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Choice Nutrition’s Dr. Evan McCarvill explains what those green leaves are all about:
Tea is a widely popular beverage, consumed all over the world, with a long and complex history. It is commonly made by pouring hot or boiling water over the cured and often fermented leaves of the Camellia sinsensis plant; an evergreen shrub native to China. It was originally consumed for centuries in China, and it was only during the Chinese Tang dynasty (618-907 CE) that it began to be promoted as a recreational drink, while previously it was first-and-foremost a medicinal drink.
It was not until about a thousand years later, in the 16th and 17thcenturies, that tea was introduced and popularized in the Western world, eventually becoming one of the cultural hallmarks of Britain.
Now, in modern times, there is a substantial body of research, consisting of more than 5600 scientific studies, supporting the medicinal benefits of this widely consumed “recreational” beverage.
The following graphic highlights some of the latest findings for the benefits of drinking tea, originally published by the American Journal of Clinical Nutrition in 2013.
About one third of a tea leaf, by weight, consists of a group of compounds called flavonoids, which are released when the leaves are exposed to boiling water. Flavonoids are known, and have been studied, for their potent anti-oxidant properties, and new research indicates that they also may help to prevent or reduce chronic inflammation. In so doing, these compounds can help mitigate vascular damage linked to chronic human conditions associated with ageing, including heart disease and cognitive decline.
Cardiovascular disease is one of the leading causes of death in Canada, having claimed the lives of 48,681 Canadians in the year 2012, according to Statistics Canada. Chronic inflammation resulting from a poor diet and/or sedentary lifestyle leads to damage and hardening of blood vessel walls, plaque-formation, and thus increased risk of stroke or heart attack.
Numerous studies from around the world indicate that regular tea consumption is linked to a reduced risk of cardiovascular disease. One meta-analysis published in 2003 in the European Journal of Clinical Nutrition looked at the relationship between intake of dietary flavonoids, which are abundant in tea, and death by heart disease. The data were derived from seven individual studies, and showed that those in the top third of dietary flavonoid comsumption had a 20 percent reduced risk of death from heart disease, compared with those in the bottom third.
Similarly, another meta-analysis, published in the American Journal of Epidemiology in 2001, looked at data from 17 population health studies, dating from 1966 to 2000, which studied directly the relationship between tea consumption and cardiovascular disease. They found that risk of heart attack was reduced by 11 percent in those who consumed at least 3 cups of tea per day.
Much of this research in heart disease is focused on black tea, which is simply the result of green tea leaves which have been fermented, but new research also demonstrates strong evidence of cardiovascular benefits with green tea.
A study out of Japan in 2006, published in the Journal of the American Medical Association, showed that there was a 26 percent reduction of risk in heart disease-related death in those who drank five or more cups of green tea per day, compared to those who drank one cup per day or less. These data come from a sample of 40,530 participants, aged 40 to 79, with no prior history of heart disease.
The reasons for why tea consumption shows such benefits for heart disease are still being explored. It is thought that tea flavonoids, in addition to antioxidant properties, work through several simultaneous mechanisms in order to improve risk factors. For instance, research also shows that the function of endothelial cells lining blood vessel walls is improved by flavonoids. The Journal of Cardiology in 2004 published a double-blind study showing that drinking a cup of black tea improved coronary vessel function, over the short-term, in healthy males.
Improved blood vessel function can thus help reduce risk of hypertension. According to a study from the Archives of Internal Medicine in 2004, moderate-strength green or oolong tea, regularly consumed, reduced risk of developing high blood pressure. These are data from 1507 subjects with no prior history of high blood pressure. In fact, for 600 of these subjects who consumed 120 milliliters of tea per day for a year or more, risk of hypertension reduced by a whopping 46 percent.
Flavonoids also seem to have an impact on cholesterol levels. High cholesterol is a significant risk factor for heart disease, contributing to plaque build-up in artery walls, thus hardening and narrowing them. The Journal of Nutrition published in 2003 a study that found that consumption of 5 servings of black tea per day reduced “bad” LDL cholesterol by 11.1 percent, and total cholesterol by 6.5 percent, in individuals with mildly elevated cholesterol. Additionally, another study in 2003, published in the Archives of Internal Medicine, showed that 240 men and women with mildly elevated cholesterol, had their LDL cholesterol reduced by 16.4 percent, and their total cholesterol reduced by 11.3 percent, by consumption of green tea extract, compared with those who took placebo.
Every organ in the body, including the brain, benefits from better blood flow, which results from healthier blood vessels. Researchers recognize the importance of the relationship between vascular health and Alzheimer disease, as well as other forms of dementia.
The number one cause of premature death in Canada is Cancer. It is estimated that about 38 percent of Canadian women, and 44 percent of Canadian men, will develop some kind of cancer at some point in their lifetimes. Cancer is a highly complex disease with a wide variety of contributing risk factors, both genetic and environmental. Research is showing that lifestyle is a key factor in this equation, and there is a mounting body of evidence showing that diet plays a crucial role in cancer prevention. Among this research is to be found evidence that tea could play a role in preventing certain types of cancer.
Flavonoids, as discussed above, act as potent antioxidants, which can help prevent DNA damage that can lead to cancer-causing mutations, as well as down-regulating certain growth-factor molecules, which may help prevent or delay tumour formation. Animal models of skin, lung, liver, and colon cancer have shown that the polyphenols (i.e. Flavonoids) in tea offer protection from chemical-based factors in cancer development. Flavonoids help to maintain normal cell growth rates, and may even help by enhancing the body’s ability to detect and naturally destroy cancerous and precancerous cells.
It should be noted that most of the promising research of tea for cancer prevention comes from animal studies. Results from some human population studies do show promise for cancer prevention, but results from other studies are mixed. So while it is not definitive, research of tea’s role in cancer prevention is an active area of research.
There was a Swedish study published in 2005 in the Archives of Internal Medicine, involving 61,057 women. Over the course of 15 years of follow-up, it was found that there was a 46 percent lower risk of ovarian cancer in women who drank at least two cups of tea per day, compared with those who did not drink tea.
The International Journal of Cancer published in 2003 a study that found a significantly reduced risk of breast cancer in women who drank green tea, compared to non-tea drinkers. Interestingly, this study involving Asian-Amerian women showed a proportional reduction of risk with the amount of green tea consumed, but showed there was no apparent reduced risk if black tea was consumed.
There was an American study published in Public Health Nutrition in 2002 showing that drinking one or two cups of tea per day resulted in a 42 percent reduced risk of colon cancer, compared to not drinking tea. It was also found that drinking more than one and a half cups of tea per day resulted in a 70 percent reduced risk of colon cancer in men.
It is also noteworthy that all the benefits of black tea and green tea that are being discussed here are associated with tea that has no added milk. Milk proteins have the effect of binding to, and rendering inert, the beneficial properties of the flavonoids which are so central to the positive effects of tea. While it is debatable just how much a little added milk will mitigate the effects of the flavonoids, the fact remains that the optimal benefits are to be obtained from tea that has no added milk or sugar.
Sometimes the most common and mundane things in our lives can prove to have great value after all. After a centuries-long tradition of drinking tea mostly for pleasure, tea is now being rediscovered for its original medicinal benefits. So drink your morning tea with confidence. It’s good for you!
DO NOT ACCEPT ILLNESS!
Yours in health,
Dr. Evan McCarvill, B.Sc., Doctor of Naturopathic Medicine (ND)
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