Download
1 / 35
360 likes | 481 Vues
An t ioxidants Mgr. Iva Klimešová, Ph.D. Antioxidant is any substance that prevents or reduces damage caused by reactive free radicals .
E N D
Antioxidant is any substance that prevents or reduces damage caused by reactive free radicals. • Free radicals are formed primarily in the body during normal metabolism and also upon exposure to environmental factors such as cigarette smoke or pollutants. • Fats, which are an integral part of all cell membranes, are vulnerable to destruction through oxidation by free radicals.
Free radicals promote beneficial oxidation that produces energy and kills bacteria. • In excess, however, they produce harmful oxidation that can damage cell membranes and cell contents. • Although oxidation reactions are critical for life, they can also be damaging; hence, plants and animals maintain complex systems of multiple types of antioxidants, such as glutathione, vitamin C, and vitamin E as well as enzymes such as catalase, superoxide dismutase and various peroxidases. • Low levels of antioxidant molecules or inhibition of these antioxidant enzymes causes oxidative stress and may damage or kill cells. • It is known that people who eat adequate amounts of fruits and vegetables high in antioxidants have a lower incidence of cardiovascular disease, certain cancers, and cataracts.
The most publicized antioxidants have been: • fat-soluble antioxidants: vitamin A and carotenoids, vitamin E, Coenzyme Q10 • water-soluble antioxidants: vitamin C, minerals – zinc, selenium, manganese
Vitamin A Vitamin A is a generic term for a large number of related compounds: • Retinol (an alcohol) and retinal (an aldehyde) are often referred to as preformed vitamin A. Retinol, retinal, retinoic acid, and related compounds are known as retinoids. • Beta-carotene and other carotenoids that can be converted by the body into retinol are referred to as provitamin A. • Hundreds of different carotenoids are synthesized by plants, but only about 10% of them are provitamin A. • The main function of vitamin A in humans appears to be that of an antioxidant, beta-carotene has stronger antioxidant activity than retinol.
Beta-carotene is less easily absorbed than retinol and must be converted to active form. • Two micrograms (mcg) of beta-carotene in oil provided as a supplement can be converted by the body to 1 mcg of retinol. • However, 6 mcg of beta-carotene from foods are required to provide the body with 1 mcg of retinol. • Retinol is found in foods from animals. Plants contain carotenoids.
good sources, (RDA = 1000 mcg, or 5000 IU, for adult males and 800 mcg, or 4,000 IU, for adult females)
The combination of vitamin A and iron seems to reduce anemia more effectively than either iron or vitamin A alone. • Toxicity: • Vitamin A toxicity is relatively rare. Symptoms include nausea, headache, fatigue, loss of appetite, dizziness, and dry skin. • Consumption of excess vitamin A during pregnancy is known to cause birth defects.
Carotenoids • The main carotenoids:Alpha-Carotene, Beta-Carotene, Beta-Cryptoxanthin, Lycopene, Lutein and Zeaxanthin • Carotenoids are yellow, orange, and red pigments synthesized by plants. • Fruits and vegetables provide most of the carotenoids in the human diet. • Orange and yellow vegetables like carrots and winter squash are rich sources of alpha- and beta-carotene. Spinach is also a rich source of beta-carotene, although the chlorophyll in spinach leaves hides the yellow-orange pigment. • Alpha-carotene, beta-carotene, and beta-cryptoxanthin are provitamin A, meaning they can be converted by the body to retinol (vitamin A). • Lutein, zeaxanthin, and lycopene have no vitamin A activity.
For dietary carotenoids to be absorbed intestinally, they must be mixed with fatty acids and incorporated into micelles. • As little as 3-5 g of fat in a meal appears sufficient to ensure carotenoid absorption. • Supplement: Because they do not need to be released from the plant matrix, carotenoids supplements (in oil) are more efficiently absorbed than carotenoids in foods.
The vitamin A activity of beta-carotene in foods is 1/6 that of retinol (preformed vitamin A). Thus, it would take 6 mcg of beta-carotene from foods to provide the equivalent of 1 mcg of retinol.
Supplementation: Although diets rich in beta-carotene have generally been associated with reduced cardiovascular disease risk in observational studies, there is no evidence that beta-carotene supplementation reduces cardiovascular disease risk. • Toxicity: Two randomized controlled trials found that high-dose beta-carotene supplements increased the risk of lung cancer in smokers.* * Michaud DS, Feskanich D, Rimm EB, et al. Intake of specific carotenoids and risk of lung cancer in 2 prospective US cohorts. Am J Clin Nutr. 2000;72(4):990-997. Holick CN, Michaud DS, Stolzenberg-Solomon R, et al. Dietary carotenoids, serum beta-carotene, and retinol and risk of lung cancer in the alpha-tocopherol, beta-carotene cohort study. Am J Epidemiol. 2002;156(6):536-547.
Vitamin E • Major sources of alpha-tocopherol in the diet include vegetable oils (olive, sunflower, safflower oils), nuts, whole grains, and green leafy vegetables. • The main function of alpha-tocopherol in humans appears to be that of an antioxidant. • The term vitamin E describes a family of eight antioxidants: • four tocopherols, alpha-, beta-, gamma- and delta-, • and four tocotrienols (also alpha-, beta-, gamma- and delta-). • Alpha-tocopherol is the only form of vitamin E that is actively maintained in the human body.
good sources (the RDA for vitamin E was previously 8 mg/day for women and 10 mg/day for men)
Toxicity: High-dose vitamin E supplementation increases the risk of hemorrhage. • Supplementation with 400 IU/day of vitamin E has been found to accelerate the progression of retinitis pigmentosa.
Coenzyme Q10 (ubiquinol) • Coenzyme Q10 is a fat-soluble compound primarily synthesized by the body and also consumed in the diet. • Coenzyme Q10 is a member of the ubiquinone family of compounds, also known as ubiquinone or ubiquinol. • All animals, including humans, can synthesize ubiquinones, hence, coenzyme Q10 cannot be considered a vitamin. • Coenzyme Q10 is required for mitochondrial ATP synthesis and functions as an antioxidant in cell membranes and lipoproteins.
CoQ10 has been widely used for the treatment of heart disease (especially heart failure), gum diseases, and also breast cancer. • Young people are able to make Q10 from the lower numbered ubiquinones such as Q6 or Q8. The sick and elderly may not be able to make enough. • Rich sources of dietary coenzyme Q10 include mainly meat, poultry, and fish. Other relatively rich sources include soybean and canola oils, and nuts. Fruits, vegetables, eggs, and dairy products are reasonable sources of coenzyme Q10. • Approximately 14%-32% of coenzyme Q10 was lost during frying of vegetables and eggs, but the coenzyme Q10 content of these foods did not change when they were boiled.
Supplementation: Although coenzyme Q10 supplements are relatively safe, they may decrease the anticoagulant efficacy of warfarin. Presently, it is unclear whether individuals taking cholesterol-lowering medications, known as statins, would benefit from coenzyme Q10 supplementation.
Vitamin C • Sources: fruits and vegetables vary in their vitamin C content • Vitamin C, also known as ascorbic acid, is a water-soluble vitamin. • Unlike most mammals, humans do not have the ability to make their own vitamin C. Therefore, we must obtain vitamin C through our diet. • Vitamin C is known as a highly effective antioxidant in living organisms.
Supplements: Vitamin C (ascorbic acid) is available in many forms, but there is little scientific evidence that any one form is better absorbed or more effective than another. Natural vs. synthetic vitamin C: • Natural and synthetic ascorbic acid are chemically identical and there are no known differences in their biological activities or bioavailability Toxicity: A number of possible problems with very large doses of vitamin C have been suggested, mainly based on in vitro experiments or isolated case reports, including: genetic mutations, birth defects, cancer, atherosclerosis, kidney stones, increased oxidative stress, excess iron absorption, vitamin B-12 deficiency, and erosion of dental enamel. • However, none of these adverse health effects have been confirmed, and there is no reliable scientific evidence that large amounts of vitamin C (up to 10 grams/day in adults) are toxic.
Zinc Shellfish, beef, and other red meats are rich sources of zinc. Nuts and legumes are relatively good plant sources. • Zinc bioavailability is relatively high in meat, eggs, and seafood because of the relative absence of compounds that inhibit zinc absorption and the presence of certain amino acids (cysteine and methionine) that improve zinc absorption. • The zinc in whole grain products and plant proteins is less bioavailable due to their relatively high content of phytic acid, a compound that inhibits zinc absorption. • The enzymatic action of yeast reduces the level of phytic acid in foods. Therefore, leavened whole grain breads have more bioavailable zinc than unleavened whole grain breads.
Supplementation: A number of zinc supplements are available, including zinc acetate, zinc gluconate, zinc picolinate, and zinc sulfate. • Zinc picolinate has been promoted as a more absorbable form of zinc, but there is little data to support this idea in humans. • Adverse effects: The major consequence of long-term consumption of excessive zinc is copper deficiency. • Drug Interactions: Zinc may decrease absorption of the antibiotic. Taking zinc supplements and these antibiotics at least two hours apart should prevent this interaction.
Selenium • The richest food sources of selenium are organ meats and seafood, followed by muscle meats. • In general, there is wide variation in the selenium content of plants and grains because plants do not appear to require selenium. • Thus, the incorporation of selenium into plant proteins is dependent only on soil selenium content. Brazil nuts grown in areas of Brazil with selenium-rich soil may provide more than 100 mcg of selenium in one nut, while those grown in selenium-poor soil may provide 10 times less. • Selenium is a trace element that is essential in small amounts, but can be toxic in larger amounts. • Humans and animals require selenium for the function of a number of selenium-dependent enzymes, also known as selenoproteins (e.g. coating of sperm is made from selenoproteins).
Supplementation: Selenium supplements are available in several forms. • Sodium selenite and sodium selenate are inorganic forms of selenium. • Selenate is almost completely absorbed, but a significant amount is excreted in the urine before it can be incorporated into proteins. • Selenite is only about 50% absorbed, but is better retained than selenate, once absorbed. • Selenomethionine, an organic form of selenium that occurs naturally in foods, is about 90% absorbed (enriched areyeast, which are also available as supplements). • It is not clear whether one form of selenium is preferable to another.
Toxicity: Although selenium is required for health, high doses can be toxic.
Manganese • Rich sources of manganese include whole grains, nuts, leafy vegetables, and teas. • Foods high in phytic acid, such as beans, seeds, nuts, whole grains, and soy products, or foods high in oxalic acid, such as cabbage, spinach, and sweet potatoes, may slightly inhibit manganese absorption. • Although teas are rich sources of manganese, the tannins present in tea may moderately reduce the absorption of manganese • Manganese is a mineral element that is both nutritionally essential and potentially toxic.
Supplements: Several forms of manganese are found in supplements, including manganese gluconate, manganese sulfate, manganese ascorbate, and amino acid chelates of manganese. • Relatively high levels of manganese ascorbate may be found in a bone/joint health product containing chondroitin sulfate and glucosamine hydrochloride • Toxicity: A single case of manganese toxicity was reported in a person who took large amounts of mineral supplements for years, while another case was reported as a result of taking a Chinese herbal supplement.
Some antioxidants are made in the body and are poorly absorbed from the intestine. • One example is glutathione, which is made from amino acids. Glutathion as a supplemet in the gut is broken down and have little effect on the concentration of glutathione in the body. • Ubiquinol (coenzyme Q) is also poorly absorbed from the gut and is made in humans body.
Food preservatives • Antioxidants are used as food additives to help guard against food deterioration. • Exposure to oxygen and sunlight are the two main factors in the oxidation of food, so food is preserved by keeping in the dark and sealing it in containers or even coating it in wax, as with cucumbers. • However, as oxygen is also important for plant respiration, storing plant materials in anaerobic conditions produces unpleasant flavours and unappealing colours. • Consequently, packaging of fresh fruits and vegetables contains an -8% oxygen atmosphere. • Some antioxidants are added to industrial products. A common use is as stabilizers in foods and lubricants to prevent oxidation. • Antioxidant preservatives are also added to fat-based cosmetics such as lipstick and moisturizers to prevent rancidity.
The body has a limited ability to excrete vitamins and minerals, so excessive consumption may cause a number of health problems, even in relatively small dosages. • Vitamin supplements may be necessary for individuals whose diets don't provide the recommended amounts of specific vitamins and especially important for pregnant and nursing women and people with specific illnesses. • However, the benefits of vitamin supplements for the general population remain uncertain.
