Evidence suggests that tea is a healthy drink. The reason for this is not fully understood, but two main lines of reasoning exist. Firstly, tea has been shown to possess weight loss effects. As body weight increases, the risk of serious disease and mortality increases, and so by facilitating the maintenance of a correct body weight regular tea consumption may protect from Western lifestyle diseases associated with weight gain. Tea is also a rich source of antioxidants, particularly catechins and tannins in green and black tea, respectively. These antioxidants have been shown to be bioavailable, and once absorbed they accumulate in the tissues of animals. Here they may protect from disease by increasing the reductive capacity of tissues. Oxidative stress is a situation by which the oxidation rate exceeds the reductive capacity of the cells. This leads to tissue damage. Increasing the reductive capacity of the cells, such as can occur through drinking tea, decreases the risk of oxidative stress and thus protects from cell and tissue damage.
Many clinical trials in humans have shown the biochemical changes that can be caused by tea drinking, both black and green. Extrapolation of these changes to an understanding of physiology and health suggests that regular tea drinking produces beneficial health effects over the long term. However epidemiological investigations (studies that investigate associations between variables in populations) are a little less clear in their interpretation. Part of the reason for this is that clinical trials are performed in closely controlled conditions whereby the external parameters are controlled (or attempted to be controlled) by the researchers. In epidemiology, the data is collected from real populations, often retrospectively, and therefore control over these variables is not possible. With regard tea drinking, it is therefore difficult to study tea drinking from an epidemiological viewpoint because tea is associated with other behaviours and foods, and it is difficult to isolate any health changes to the tea alone.
Two of the often forgotten modifying effects of tea drinking are the addition of milk and sugar. These factors can be problematic when analysing tea drinking for a number of reasons. Milk is associated with a reduced in body weight, as is all dairy consumption. Most black tea drinkers take milk and so separating the effects of the milk from the tea is very difficult. Green tea is also associated with a reduced body weight and green tea is not taken with milk. Therefore it would be reasonable to conclude that tea does have weight loss effects, but these effects may work synergistic with milk with respect to black tea. Black tea is also taken with sugar in many cases and this confounds the association between drinking tea and improvements in health because added sugar is associated with poor health outcomes. There is also the problem that people’s tastes tend to change and although they may drink tea with sugar for much of their lives, they may then switch to tea without sugar for the rest of their lives.
Tea drinking is therefore very difficult to analyse from a nutritional point of view with respect to health outcomes. Of all the foods that can be consumed, the evidence for tea as being a health providing food is probably more established and more extensive that for any other food. However, the exact effects of tea drinking are still highly controversial. Animal studies can be more tightly controlled than human studies and these have probably provided the most interesting and usable data with regard to health benefits. Tea does appear to provide substantial benefits if drunk regularly, and although there is a perception that green tea is superior in this respect, studies now confirm the black tea may confer just as many health benefits as green tea. The differing chemical compositions of black and green teas add another level of complexity to the situation. In many respects tea is therefore similar to red wine in that it is a highly complex chemical soup that is associated with behavioural factors that make it difficult to study.
RdB
