Did you know that after water, tea is the most commonly consumed beverage in the world? This ancient drink is as popular today as it was hundreds of years ago. Iced or hot, tea is in high demand. In the United States alone, more than 50 billion servings (that’s a whopping 2.25 billion gallons) of tea were consumed last year, according to trends data provided by the Tea Association of the USA Inc. (2007). And while many new types and colors of tea are being introduced to the market now, black tea remains the most popular, accounting for approximately 78% of the world’s total tea consumption (Shixian et al. 2006).
One reason why so many consumers are switching to tea is its reputed health benefits. This article will examine some of the studies indicating that tea may improve heart health and reduce the risk of certain diseases, such as diabetes.
Reading Tea Leaves
All teas originate from the leaves of the plant Camellia sinensis, a warm-weather evergreen (Kuriyama et al. 2006). How the fresh leaves are ultimately processed and come into contact with oxygen determines the variety of tea that is produced (Tea Association of the USA Inc. 2007). For example, green tea is not oxidized at all, Oolong tea is oxidized for 2-3 hours, and black tea is allowed to oxidize for up to 4 hours (Tea Association of the USA Inc. 2007).
It should be noted that herbal teas are not derived from the Camellia sinensis plant and are thus not the same as traditional teas. Because herbal teas are combinations of the leaves, roots, bark, seeds and flowers of other plants, they are not thought to bestow the same health benefits as conventional tea.
The Health Connection
Over the past two decades, studies have shown that both green and black teas contain an arsenal of naturally occurring disease-fighting compounds called flavonoids embedded in their leaves. “It is the flavonoids, specifically catechins, in tea that are so favorable to health,” according to Jeffrey Blumberg, PhD, FACN, CNS, professor of nutrition at Tufts University in Massachusetts.
Tea flavonoids have been credited with helping protect the human immune system and reducing the risk of various diseases, including cardiovascular disease (CVD), bone and oral diseases, diabetes and certain cancers. The antioxidant properties of tea flavonoids are believed to fend off free-radical damage to cells, which can lead to many chronic diseases in the body. An average cup of brewed green or black tea contains 150-200 milligrams (mg) of flavonoids (Merken & Beecher 2000).
With the Dietary Guidelines for Americans 2005 recommending that the public consume more plant-based foods, such as fruits and vegetables, nutrition experts contend that tea should be included among those foods. “Tea is a plant food and almost exclusively a catechin beverage,” notes Blumberg.
Tea as a Cancer Fighter
Although more conclusive evidence is needed, studies have found a lower risk of certain cancers in people who drink green or black tea. The tea flavonoids may inhibit the oxidative changes that free radicals and some carcinogens cause in DNA (Weisburger 1999). Specifically, study results suggest that it is tea’s high antioxidant polyphenol content that may serve as the mechanism of action. Research has shown that the antioxidant is believed to inhibit the growth and destroy cancer cells—while leaving healthy tissue alone.
Findings from a prospective cohort study of 8,552 general residents of Saitama Prefecture, Japan, revealed that drinking more than 10 cups of green tea per day (versus less than 3 cups per day) significantly reduced death rates from cancer (Nakachi, Eguchi & Imai 2003). In other research, black tea was found to boost the immune system in animals with cancer, and like anticancer drugs, did not promote the proliferation of cancerous cells at the same time (Bhattacharyya et al. 2004).
Studies have also indicated that tea may help reduce the risk of ovarian cancer. In a Chinese study, researchers found that the risk of this type of cancer declined when participants increased the frequency and duration of their overall tea (especially green tea) consumption (Zhang, Binns & Lee 2002). Likewise, green and black tea consumption was linked to decreased ovarian-cancer risk in a large, long-term, population-based study of 61,057 Swedish women aged 40-76 years old (Larsson & Wolk 2005). Over the course of a 15-year follow-up, those women who drank two or more cups of tea daily had a 46% lower risk of ovarian cancer compared with those who drank no tea at all. Even the slightest tea intake (< 1 cup per day) was linked to an 18% reduction in risk, while 1 cup per day decreased the risk of ovarian cancer by 24% (Larsson & Wolk 2005).
Tea and Heart Health
In the arena of cardiovascular health, the studies on tea are perhaps the most robustly researched and conclusive. Evidence indicates that the antioxidant properties of tea flavonoids may help lower CVD by decreasing lipid oxidation (Weisburger 1999); may reduce the incidence of heart attacks and stroke (Hertog et al. 1993; Keli et al. 1996); and may beneficially impact blood vessel function (Duffy et al. 2001).
Recent data from The Ohsaki National Health Insurance Cohort Study revealed a cardiovascular benefit among green-tea drinkers, especially in women. The data showed a 16% lower risk of all-cause and CVD mortality over 11 years of follow-up in participants who consumed five or more cups of tea per day, with the inverse association being more prevalent in female subjects (Kuriyama et al. 2006). The researchers’ rationale for the mortality outcome differences between the genders was cigarette-smoking rates. Men were more likely to smoke, and the inverse associations between green-tea consumption and CVD mortality appeared to be more pronounced in participants who had never smoked.
Tea’s Role in Diabetes Defense
With rates of type 2 diabetes increasing globally, nutrition researchers are examining how the caffeine in tea may affect the risk for getting diabetes. Since population-based studies in Japan have shown a twofold increase in the prevalence of diabetes during the past two decades, researchers have focused on the effects of green tea, the most commonly consumed tea in Japan (average consumption per capita is two cups per day) (King, Aubert & Herman 1998; Ohmura et al. 1993).
One large cohort study involved 17,413 middle-aged and older men and women in 25 communities across Japan. The researchers found that participants who consumed six or more cups of green tea per day actually lowered their risk of diabetes by 33% (compared with those who consumed none) (Iso et al. 2006). There appeared to be a dose-response relationship: in other words, the more caffeine the participants consumed, the lower their risk level was for diabetes. The subjects in this study had a median caffeine intake of 416 mg a day from both green tea and coffee (Iso et al. 2006).
Although this research focused on the caffeine in green tea, it should be noted that plants containing flavonoids have long been used to treat diabetes in Indian medicine (Waltner-Law et al. 2002). Green-tea flavonoids have been shown to have insulin-like activity, as well as insulin-enhancing activity (Cheng 2006a; Anderson & Polansky 2002; Cheng 2006b).
“Although there has been some experimental work on tea’s effects on insulin sensitivity, the research is in the early stages,” explains Blumberg. Even though the research has not indicated a standard recommendation for tea as an antidiabetic agent, experts contend that more is better—as is the case with all plant-based foods.
Be Careful What You Add to Your Tea
Sad to say, the English tradition of serving tea with milk has now come under fire. Some experts contend casein, a protein component in milk, binds with tea flavonoids and lowers their rate of absorption in the body. However, one study that looked at six brands of black tea found that there was similar antioxidant capacity even when milk was added, and the researchers concluded that the addition of milk did not affect antioxidant absorption rates or quantities (Kyle et al. 2007). Another study found that adding milk to black tea did not hinder its ability to alter the antioxidant status of subjects or its ability to prevent oxidative damage (Reddy et al. 2005).
“The state of the science is mixed (on this),” according to Blumberg. However, Blumberg does point out that if the health benefits of tea are based on a dose-response relationship, then diluting tea with milk would equate to a smaller quantity of catechins being ingested—and thus fewer benefits overall.
And do think twice before you go heaping teaspoons of sugar into your cup of tea. While sweetened tea is a staple of the American South, most people now realize that the addition of sugar to any beverage or food simply amounts to useless calories and potential dental problems. If unsweetened tea simply isn’t for you, consider adding a small amount of sweetener to the brew.
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