative stress results from an imbalance in free radicals with their detoxifying and repair systems. Another factor that can cause oxidative stress is a high level of antioxidants, as some will revert to acting as pro-oxidants.
Key Takeaways
- Health claims of antioxidant supplements are not backed by scientific evidence and there is some evidence suggesting they cause harm.
- Consuming antioxidant-rich foods is scientifically supported to reduce the risk of chronic diseases.
- Antioxidants have specific functions, and they can act in concert with each other to protect against free radicals.
- Some antioxidants are involved in the regeneration of other antioxidants.
- Many phytochemicals act as antioxidants, but they perform several other functions as well.
- Dietary antioxidants and phytochemicals in the proper doses are beneficial to health but can cause harm in excess.
Discussion Starters
- In regard to doses, discuss why it is better to obtain antioxidants from the diet and not from supplements. When might supplements be advantageous?
- With more scientific evidence in hand, debate whether or not the supplement industry requires more regulation.
References
1. Institute of Medicine (IOM). Food and Nutrition Board. 2000. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium and Carotenoids. Washington, DC: National Academy Press.
2. Kristal, A. R., A. K. Darke, J. S. Morris, et al. 2014. Baseline selenium status and effects of selenium and vitamin E supplementation on prostate cancer risk. The Journal of the National Cancer Institute 106(3):djt456.
3. Stabler, S. P. 2012. Vitamin B12. In: Erdman, J. W., I. A. Macdonald, and S. H. Zeisel, eds. Present Knowledge in Nutrition, 10th ed., pp. 343–358. Washington, DC: ILSI Press.
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