Antioxidant foods in diets and their main sources
An antioxidant is any substance that, when present at low concentration as compared to those of an oxidizable substrate, significantly delays or prevents oxidization of that substrate. There are many types of antioxidants in nature and the plant kingdom hosts a huge number of species containing many substances with antioxidant activity and consumed in various ways. There are fruits, herbs and spices, vegetables, olive and olive oil, grapes, wines and juices whose chemical composition consists of different antioxidant compounds, classified according to different categories. Here is a classification of the main natural dietary antioxidants.
Polyphenols are the most important group of antioxidants in the plant kingdom, with more than 8000 phenolic compounds identified from various sources: fruit, vegetables, grains, herbs, spices and decoctions. They are usually divided in flavonoids and non-flavonoids:
A strong structural variability is the main chemical feature of polyphenols. They are particularly effective in preventing lipoprotein oxidization, above all the LDL carrying “bad cholesterol”. Moreover, they have shown an anticancer therapeutic activity, by altering the activation mechanism induced by carcinogens; they have also positive effects on cardiovascular diseases and on many age-related pathologies, besides their anti-inflammatory and anti-viral characteristics. The best-known polyphenols are quercetin found in onions, resveratrol commonly found in berries, leaves and canes of grapes as well as in red wine and lesser in white wine, and isoflavones which are natural constituents of soybeans. All foodstuffs in the plant kingdom are rich in polyphenols: in particular, the highest concentrations are commonly found in green tea, cocoa, wild berries, citruses, cherries, pollen, cold-pressed olive oil, garlic, onion, radicchio, broccoli and tomatoes. The lowest polyphenols concentration is in watermelons and in avocadoes. The content of polyphenols in the same foodstuff varies greatly according to different cultivation techniques, phases of ripeness and the time-lapse between harvest and consumption.
Although many classes of phenolic molecules, like quercetin, are present in most of foodstuffs derived from plants, some other classes constitute a distinctive feature of a single food, like flavanones in lemon or isoflavones in soybeans. In the same product, however, many polyphenols are usually represented: as an example, apples contain flavanols, hydroxycinnamic acids, quercetin glycosides and anthocyanins.
Among simple phenols, the most common are carvacrol, thymol and eugenol. The first two species are found in high amounts in the essential oils derived from plants belonging to the genus Origanum, Thymus and Satureja. Olives and olive oil contain a high amount of tyrosol, hydroxytyrosol and oleuropein which, along with tocopherols, constitute most of the nutritional importance of olive oil.
Carotenoids are a group of tetraterpenes widely distributed in nature, giving fruits and vegetables a characteristic colour, ranging from light yellow to dark red. Carotenoids are the precursors of Vitamin A, which has a strong antioxidant activity and is essential for twilight vision and for cell differentiation. The recommended daily intake of Vitamin A is ranging from 400 RE (retinol-equivalents) in children to 600-700 RE in adults and until 900 RE in breastfeeding mothers. Typical food sources of Vitamin A include apricots, oranges, persimmons, watermelons, mango, pawpaw, medlars, agretti, asparagus, Swiss chard, broccoli, carrots, cabbages, chicory, lettuce, pepper and chilli peppers, tomatoes, parsley, radicchio, rocket (arugula), spinach, pumpkin; high amounts of Vitamin A are also found in eggs and in cod-liver oil.
Vitamin C and E
Vitamin C (ascorbic acid) is one of the most powerful natural antioxidants, thanks to her high capacity to oxidize to dehydroascorbic acid: the resulting reaction reduces the α-tocopherossilic radical to Vitamin E, folic acid in its two-coenzyme forms and ferric ion to ferrous, thus promoting the absorption of plant-derived iron. The recommended daily intake of Vitamin C is 60 mg; an excessive intake causes an increase of urinary excretion of oxalate, resulting in the formation of kidney stones. Vitamin C is widespread among plant foodstuffs, including citruses, strawberries, kiwi, raspberries, mango, watermelons, pawpaw, currants, broccoli, cabbages, turnip tops, endive, lettuce, pepper and chilli peppers, rocket (arugula), celery and spinach.
Vitamin E (α-tocopherol) is found mainly in vegetable oils, among which olive oil and sunflower oil are the most important, but also in lesser extent in seeds, in milk and eggs, in avocadoes and in nuts. Soybean oil contains mainly γ-tocopherol, with a lower biological activity. Food industrial processes cause a loss of about 80% of the natural content of Vitamin E.
Essential oils are natural blends of volatile compounds, belonging mainly to the terpenes group; they are found in a special category of plants called aromatic plants. Several families of plants such as Asteraceae, Apiaceae, Cistaceae, Lamiaceae and Verbenaceae contain great amounts of these compounds, mainly in aerial parts commonly used as herbs and spices or consumed as vegetables. Among the first ones, oregano, marjoram, dittany, savory and thime are the best known, rich in thymol, carvacrol, γ-terpinene and p-cymene. Limonene is a monoterpene commonly found in the peel of citrus, fruits and in lemongrass. Rosemary, sage and mountain tea contain α-pinene as main essential oil.
Parsley and dill leaves contain β-Phellandrene, whereas eucalyptol and camphor are characteristic compounds in sage and rosemary. Cinnamaldehyde is the predominant oil in cinnamon, eugenol is found in clove and basil oil, whereas myristicin is a characteristic oil compound in parsley. Garlic possesses b