Tea is the second most-consumed beverage in the world, after water. All tea comes from the Camellia sinensis plant, but different ways of harvesting and processing produce different teas.
To make black tea, the leaves are wilted, bruised, rolled, and fully oxidized. Black tea accounts for 75 percent of the tea consumed in the world.

Oxidization happens when the leaves are exposed to the air for long periods. Enzymes break down the chemicals in the leaves, producing their brown coloring and familiar smell.

Green tea, in contrast, is made from leaves that are not oxidized.

Oxidization may give black tea nutritional benefits that are not present in green tea, such as reducing the risk of several cancers, protecting the heart against atherosclerosis, and maintaining a healthy blood pressure.

Benefits
Most studies on the potential health benefits of tea have focused on green tea.

However, taking into account the oxidization process involved in making black tea, some studies have investigated the unique benefits that this may provide.

Few studies on black tea are conclusive, as the tests have involved giving animals larger doses than would normally be consumed in an average human diet.

Food and drink companies may overemphasize the health benefits of antioxidants to support sales.

Atherosclerosis

The oxidation process in black tea, which lends the leaves their brown colour, may be linked to black tea's potential health benefits.
Atherosclerosis is a buildup of plaque in the walls of any artery in the body. It can lead to coronary heart disease and chronic kidney disease. Free radicals heavily contribute to this condition.

A 2004 study on hamsters by researchers at the University of Maryland linked the antioxidants available in green and black tea to combatting the free radicals that cause atherosclerosis.

Three cups of black tea per day were estimated to reduce the risk of atherosclerosis by 11 percent.

A review published in the European Journal of Clinical Nutrition suggests that drinking 3 or more cups of tea a day might offer protection against coronary heart disease.

Decreasing cancer risk

Findings cited by the National Cancer Institute suggest that the polyphenols in tea may decrease tumor growth. Laboratory tests and animal studies suggest they may protect against damage caused by ultraviolet B (UVB) radiation.

Black tea has been linked to cancer in a similar way to green tea, although it affects fewer types of cancer. Studies have also indicated that black tea may have a positive impact on bladder, lung, and prostate cancer.

As is the case in many studies related to tea and cancer, the results are inconclusive.

Black tea has also been shown to stimulate genes that reduce the sensitivity to chemotherapy treatment in the cell. This suggests that back tea could weaken the effect of cancer treatment.

Separate studies have found conflicting outcomes as far as black tea and cancer are concerned, noting that black tea both increased and decreased the risk of lung cancer in differing studies.

Researchers have not been able to explain how antioxidants and cancer cells interact to reduce the risk of cancer development.

However, one team concluded that drinking six cups of tea a day could enhance antioxidant status.