The Effects of Plant Bioactives on Human Health
Authors: Nitin Kumar Garg1, Rakesh Kumar Prajapat2, Chirag Maheshwari1, Ashish Marathe1
1Ph.D Research Scholar, Division of Biochemistry, IARI, New Delhi-110012
2Ph.D Research Scholar, NRCPB, IARI, New Delhi-110012


Bioactive compounds in plants can be defined as secondary plant metabolites eliciting pharmacological or toxicological effects in human. Secondary metabolites are produced within the plants besides the primary biosynthetic and metabolic routes of compounds aimed at plant growth and development, such as carbohydrates, amino acids, proteins and lipids. The secondary bioactive compounds in plants appear to be randomly synthesized - but they are not useless junk. The specific functions of plant bioactives are diverse, but they may protect against free radicals generated during photosynthesis, protecting plants from being attacked by insects or being eaten by herbivores, protecting from UV light and microbial infection. They may also attract pollinators or seed dispersers. Plant bioactives are found in a vast range of foods which are consumed as human diet. They may be consumed via the leaves, stems, roots, tubers, buds, fruits, seeds and flowers of whole plants, or consumed in plant derived foods and drinks such as chocolate, tea, coffee and fermented foods such as wine and bread.

Main classes of plant bioactives

Plant bioactives are classified in different classes according to their health benefits. They are classified as :

1. Flavonoids :

Flavonoids are a large group of 'phenolic' bioactives that consist of a central three-ring structure and found in a range of plant derived foods, mainly in the skin of fruit such as grapes, and in the epidermis of leaves such as tea leaves. Flavonoids include a large range of bioactives like flavan-3-ols(flavanols), anthocyanins, flavonols, flavones, isoflavones and flavanones. The flavonoids can occur as a glycosides. All compounds contain phenol-groups involved in an effect as general antioxidant.

Flavan-3-ols (flavanols) found in tea, apples, apricots and cherries. It includes catechins and the larger proanthocyanidins, which are formed from catechin units. In green tea epicatechins are the major polyphenolic compounds, and the most significant active component is epigallocatechin gallate (EGCG). In breast cancer cell lines, EGCG inhibits cell shedding (indicative of metastasis), hepatocyte growth factor signaling, and cell motility; causes cell arrest in S phase. Red wines contain flavan-3-ols that originate from the seeds of black grapes and dark chocolate is a rich source of flavan-3-ols derived from roasted cocoa seeds.

Anthocyanins offer protection against cancer, inhibiting the initiation and progression stages of tumor development. They also reduce inflammatory inducers of tumor initiation, suppress angiogenesis, and minimize cancer-induced DNA damage in animal disease models. Anthocyanins also protect against CVD and age-related degenerative diseases associated with metabolic syndrome. It is present at relatively high levels in fruits such as blueberry, blackberry, cranberry, strawberry, and raspberry.

Flavonols are present in vegetables like onion and fruits like apple. Dietary flavonols inhibit LDL oxidation and so reduce the primary risk factor for atherosclerosis and related diseases. Quercetin, kaempferol and myricetin are all examples of flavonols.

Flavones are structurally very similar to flavonols but they are not widely distributed throughout the plant kingdom. Apigenin and luteolin are the examples of flavones. These are found in parsley, thyme and celery.

Isoflavones are produced almost exclusively by members of the legume family. Isoflavonoids in the diet is linked to reduced incidence of CVD, breast and prostate cancers, osteoporosis, and associated complications. The major sources of dietary isoflavonoids for humans are soybean products such as tofu.

Flavanones are abundant in citrus fruits. Hesperetin, for example, is commonly found in citrus peel and naringenin imparts intensely bitter flavours to grapefruit peel.

2. Plant Sterols:

Plant sterols have a similar structure to dietary cholesterol but are not absorbed by the human gastrointestinal tract, and so have the ability to inhibit the absorption of cholesterol from the diet and facilitate its elimination from the body. The principal plant sterols are Beta-sitosterol, campesterol, and stigmasterol. Some vegetables contain high levels of sterols; for example, broccoli contains 367-390 mg per gram fresh weight, carrot contains 153-160 mg per gram fresh weight, and cauliflower contains 310-400 mg per gram fresh weight.

3. Carotenoids:

These belong to the group of bioactives known as the terpenoids. Carotenoids are structurally diverse compounds that are classified as tetraterpenes. In plants, carotenoids work alongside chlorophyll in photosynthesis, extending the range of light that can be absorbed by the photosynthetic pigments, and giving plant foods that contain carotenoids their distinctive red or orange colours (e.g. lycopene in red tomatoes, lutein in yellow peppers and a- and Beta-carotene in orange carrots). Lycopene is believed to confer protection against CVD, specifically protecting against LDL oxidation and reducing the risk of cerebral infarction, acute coronary events, and stroke. The richest dietary source of lycopene is tomato, which contains 8-40 ug per gram fresh weight, although it is also consumed in ruby grapefruit, papaya, guava, and watermelon. Dietary carotenes (Alpha, Beta, and Gamma) from plants can be converted into vitamin A (retinol). Dietary provitamin A is required for synthesis of retinal (a hormone-like growth factor) and for scotopic and color vision by the retina. Lutein is a carotenoid synthesized only by plants; it is found in high quantities in green leafy vegetables such as spinach and kale. In humans, it is concentrated in the macula, a small area of the retina responsible for central vision, where it is thought to keep the eyes safe from oxidative stress.

4. Glucosinolates:

Cruciferous vegetables, such as sprouts, broccoli, cabbage and watercress contain the plant bioactives glucosinolates. When cruciferous vegetables are chopped, crushed or chewed, the mechanical action breaks down the walls of the plant food cells, causing the glucosinolates contained within the cells to come into contact with the enzyme 'myrosinase', which is contained within a different area of the plant cell. The action of myrosinase on glucosinolates causes their conversion to isothiocyanates and/or other products, which provide the characteristic hot and pungent flavours of many of our cruciferous salad crops. The potential health promoting effect of isothiocyanates on the body seems to be due to their metabolism in the colon by gut bacteria. Once the biologically active isothiocyanates have been released into the small intestine, they are available for interactions with the colonic epithelial cells and for uptake into the circulation via the colonic mucosa. Thus, Glucosinolates protect from lung, stomach, colon and rectal cancers.

References:

Martin, C., Zhang, Y., Tonelli, C. and Petroni, K. 2013. Plants, Diet and Health. Annual Review of Plant Biology. 64: 19-46.

About Author / Additional Info:
I am a first year Ph.D research scholar in division of biochemistry, IARI, New Delhi