Beneficial effects of natural plant polyphenols on the human body have been evaluated in a number of scientific research projects. Bioactive polyphenols are natural compounds of various chemical structures. Their sources are mostly fruits, vegetables, nuts and seeds, roots, bark, leaves of different plants, herbs, whole grain products, processed foods (dark chocolate), as well as tea, coffee, and red wine. Polyphenols are believed to reduce morbidity and/or slow down the development of cardiovascular and neurodegenerative diseases as well as cancer. Biological activity of polyphenols is strongly related to their antioxidant properties. They tend to reduce the pool of reactive oxygen species as well as to neutralize potentially carcinogenic metabolites. A broad spectrum of health-promoting properties of plant polyphenols comprises antioxidant, anti-inflammatory, anti-allergic, anti-atherogenic, anti-thrombotic, and anti-mutagenic effects. Scientific studies present the ability of polyphenols to modulate the human immune system by affecting the proliferation of white blood cells, and also the production of cytokines or other factors that participate in the immunological defense. The aim of the review is to focus on polyphenols of olive oil in context of their biological activities.
Keywords: olive oil, Olea europea, polyphenols, oleuropein, hydroxytyrosol, anticancer therapy
1. Beneficial Effects of Polyphenols
As the name suggests, polyphenols are natural, synthetic, or semisynthetic organic compounds with multiple phenolic groups in the structure. It means that polyphenols typically contain one or more aromatic rings with hydroxyl groups attached to them [1,2]. There is a growing body of evidence for beneficial roles of natural plant polyphenols in the human body. Natural bioactive polyphenols are compounds of varied chemical structures. Polyphenols are arguably the largest group of chemical substances in the plant kingdom. There are more than 8000 different polyphenolic structures known, including several hundred isolated from edible plants [3,4]. Their sources are, among others, fruits, vegetables, nuts and seeds, roots, bark, leaves of different plants, herbs, whole grain products, processed foods, as well as tea, coffee, and red wine. These compounds are characterized by a broad spectrum of biological activities. The beneficial impact of vegetables, fruits, and herbs on human health has been well known for centuries. Today, we understand the reasons for that as many plant-derived products are rich in nutrients, vitamins, minerals, and very importantly, bioactive polyphenols. Some vitamins as well as polyphenols present powerful antioxidant and anti-inflammatory properties that make them natural and efficient anticancer agents to be found in a well-balanced diet. Unlike vitamins and minerals, polyphenols are not the essential elements of the primary plant metabolism. Natural polyphenolic compounds are rather products of the secondary plant metabolism. Anyhow, they do play critical metabolic roles in the human organism [2,5,6,7].
Polyphenols were determined to reduce morbidity and/or slow down the progression of cardiovascular, neurodegenerative, and cancer diseases. The mechanism of action of polyphenols strongly relates to their antioxidant activity. Polyphenols are known to decrease the level of reactive oxygen species in the human body. Other than that, health-promoting properties of plant polyphenols comprise anti-inflammatory, anti-allergic, anti-atherogenic, anti-thrombotic, and anti-mutagenic effects [8]. There is a body of research presenting their ability to modulate the human immune system by affecting the proliferation and activity of white blood cells, as well as the production of cytokines or other factors that participate in the immunological defense [9].
A daily intake of polyphenols ranges from 0.1 to 1.0 g per day [10] with the main dietary source being fruits and vegetables, as well as herbs, spices, seasonings, coffee, tea, or wine [11]. The polyphenols of olive oil are especially interesting with respect to their well-established beneficial effects on human health and metabolism, as well as the popularity of olive oil in many different diets, and specifically the Mediterranean cuisine. Herein, we mostly focus on the anticancer properties of polyphenols available from olive oil.
2. Chemical Composition of Olive Oil
The chemical composition of olive oil varies depending on the extraction technology that is applied in order to obtain oil form the fruits (Figure 1). The process of extraction of olive oil depends on crushing olives and then separating the oil from the fruit pulp under elevated pressure. Additionally, olive oil can be extruded, post-pressured, re-pressed with or without the use of hot water. The olive oil obtained from this kind of process is usually characterized by stronger color intensity, weaker aroma, and a higher content of free fatty acids