The main role of antioxidants is to prevent, retard and / or reverse reactions leading to the oxidation of biological substrates (proteins, lipids and nucleic acids). Although antioxidants can act through various mechanisms, mostly by stabilizing a free radical through the donation of an electron, or a hydrogen atom. As a result of such interaction, free radicals lose their reactivity and antioxidants are oxidized.
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On the other hand, certain antioxidants can also act by inhibiting the formation of
pro-oxidant species , favoring the removal of such species, or facilitating the reduction of those biological substrates that have already been targeted for oxidation.
ANTIOXIDANTS IN FOOD: MAIN SOURCES AND THEIR CONTENTS
The presence of natural antioxidants in food is important, not only because these compounds help to define the organoleptic characteristics and preserve the nutritional quality of the products that contain them, but also because, when ingested, they help to preserve -in a considerable way- the health of the individuals who consume them. Indeed, the recommendation to increase the intake of foods rich in natural antioxidants is currently considered one of the most effective ways to reduce the risk of developing those chronic noncommunicable diseases that most limit the quality and life expectancy of The world population.
In this section, the following questions are addressed, among others: What are the main dietary sources of antioxidants? What is the chemical nature of the antioxidants that are most abundant in foods? and How to access information about the content and antioxidant activity of foods?
Faced with the question " Which food would bring more antioxidants to the body ?", The following considerations should be kept in mind: (i) the greater the antioxidant content (mg / 100 g of weight) of a given food, the greater the contribution of antioxidants that the ingestion of said food supposes to the organism; (ii) for a food with a given antioxidant content, we will have that the greater the amount of food eaten, the greater the total amount of antioxidants that could enter the body. Then, to define what food could suppose a greater contribution of antioxidants to the organism, it will be necessary to consider both the content of antioxidants present in it, as well as the size of the portion (g) of the food that regularly characterizes its intake.
In attention to both considerations, ideally, to ensure a greater contribution of antioxidants to the body , we should choose to consume those foods that along with being "rich in antioxidants" are, in addition, regularly consumed in larger portions. The truth, however, is that along with the above, other considerations should also be kept in mind. For example, the one that not necessarily all the antioxidants present in a food will be absorbed in equal magnitude after its arrival in the gastro-intestinal tract (from where they are absorbed, to be transported through the blood to the various organs and tissues where they would act) . Indeed, for a given individual, the bioavailability of a given antioxidant (fraction of the amount initially ingested that eventually reaches the blood), will depend, among other factors, on:
- (1) the chemical structure of each antioxidant in question (eg absorption efficiency of tocopherols relative to carotenoids or flavonoids),
- (2) of the matrix in which each antioxidant is part of the food (eg whole fruit with respect to juice, lyophilized or microencapsulated thereof) and
- (3) of the state in which the food to be ingested is found (eg, raw with respect to cooked, natural with respect to processed).
Clearly, for the consumer of a given food it is not possible to directly affect these last three factors. However, in the perspective of ensuring a greater contribution of antioxidants to the body, the consumer -when duly informed- will have the possibility of leaning towards those foods that contain more antioxidants. For this, the following two questions are addressed below:
What are the antioxidants that are most abundant in food ?
What are the main dietary sources of said antioxidants ?
Regarding the first question, in short, among the antioxidants that abound most in the diet include: ascorbic acid, vitamin E, carotenoids, and polyphenols. For each of these antioxidants are described below, aspects related to its chemistry, with some of its functions, with the recommended daily doses, and with the main foods that contain them:
Ascorbic acid . Ascorbic acid or Vitamin C (Figure I) is a water-soluble compound that fulfills important functions as an antioxidant in the body. As such, it has the potential to protect proteins, lipids, carbohydrates and acids nucleic acids (DNA and RNA) against oxidative damage caused by various free radicals and reactive species. To access information related to the antioxidant properties of ascorbic acid, its main biological actions and health benefits, please go to the section "Antioxidants and health: Scientific evidence". From a nutritional point of view, ascorbic acid is an essential nutrient. However, unlike most mammals and other animals, humans do not have the ability to synthesize vitamin C, so they must obtain it through the diet. Vitamin C is necessary for the synthesis of collagen (a structural component of blood vessels, tendons), ligaments, and bones. It also plays an important role in the synthesis of noradrenaline, carnitine (necessary to obtain energy from the metabolism of lipids), and possibly in the metabolic conversion of cholesterol into bile acids. Severe vitamin C deficiency can lead to scurvy. Although it is now known that such a condition can be prevented and / or reversed with a minimum dose of 10 mg of vitamin C per day, the recommended daily doses
(RDA, Recommended Daily Allowance) in the US are markedly superior.
Table I shows the RDA values according to age and gender. The calculation of the RDA continues to be based mainly on the prevention of the disease associated with vitamin C deficiency, rather than the prevention of chronic diseases and the promotion of optimal health. It should be noted that the recommended intake of vitamin C for smokers is 125 mg / day, that is, 35 mg / day higher than that of non-smokers. This is due to the fact that smokers are under greater oxidative stress, as a result of tobacco consumption (cigarette smoke), and that they generally have lower blood levels of vitamin C.
Learn the opinion of experts from the Linus Pauling Institute , (Oregon, USA) about the population's requirements for vitamin C.
Vitamin C and its food sources. Fruits and vegetables are, in general, a good source of vitamin C. Although the content of ascorbic acid in such foods can vary greatly depending on the species and variety of the fruit or vegetable (Table II), with a daily intake of five servings of fruits + vegetables (2 ½ cups, equivalent to 400 g) it is possible to ensure an approximate intake of 200 mg of this vitamin. While it is always more desirable to consume ascorbic acid in the form of the foods that contain it (as they provide not only vitamin C but also numerous other nutrients, fibers and microminerals), it should be noted that there is no chemical difference between ascorbic acid present in natural form in foods (whose isomer is L) and synthetic L-ascorbic acid.
If you would like to know the ascorbic acid content in more detail, and for a larger number of foods, please refer to the USDA Food Compound Vitamin C Database
Vitamin E The term vitamin E comprises two chemically closely related types of molecules: tocopherols and tocotrienols. From a structural point of view, both molecules include a hydroxyl group attached to a C-6 of a ring aromatic which is, in turn, attached to an oxygenated heterocycle. From said heterocycle (C-2) a long hydrocarbon side chain is born which, in the case of tocopherols, is totally saturated (phytyl chain), while in the tocotrienols it exhibits three instaurations. Both tocopherols and tocotrienols occur in the form of alpha, beta, gamma and delta isomers. Figure II shows the chemical structure of tocopherols.
In food, the concentration of tocopherols is substantially higher than that of tocotrienols. Within the tocopherols, the gamma isomer is more abundant than alpha, at least in the Western diet (especially in the North American). However, the levels of alpha-tocopherol in the blood are approximately ten times higher than those of gamma-tocopherol. The latter is due to the fact that the human liver has a tocopherol transfer protein that does not respond to gamma but only to the alpha isomer, which allows storage, incorporation into lipoproteins and subsequent transport and distribution of it to other tissues. In addition, relative to the alpha isomer, the other tocopherols are actively bio-transformed (degraded) in the organism, which does not allow their accumulation.
Until now, scientific evidence indicates that the main biological function of alpha-tocopherol in humans would be to act as an antioxidant. The fat-soluble nature of alpha-tocopherol (mainly given by the phytyl side chain) allows it to reach higher concentrations in lipid environments. The latter leads to its antioxidant properties manifesting mainly at the level of cell membranes (eg plasma, mitochondrial), preventing and / or retarding the oxidation of the lipids of such structures, and lipoproteins such as LDL (low density lipoprotein, Low Density Lipoprotein). The oxidation of LDL by reactive species (free radicals and other oxidants) is a key event in the process of atherogenesis since it is conducive to the formation of atheromas or plaques in the vascular sub-endothelium.
The ability of alpha-tocopherol to efficiently intercept the propagation of lipoperoxidation is not limited to its action only in biological systems. In fact, alpha-tocopherol is also very effective when it is used as an antioxidant to prevent or retard the oxidative rancidity that affects lipids and fats in food.
When a molecule of alpha-tocopherol "neutralizes" a free radical (especially of the lipoperoxy type), whether in a biological or abiotic system, it does so by donating its phenolic hydrogen atom (HAT) to said radical. As a result, the alpha-tocopherol molecule becomes a free radical called tocopheryl, which, as expected, lacks antioxidant activity. However, when such a reaction takes place in the body, other antioxidants, such as vitamin C, would be able to react with the tocopheryl radical, regenerating the original antioxidant capacity of alpha-tocopherol. Aspects related to the above and with the role of alpha-tocopherol in the oxidation of LDL and in the reduction of the risk of development of cardiovascular diseases are included in the section
"Antioxidants and health: Scientific evidence".
The most commonly found form of alpha-tocopherol in foods is RRR-alpha-tocopherol (also known as "natural tocopherol" or d-alpha-tocopherol or ddd-alpha-tocopherol). Synthetic alpha-tocopherol, which is labeled all-rac- or dl-alpha-tocopherol, has only half the biological activity (as a vitamin) of the RRR-alpha-tocopherol isomer.
Table III shows the recommended daily doses of vitamin E (RDA applied in the USA), according to age and gender. It is important to note that the values of RDA indicated in said table are based on the prevention of symptoms of vitamin E deficiency and not on the doses that would be required to promote health and prevent chronic diseases. The reason for the latter is that the available scientific evidence is still insufficient to support the recommendation to increase such intake of tocopherol beyond the RDA.
The main sources of alpha-tocopherol in the Western diet They include vegetable oils (marigold, safflower, olive), nuts, almonds, and green leafy vegetables. The eight forms of vitamin E (alpha, beta, gamma and delta of tocopherols and tocotrienols) are found in varying amounts in foods (Table IV).
For more information on the content of tocopherols in foods, please refer to the USDA Food Vitamin Composition Database
Carotenoids Carotenoids are pigments synthesized by plants, where they act as "quencheadores" (deactivators) of singlet oxygen. The latter is a ROS formed during the process of photosynthesis. Although singlet oxygen has a very minor importance in the development of oxidative stress generated by the human organism, as indicated below, the antioxidant activity of carotenoids is not limited to the removal of said ROS.
In our diet, carotenoids concentrate mostly (in the form of all-trans isomers) in fruits, vegetables and cereals, giving them yellow, orange or red colors. From a structural point of view, carotenoids are classified into: carotenoids, represented by alpha-carotene, beta-carotene and lycopene, and in xanthophylls, represented by beta-cryptoxanthin, lutein and zeaxanthin. Xanthophylls are carotenoids that include one or more oxygen atoms in their structures.
Figure III shows the chemical structure of the carotenoids most commonly present in food.
Alpha-carotene, beta-carotene and beta-cryptoxanthin are pro-vitamin A type carotenoids, which means that they can be converted into retinol or vitamin A in the body (essential to ensure normal tissue growth, and for a proper functioning of the immune system and vision). The pro-vitamin A function of said carotenoids is the only function currently recognized as essential. Lutein, lycopene and zeaxanthin do not act as pro-vitamin A.
In general, carotenoids exhibit low bioavailability. The latter is partly due to the fact that these compounds are mostly bound to proteins in their phyto-alimentary matrices. The processes of cutting, homogenization and cooking of foods rich in carotenoids generally increases their bioavailability. In order for the carotenoids of the diet to be absorbed at the intestinal level, they must first be released from the food matrix and incorporated into mixed miscelas (mixture of bile salts and various types of lipids). Therefore, a minimum amount of fat (3-5 g) in a meal is required to ensure efficient intestinal absorption. For example, in the case of lycopene, its bioavailability from the tomato is substantially increased when subjected to a cooking process in oil.
Carotenoids and their food sources . Yellow or orange vegetables, such as carrots and squash, are a very good source of alpha and beta-carotene. For its part, spinach is also a good source of beta-carotene, although chlorophyll masks the yellow-orange pigment present in its leaves.
Some foods that are a good source of alpha-carotene and beta-carotene are the following (Table V):
- Some foods that are a good source of beta-cryptoxanthin are the following (Table VI):
- Some foods that are a good source of lycopene are the following (Table VII):
- Some foods that are a good source of lutein + zeaxanthin (given the complexity of their analysis separately, the determinations often express the sum of the content of these two xanthophylls) are (Table VIII):
For more information about the content of carotenoids in these and other foods, go to: http://www.nal.usda.gov/fnic/foodcomp/search/
The polyphenols . Polyphenols are compounds that are bio-synthesized by plants (their fruits, leaves, stems, roots, seeds or other parts). All polyphenols exhibit antioxidant properties. These compounds account for most of the antioxidant activity exhibited by fruits, vegetables and certain infusions and natural beverages habitually consumed by the population. From a chemical point of view, all the polyphenols exhibit in their structure, at least, one or more hydroxyl groups (HO-) linked to an aromatic ring, that is, they present some phenolic group. In turn, among the polyphenols it is possible to distinguish two subtypes of compounds:
- I) The flavonoids , whose structure (diphenylpropane, C6-C3-C6, Figure IV) comprises two aromatic rings (A and B) which are they are linked together by a heterocycle formed by three carbon atoms and one oxygen atom (C), and for which more than five thousand compounds have been described in the vegetable kingdom. As described below, in turn, the flavonoids are subdivided into the following six groups of compounds: anthocyanidins, flavanols, flavanones, flavonols, flavones and isoflavones.
- II) The so - called non-flavonoids (a few hundred), which mainly comprise mono-phenolic alcohols (eg hydroxytyrosol), simple phenolic acids and stilbenes (eg resveratrol). In the case of simple phenolic acids, which make up the majority of the non-flavonoid polyphenols, there are benzoic acid derivatives (eg protocatécuico, gallic, vanillic, p-hydroxy-benzoic) and those of cinnamic acid (chlorogenic) , coffee, ferulic, p-cumárico).
While all polyphenols exhibit antioxidant properties, it has been established that some of these compounds also exhibit, among others, anti-inflammatory, antiplatelet, anti-bacterial, estrogenic activity and activity-modulating properties of numerous enzymes, including that of certain digestive enzymes. Some of these aspects are addressed in the section "Antioxidants and health: Scientific evidence".
The ability of polyphenols to act as antioxidants, both flavonoids and non-flavonoids, depends primarily on the presence of HO- groups in their structure. Being linked to a benzene ring, the groups hydroxyl confer to polyphenol the ability to act, either as a donor of a hydrogen atom (HAT) or as an electron donor (SET) to a free radical (or other reactive species). In the case of flavonoids, in particular, some can also act as antioxidants through a mechanism that involves their ability to react with (chelating) certain transition metals (such as copper and iron). Such a reaction prevents the formation of hydroxyl free radicals (from hydrogen peroxide in the Fenton reaction) and superoxide (from molecular oxygen) that would otherwise catalyze both metals to be in their free and reduced state (that is, redox-active). Therefore, the flavonoids that exhibit catecholic hydroxyls in ring B of their structure also promote an antioxidant effect through the aforementioned mechanism (eg quercetin, Figure V).
Flavonoids are usually found in nature as conjugated compounds, that is, linked to different sugars (such as glucose, fructose), or in the form of free compounds (called aglycones). The proportions of free flavonoid and conjugate will depend on the type of food in which they are found. In turn, the gastrointestinal tract will be exposed to a different ratio of free / conjugated, depending on the state of cooking of the food, and the action they have had throughout the process of digestion, various enzymes capable of hydrolyzing the sugars. The latter is not less, therefore, the physicochemical properties (which define solubility and potential to be absorbed) of the polyphenols can be markedly affected by the presence of said sugars. It should be noted, however, that absorption is not a fundamental process when the action of these compounds is exerted directly on the lumen of the gastrointestinal tract; for example, modulating the activity of some digestive enzyme, or acting as an antioxidant in a direct way on ROS present in the lumen. It is, however, when the action of the flavonoids is exerted in a systemic way, that is, in organs, tissues or cells which can only be accessed after gastrointestinal absorption and distribution from the blood to such tissues. In general terms, the conjugated form of polyphenols is more polar (water-soluble) and therefore less absorbable (and bioavailable). At the level of the large intestine, the bacteria that normally colonize the colon also play an important role in the metabolism of flavonoids, favoring their absorption by promoting the hydrolysis of glycosidic bonds. Indeed, different individuals may differ in their ability to hydrolyze a particular flavonoid (and then to absorb it) depending on the differences they have in their colonic microflora.
Is the intake of polyphenols "essential" for the conservation of health? There is still no evidence that the consumption of polyphenols is "essential" for the conservation of health, and therefore there are no recommended daily dose (RDA) values of these compounds. However, abundant scientific literature reports several health benefits associated with a higher consumption of foods rich in polyphenols, such as certain fruits, vegetables, legumes and cereals. In addition, increasingly, the evidence shows that the consumption of products rich in polyphenols, such as cocoa (in the form of dark / bitter chocolate), green tea (in beverages / juices that contain it) or red wine (in the form of moderate) has effects that would be potentially favorable for the conservation and / or normalization of relevant physiological parameters or indicative of cardiovascular health.
To access information related to some of the health benefits associated with the consumption of foods rich in polyphenols, please go to the section " Antioxidants and health: Scientific evidence "
Although the proportion of flavonoids with respect to polyphenols that are not flavonoids can vary significantly between one food and another, in the case of fruits and vegetables, flavonoids are, in general, the polyphenols that are most abundant in these foods. Along with the latter, it is worth mentioning that the scientific literature that implicates polyphenols as health protection factors mainly comprises polyphenols of the flavonoid type. Table IX describes, for each of the six groups of flavonoids mentioned above (anthocyanidins, flavanols, flavanones, flavonols, flavones and isoflavones), the main compounds that constitute each group of flavonoids, and gives examples of those foods that concentrate most such flavonoids
What drinks could be a good source of antioxidants? For the flavonoids described in the table, along with fruits and vegetables, certain beverages can also be a source of antioxidants. Examples of beverages that concentrate antioxidants, and also because of their frequency of consumption could suppose a very good contribution of some of these compounds, are green tea (and to a lesser extent black tea), coffee (especially grain) and Red wine. It should be noted, however, that the consumption of these beverages as a form of ingesting antioxidants should be limited to adults, and if they are abundant and sustained, they should consider the potential inconvenience that the obligatory co-ingestion of caffeine can suppose in the case of tea and regular coffee, compounds capable of affecting the bioavailability of dietary iron in the case of tea, and the consumption of alcohol and the corresponding calories in the case of red wine. In this last case, it is necessary to clarify that although red wine can be a good source of certain polyphenols, many fruits and vegetables also contain the same antioxidant compounds, being fruits and vegetables, in addition, an excellent source of other polyphenols and , above all, of numerous nutrients (proteins, lipids, fatty acids, vitamins, micro- and macrominerals, fibers, etc.) that are not present in such a beverage. Clearly, unlike red wine, the consumption of fruits and vegetables can be promoted on the basis of their nutritional wealth and without the ethical and public health reserves that merit the real risks that would imply the promotion of a greater consumption of red wine as a important way to "gain health".
In spite of these last considerations, the consumption of tea, coffee and also of red wine constitutes a practice deeply rooted and transversal in our society. For example, in the case of tea (in its various types), its consumption accounts worldwide for the second most consumed beverage, after water. Without excluding certain Latin American populations, in the case of Chile, it is necessary, together with the aforementioned beverages, certain infusions of herbs or "digestive water", prepared from plants regularly used by the population, such as boldo, bailahuén and rosa mosqueta (among others), could also be an interesting source of antioxidants for the population. It should be clarified, however, that unlike tea, coffee and red wine, the real impact that the consumption of such herbal infusions can have on the antioxidant status of the organism or other relevant biological parameters has not yet been evaluated. the preservation of human health.
More information about what antioxidant compounds characterize and what is the antioxidant level of beverages such as green tea, coffee, red wine, and certain "digestive water"? will be presented promptly on this site. We already appreciate your eventual interest in this topic.
Antioxidants as preservatives of processed foods.
What are the main antioxidants used in the preservation of food? The antioxidants used in the preservation of food can be classified, according to their origin, into two types: natural and synthetic. Among the natural antioxidants that are most commonly used as preservatives are: ascorbic acid, alpha-tocopherol and various derivatives of rosmarinic acid. Such compounds can be obtained: by direct extraction from their natural sources (where they exist in abundance), or by chemical synthesis. Among the non-natural or synthetic antioxidants most used by the food industry are: butyl-hydroxytoluene (BHT; E 321), butyl-hydroxyanisole (BHA; E 320), tert-butyl-hydroquinone (TBHQ), ethoxyquin (EQ) , propyl gallate (E 310) and metal chelators such as EDTA and citric acid.
If you would like to access more information regarding the following antioxidants, we invite you to click as appropriate: BHT , BHA , TBHQ and EQ .
Are there risks associated with the chronic consumption of synthetic antioxidants? The safety of most synthetic antioxidants (such as BHA, BHT, EQ, TBHQ and certain gallates) has been increasingly questioned, especially in the last 2 decades, as a result of studies that show that, when administered in a prolonged manner and in high doses, some of these compounds can be mutagenic and / or carcinogenic in experimental animals. However, currently, given the effectiveness, low cost and still controversial evidence of real risk in humans, synthetic antioxidants are still used by natural antioxidants as main preservatives in the food industry. Although more research is still required, the evidence existing today indicates that, when used in doses lower than the ADI (Acceptable Daily Intake), the prolonged use of those synthetic antioxidants that are considered GRAS (Generally Recognized As Safe) are not it should pose an appreciable risk to the health of the population ( for more information ).