Tea: Health and HydrationTea has been considered a refreshing beverage for centuries. Tea is thought to have health-promoting properties – is this supported by the latest scientific evidence? Is tea a healthy source of fluids?
History of tea and tea varieties
Tea is a beverage that has been consumed for centuries. After water, it is the most consumed beverage worldwide. (1) Diagram 1 shows the per capita consumption of tea in some European countries.
Diagram 1: Per capita tea consumption in Europe (2)
Tea is made by pouring hot water over the leaves of the tea plant, Camellia sinensis. This process is called infusion. Herbal or fruit teas, despite their name, are not strictly speaking teas, because they come from other types of plants (for example, linden or chamomile).
Different methods are used for the production of the numerous tea varieties:
– Black teas, the most common in Europe (3), are produced by fermentation (d. h. the enzymes of the tea leaves are destroyed in a temperature-controlled room) and subsequent drying of the tea leaves is produced. During these processes the special polyphenols are released, which provide the typical color and taste. (4) – Green (unfermented) tea is produced by steaming it before drying to reduce oxidation by enzymes – this preserves the color of the leaf and gives it its characteristic flavor. – Oolong tea is intermediate between green and black teas in terms of fermentation and flavor – its leaves are only partially oxidized.
Black and green teas (and oolong) contain different types of polyphenols, but the total polyphenol content is similar. (5)
Tea can be drunk simply as an infusion, or milk or lemon or sugar can be added. However, the extent to which this affects the bioavailability of the various polyphenols, and by what mechanisms, has not yet been adequately researched.
Is tea healthy?
Much more research has been published on green tea than on black (and other) teas. A recently published comprehensive evaluation that examined the relationship between food and beverage groups and diet-related chronic diseases found that tea is the most protective of all commonly consumed beverages. Individuals who reported the highest tea consumption had a significantly lower risk of developing type 2 diabetes (by 16%), cardiovascular disease (by 28%), and cancer (by 34%), while other studies have found no clear relationship between tea and cancer. (6)
Another large study found that each additional daily cup of tea resulted in a 9% reduction in all-cause mortality.7 Many of the presumed benefits have been attributed to the specific polyphenols contained, known as flavonoids, which have beneficial effects on blood vessels because of their antioxidant and anti-inflammatory properties; however, caffeine and fluorides have also been reported to be important. (3,5)
Large-scale studies on tea and cardiovascular disease found a relationship between black and green tea consumption and significantly lower LDL cholesterol, blood prere and stroke risk. (8)
Regarding weight control, two recent studies found that increased consumption of black tea resulted in a small but definite weight loss when people stayed on their normal diet. (9,10) A previous review of 18 studies found no significant effect of green tea on weight. 11) Thus, the interaction of weight control. Tea consumption not yet conclusively clarified.
The flavonoids in tea may protect against dental caries, as mechanisms such as antiviral and antimicrobial effects are attributed to them.8 In addition, tea contains fluoride, which is beneficial to the health of teeth by making them less susceptible to tooth decay.12 Nevertheless, good general dental hygiene is the most important thing to keep teeth healthy.
Caffeine in tea and how much to drink
The caffeine content of tea depends largely on how long it steeped; it ranges from 1 to 90 mg per 100 ml. (8) Herbal or fruit teas, on the other hand, do not usually contain caffeine. Studies have found that light caffeine consumption may be beneficial to physical endurance and alertness; on the other hand, too much caffeine may affect sleep. (13,14) Another known effect of caffeine is that it is a mild diuretic (which dehydrates the body). However, studies have shown that this is only a concern with very high doses of caffeine. For example, consumption of 6-8 cups of tea does not affect fluid balance. (15)
A recent EFSA report (16) concluded that a healthy adult can consume a single dose of caffeine of up to 200 mg without experiencing health problems; the guideline values given in Table 1 indicate that up to 8 cups of tea per day are safe for adults (different values apply to pregnant women). Children older than 24 months of age can drink 1-2 cups of unsweetened tea without compromising health. For all the concern about the effects of tea polyphenols on iron bioavailability, reports have shown that tea consumption has little effect on iron levels in adults and children.