**1. Introduction**

Tea is produced from *Camellia sinensis* leaves using various methods and is one of the most commonly used beverages throughout history due to its consumption and health properties [1, 2]. Commercial tea is divided into three main groups: full fermented black tea, semi-fermented oolong tea, and non-fermented green tea according to the production method [3]. Besides these groups, there is non-fermented white tea that is produced and consumed mostly in Asia and has a higher price than other types of tea [4]. Black tea and green tea are the most well-known and consumed types. About 78% of all tea consumed in the world is black tea, 20% is green tea and 2% is oolong tea [5]. The physical and chemical properties of fresh tea leaves vary depending on many factors such as geography, climate, soil, leaf age, and cultivation [6, 7]. Fresh tea leaf consists of polyphenol by 36%, carbohydrate by 25%, protein by 15%, lignin by 6.5%, ash by 5%, amino acid 4%, lipid by 2%, organic acid by 51.5%, chlorophyll by 0.5%,

and carotenoid and volatile compounds by less than 0.1% in dry matter [8]. Fresh tea leaf is considered to be one of the plants with the richest amount of phenolic compounds [9]. **Table 1** shows the phenolic compound contents of different types of tea (**Table 1**).

Tea is also an important source of caffeine which is found naturally in coffee, cola, chocolate, cocoa or added as an additive to various energy drinks, medicine and cosmetics [11], is the most consumed drink in the world, whose use is increasing each passing day, and is considered to be a psychoactive substance [12, 13].

Caffeine is a member of a group known as purine alkaloids which also contain 3,7-dimethylxanthine (theobromine), 1,7-dimethylxanthine (paraxanthine), and methyluric acids. Its chemical formula is C8H10N4O2 and its systematic name is known as 1,3,7-trimethylxanthine [14].

Caffeine is largely absorbed by the stomach and small intestine after being taken into the body, quickly moves into body cells and reaches the highest level in 15 to 60 minutes after crossing the blood-brain barrier. The half-life of caffeine varies depending on factors such as puberty, pregnancy, and disease, but it is considered to be 5–6 hours in a healthy individual [15, 16]. Metabolites such as paraxanthine, theobromine, and theophylline are released as a result of caffeine metabolism. Only 10% of caffeine is excreted from the body without being metabolized [17].

Caffeine is available at different levels in many foods that we often consume in our daily lives. Therefore, it can be easily said that individuals consume caffeine regularly every day. However, the prediction that caffeine intake into the body for nutrition and short, medium and long-term exposures can lead to various health problems has led the scientific world to do research in this area. This study aims to examine the studies on the correlation between caffeine intake of individuals and health, and to emphasize the importance of tea for health.


#### **Table 1.**

*Phenolic compounds of different types of tea [10].*
