The Beneficial Effects of Tea on Bodily Systems: Good or Bad?

Introduction Tea is regarded as a popular beverage in different cultures. Apart from its flavor and stimulant effect, tea compounds may be highlighted for human health. For example, antioxidant, anti-inflammatory, and antitumor effects of tea contents, among others, can be beneficial for preventing all kinds of cancer, coronary artery, and inflammatory diseases. Similarly, tea is the most globally used beverage after water and makes up 78% of all drinks (1-3). Its history goes back to 500 years ago in China and the north of India (4). In addition, tea, which is scientifically known as camellia sinensis from the Aceae family, is a large shrub plant with evergreen leaves and is native to East Asia where it is cultivated widely. Further, this beverage has dark green, leathery leaves, and white aromatic flowers and its plant (Figure 1) can be as high as 10 meters but its shrubs are kept at a 1-1.5-meter height for easy harvest (5). Tea is currently cultivated in more than 30 countries with approximately three billion kilograms of production and consumption worldwide. Furthermore, it is classified into 3 categories based on the production method. Out of the total global produced tea, 78% is black tea, mainly produced in India and Sri Lanka, and 20% is green tea which is mostly cultivated in China, Japan, the Middle East, and America while only 2% is oolong which is mainly produced in China through partial fermentation (6-9). Caffeine, ascorbate, flavonoids, and anti-inflammatories are the effective compounds of tea that are appreciated for their positive effects on human health (4). The chemical compound of tea is complex because of numerous contents which are produced through the curing process, namely, the conversion of green tea to black tea and drying. Moreover, tea leaves contain varying amounts of polyphenol and catechin compounds. The main polyphenols (PPs) are flavonoids like catechin, epigallocatechin gallate, and proanthocyanidin (Figure 2). Other contents (Figure 3) include tannins, caffeine (0.41% or 10-50 mg), low amounts of xanthine, protein (15%20%), fiber, sugar (5%), vitamins B, ascorbic acid (exists in fresh leaves but it is destroyed during the processing for black tea), amino acid, and fat (10). Both green and black teas are produced from a single plant while they are different in terms of processing. For black and white tea, fresh leaves are oxidized to lose 55% of their weight. During the oxidation, the existing enzymes turn the simple PPs into more complex ones with less effective compounds and these complex PPs give a specific color and taste to black and white tea. Additionally, high fermentation reduces PPs while increasing the caffeine content. Black tea has 2-3 times more caffeine than green tea as well (11). Abstract Objectives: Using food to prevent diseases has become the focus of different researchers in recent years. Tea is a natural source of caffeine, flavonoids, ascorbate, theanine, and antioxidants. The purpose of this study was to provide a review of the benefits and harms of tea for different body systems. Methods and Materials: The present review study was conducted by searching through the websites and assessing the results in the literature using a library research method. The data were collected for publications during 2000-2018. Results: Various epidemiological studies concluded that tea and its polyphenolic contents have beneficial effects on different bodily systems including the cardiovascular system, diabetes control, and different types of cancer. However, some studies argue that tea may have harmful effects on the bodily systems. Conclusions: Overall, tea is considered beneficial to human health. It can also be recommended as a healing beverage. Nevertheless, conflicting findings are reported about the risks of tea as well. Therefore, more studies are needed to further explore the benefits and possible harms of different types of tea.