Polyphenols: what they are, where they are found and what their nutritional properties are
What do apples, berries, citrus fruits, plums, broccoli, cocoa, tea and coffee have in common? They are all foods rich in polyphenols, substances of natural origin known for their antioxidant properties, capable of helping keep cholesterol levels under control and preventing some illnesses. Let’s get to know them better.
What polyphenols are and how they are classified
Polyphenols are organic compounds naturally produced by plant metabolisms (and therefore defined as “secondary metabolites”).
Polyphenols are responsible for the astringency, smell, oxidative stability, flavor and above all color of foods: the typical browning reaction observed in many foods of plant origin is in fact caused by the transformation of phenolic compounds into dark pigments by enzymes (polyphenol oxidase), responsible not only for the color change but also for other “unwanted” effects such as the alteration of flavor and the loss of nutrients in foods such as fruit and vegetables.
In plants, phenolic compounds perform a support function (by the lignins) and protect against insects and against fungal and bacterial infections.
These plant products accumulate in plant organs (roots, stems, fruits and flowers, depending on the characteristics of the species), with a greater presence in the fruit peel. The reasons for this preferential location are to be attributed to the effect of light on the metabolism of phenols (light, together with oxygen, alters their composition), as well as the protective function of phenols against ultraviolet radiation and other stress factors.
Polyphenols are a highly diverse class of compounds that includes more than 8,000 compounds divided into several subgroups, ranging from fairly basic substances (such as phenolic acids) to complex molecules (such as tannins).
All plant phenolic compounds derive from a common intermediary, phenylalanine, or from a close precursor, shikimic acid.
Polyphenols can be classified into different groups based on their chemical structure (in particular based on the number of aromatic rings they contain and on the basis of the structural elements that bind these rings together). The main classes include:
Phenolic acids – They are divided into two sub-categories:
- compounds derived from benzoic acid,
- compounds derived from cinnamic acid.
Flavonoids – They represent the most studied group of polyphenols and include the following sub-categories:
- flavanols (catechins and proanthocyanidins).
Over 4,000 varieties of flavonoids have now been identified, many of which are responsible for the intense coloration of flowers, fruits and leaves. Quercetin, myricetin, and catechins are some of the more common flavonoids.
- Stilbenes – Most stilbenes in plants act as antifungal compounds, produced in response to infection or injury. The most studied is resveratrol.
The distribution of these compounds in tissues and plant cells is not uniform, but varies according to their solubility level: insoluble phenols are found in the cell walls, while soluble phenols are mainly present in storage bubbles called “vacuoles”.
In plants, the polyphenol content is influenced by a number of factors, including the degree of ripeness at the time of harvest, environmental factors and processing and storage methods.
Which foods contain polyphenols
Polyphenols are natural compounds found in fruit (grapes, apples, pears, cherries and berries are particularly rich in them), vegetables, grain, dried legumes, tea, coffee, chocolate, nuts, seeds, flowers, red wine, and condiments (i.e. extra virgin olive oil).
- PHENOLIC ACIDS – are contained in a variety of fruits and vegetables, such as cabbage, onions and broccoli. In edible plants, compounds derived from benzoic acid are the least common, with the exception of some red fruits, black radish and onions (where they can reach a concentration of several tens of mg per kg of fresh weight).
- FLAVONOIDS – are particularly present in fruit peel and in essential oils of some citrus fruits (like tangerines), in spinach extracts, strawberries and blueberries and broccoli.
Onions contain high amounts of quercetin and tea is a good source of catechins.
- STILBENES (resveratrol) – present mainly in grapes and their derivatives (red wine in particular contains it in large quantities), peanuts and red fruit.
- LIGNANS – particularly abundant in flax seeds (which contain up to 3.7 g/kg of dry weight), sesame seeds and many grains. High concentrations of lignans are also found in olive oil.
The quantity of polyphenols in foods of plant origin, such as fruit, vegetables, legumes, wine and tea, as well as within a specific food (for example apples), is highly variable and can be affected by numerous factors, such as the production method (organic or conventional), annual climatic variations and processing/storage after harvesting.
Beneficial effects of polyphenols
Insofar as nutritional properties, the beneficial effects of polyphenols are the subject of increasing attention by nutritionists, researchers and experts; in particular for their antioxidant action.
Polyphenols introduced via the diet pass intact through the intestine, modifying the intestinal microflora, probably thanks to a mechanism that inhibits pathogenic bacteria and at the same time stimulates bacteria beneficial for the organism.
Long-term consumption of a diet rich in plant polyphenols helps protect against numerous illnesses, in particular, the properties attributed to polyphenols include:
- protective effect on the heart;
- antitumoral action;
- antidiabetic action;
- anti-aging action;
- protection against asthma and infections;
- protective action against osteoporosis and neurodegenerative illnesses.
Polyphenols are powerful antioxidants, as they are able to complement the functions of vitamins and enzymes in protecting against oxidative stress caused by free radicals (in particular by reactive oxygen -ROS, involved in the metabolic processes of the cell), although most of the evidence regarding the antioxidant effect of polyphenols is based on so-called “in vitro” studies, i.e. performed in test tubes and not on living organisms.
In a study on the effects of the administration of yogurt enhanced with olive polyphenols, a considerable reduction in weight and in body mass index and in bad cholesterol concentrations (LDL*) was observed, among other effects, after only two weeks of treatment.
*LDL: Low density lipoprotein
However, due to inadequate consumption of fruit and vegetables, a significant percentage of the population in Western countries does not ingest sufficient quantities of food polyphenols. With minimal consumption of fruits and vegetables, food supplements could provide some beneficial effects similar to those of dietary polyphenols.
To date, the EFSA (European Food Safety Authority), has approved the inclusion of hydroxytyrosol (main component of the phenolic fraction of extra virgin olive oil -EVOO-) in the category of polyphenols, defining it as a substance “able to protect fats from the harmful effects of oxidative stress “.
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