Antioxidant-Rich Vegetables: Impact on Human Health

*Anne Adebukola Adeyanju, Omolola Rebecca Oyenihi and Oluwafemi Omoniyi Oguntibeju*

## **Abstract**

Antioxidants are valuable ingredients present in vegetables. Vegetables are essential and crucial in human's health and diet because of their minerals, antioxidant vitamins, phytochemical compounds, and dietary fibre content. This is the reason why an adequate consumption of vegetables has been linked with reduced risk and protection against various chronic diseases. Notably, each vegetable belongs to a group that contains a unique quantity of phytochemical compounds, which distinguish them from other groups and even within their group. The exact mechanisms by which the consumption of vegetables protects against human diseases are yet to be fully understood. However, the phytochemicals present in vegetables could be responsible for attenuating some of them. These phytochemicals are strong antioxidants that reduce the risk of chronic diseases by mounting resistance against the generation of free radicals and their damage. They are also involved in the modification of metabolic activation, detoxification of carcinogenic compounds, or attack of tumour formation in cells. This review highlights the inherent antioxidant potentials of vegetables, their roles as an excellent source of antioxidants and their impact on human health and diseases. Information provided in this review will provide more insight into the roles of antioxidants present in vegetables.

**Keywords:** antioxidant, vegetable, phytochemical, disease, free radical

### **1. Introduction**

These days, the new trend of consumption of foods in our society is for the "natural and healthy," which includes vegetables and fruits in the diet. "Western" type diets involve a high intake of food with much calories and animal protein with low consumption of vegetables. The resultant effect of this type of diet with physical inactivity is the development of diseases such as diabetes, obesity and cancer [1]. Therefore, the search for vegetables continues to gain attention as they are known to contain bioactive compounds with medicinal values that can offer healthy diet, build up the body's defences and help to prevent diseases [2].

Vegetables are large class of plants serving numerous purposes in the medicine, food and beverage industry. Their leaves, stems, seeds and flowers are useful for feeding, as flavours and colourants. They are plants of great importance in our diet. They are protective foods that are of great benefit in maintaining good health, building and repairing the body and preventing diseases because of the large number of essential nutrients they contain. They are rich in antioxidants like

carotenoids, ascorbic acid, flavonoids, folic acid and minerals like calcium, iron, phosphorus, phenolic compounds, proanthocyanidins, vitamins and saponins [3]. These phytochemicals exhibit multiple biological effects such as antioxidant, antiinflammatory, antimicrobial and anti-cancer activities. Particularly, the antioxidative activity of phenolic compounds is highly recognised and this is attributed to their chain-breaking and free radical scavenging abilities, which remove free radical intermediates, thereby offering protection against the production of reactive oxygen species [3]. Excessive production of reactive oxygen species (ROS) can cause oxidative damage to biological macromolecules such as nucleic acids and proteins [4].

In rural areas, vegetables form an integral component of the food and nutrition of the local population as they are traditionally reckoned for their medicinal, therapeutic and nutritional values since time immemorial. They are either consumed as raw or cooked as traditional delicacies and the sales from the surplus of these vegetables serve as an additional income to many families. Moreover, to this set of population, vegetable consumption gives variety to their food and add flavours to the diet. It is rich in various nutritive elements and can make up for the dietary shortfalls of vitamins and minerals necessary for the human diet. Generally, malnutrition and food shortages are prevalent among the rural population. Thus, the cultivation of vegetables will contribute to increased food production, balanced nutrition, food and health security and poverty eradication for them.

Study revealed that before the nineteenth century, there has been a particular interest in vegetables and herbs because of the beneficial effects of phytochemicals present in them [5]. They were being used for therapeutic purposes until synthetic drugs were developed. Their consumption has drawn great attention due to the discovery of the fact that, their regular intake has an alliance with declined rates of heart diseases, cancers, diabetes, and other degenerative diseases. Protection that vegetable offer has been ascribed to the presence of antioxidative compounds such as α- tocopherol, ascorbic acid [6] and phytochemicals like carotenoids, flavonoids, lycopene, phenolics and β-carotene [7–9]. These compounds are free radical scavengers, hydrogen-donating compounds, metal ion chelators with ability to inhibit generation of free radicals and reactive oxygen species (ROS) [10].

Thus, this review draws attention to the bioactive compounds present in our vegetables, their biological importance and elucidation of their roles in disease prevention and health.

#### **2. Antioxidant potential of vegetables**

Antioxidants are defined as substances which when present at low concentration compared to those of an oxidizable substrate [11], significantly delay or prevent the oxidation of that substrate. Some of the mechanisms of action of antioxidants involve prevention of lipid peroxidation, oxidative damage to membranes, glycation of proteins and inactivation of enzymes caused by free radicals. Oxidative stress can arise due to the generation of ROS including free radicals and non-free radicals. Evidences have shown that they have roles in the development of several diseased conditions such as lipid peroxidation, protein oxidation, DNA damage and cellular degeneration [12, 13].

Normally, during cellular metabolism, free radicals and other reactive oxygen species are continuously released in the body. They can also be produced from sources such as drugs, food, exhausts and other pollution from the environment. Organisms are endued with endogenous and exogenous antioxidant defence systems against free radical generation. However, when free radical produced in

the body overwhelms the antioxidant system, oxidative stress ensues. This has implications in the aetiology of several pathological conditions [14]. This is the reason for the special attention being given to the use of antioxidants especially of natural origin.

Antioxidant phytochemicals have been recognised for the role they play in the prevention and management of chronic diseases [15]. Phytochemicals are proven to have antioxidant capacities in humans*.* Consumption of vegetables with high contents of these compounds is liable to raise the antioxidant capacity of the body system. For instance, serum total antioxidant capacity was found to be elevated significantly following the consumption of spinach in elderly women [16]. Further, a study has also supported a significant increase in antioxidant capacity caused by the daily consumption of 10 servings of fruit and vegetables for a period of 15 days [17].

Given the growing prospect observed in these phytochemicals, there is a need to identify and quantify them, elucidate their mechanisms of action and assess their potential health benefits. This information could serve as a basis for intervention strategies.

The phytochemical content of some vegetables like Broccoli, Brussels sprout, green cabbage have been revealed [18]. α- and β-carotene are richly present in broccoli (1 and 779 mg/100 FW), carrot (4.6 and 8.8 mg/100 FW), tomato (112 and 393 mg/100 FW), pea (19 and 485 mg/100 FW), and sweet pepper (59 mg/100 FW). However, some factors such as level of growth, handling during post harvesting, and processing could contribute to the significant variation that sometimes occur in structure and function of phytochemicals from vegetable to vegetable [19]. Vegetables such as beans, broccoli, cabbage, cauliflower, cress, pea, spinach, spring onion, and sweet peppers are reported to be rich in ascorbic acid [20]. Asparagus, Brussels sprout, cabbage, carrot, cauliflower, kale, lettuce, spinach, sweet potato, and turnip are abundant in vitamin E [18] while red pepper is high in vitamin C content (144 mg/100 g) [21].

Furthermore, the structure can influence the antioxidant capability of a phytochemical in vegetables. For instance, the antioxidant properties of flavonoids depend on their C-ring structure. Flavonoids like rutin and luteolin with full substituted C-ring and an ether bonded with three oxygen display superior free radical scavenging capacity and higher reaction rate when compared to flavonoids that lack one or more C ring structural elements. Also, individual phenolic units in vegetables have been found to show better antioxidant activity than total phenolics. Generally, the high scavenging property of vegetables with phenolic content may be as a result of hydroxyl groups existing in the phenolic compounds' chemical structure that can provide the needed component for radical scavenging activity (**Figure 1**). The hydrogen atoms of the adjacent hydroxyl groups, situated in various positions of the rings A, B and C, the double bonds of the benzene ring, and the double bond of the oxo functional group (-C**=**O) of some flavonoids, offer them their high antioxidant activity. These structural features are shown in the

**Figure 1.** *Free radical scavenging of flavonoids where R is the radical and Fl-OH is the flavonoid.*

benzene rings of flavonoids as shown in **Figure 2**. The antioxidant ability centre of phenolic acids is in the phenolic hydroxyl group, such that the positioning of phenolic hydroxyls is directly related to their antioxidant activity [22].

The impartation of colours is also an important factor in the antioxidative activity of phytochemicals in vegetables (**Table 1**). It is said to be directly related to the pigment content such as carotene, phytoene, chlorophyll, lycopene and anthocyanin, and their relative quantities at different maturity stages. It is an important trait that largely reflects quality, type of phytochemical as well as antioxidant activity [30]. Jaganath and Crozier [31] inferred that colour difference as observed in vegetables and fruits is an indication of accumulation of phytochemicals such as flavonoids and carotenoids. Red pigment conferred on red bell peppers is linked to a large content of lycopene, a member of the carotenoid family, localised in the prostate gland. Lycopene is a powerful antioxidant that has a connection with lessened risk of some cancers, especially prostate cancer [23], and protection against heart attacks. The yellow or orange colouration noticed in vegetables such as spinach, sweet potatoes and carrots represents produce rich in both α and β-carotene. They are also members of the carotenoid family where β-carotene can be converted to vitamin A in the body, a nutrient that plays a crucial role in vision. The mechanism involves the cleavage of β-carotene into two molecules of vitamin A which is then

#### **Figure 2.**

*Structural characteristics of flavonoids with high antioxidant activity; presence of hydroxyl groups and double bonds (circled) in the benzene rings.*


#### **Table 1.**

*Impartation of colours in vegetables and their health benefits.*

converted to II-cis-retinal and thereafter combines with opsin to form a protein called rhodopsin. When light hits the rods, metarhodopsin is produced [24]. Betacarotenes in vegetables are also involved in improving immune function, skin and bone health as well as prevention of cancer [25].
