During a tea nutrition study,
physiologists David J. Baer
and David Paul (in the
background, Baer is on the
left) review calorimetry
data from a study volunteer
inside a calorimeter.
The calorimeter is a room
having a closed environment that allows the researchers
to measure how tea influences metabolism and energy expenditure.
It is universally accepted that caffeinated tea raises metabolic rate
because caffeine is a stimulant. "The interesting part of our study,
which agreed with findings from a similar study in England, was that
when you drink tea you turn on the fat-burning spigot a little bit more
than when you drink caffeinated water," says Rumpler. Some scientists
speculate that caffeine and EGCGa highly active catechin in teamay
act together to increase fat oxidation.
"Anecdotal evidence over time, particularly in China, points to
a relationship between green tea consumption and weight loss,"
says Rumpler. "But until we do a really comprehensive study in
which we have humans drink tea and see whether they lose weight, we
can't actually say that green tea makes people lose weight. What we
can say is that it raises metabolic rates and increases fat oxidation
rates. Those are two things that are predictive of weight loss."
Coping with Confounding Variables
In September 2002, ARS and other researchers met at the U.S. Department
of Agriculture in Washington, D.C., for the Third International Scientific
Symposium on Tea and Human Health. The symposium was hosted by organizer
Jeffrey B. Blumberg, associate director of the Jean Mayer USDA Human
Nutrition Research Center on Aging at Tufts University in Boston, Massachusetts.
Blumberg is also director of HNRCA's Antioxidants Research Laboratory.
Lead cook Sue Burns
instructs volunteers before
they eat dinner and drink
tea, which is part of a
tea nutrition study.
One issue discussed there was inconsistency among early studies of
tea. "Confounding"a situation in which findings are
affected by a variety of uncontrolled factorscan occur. "Some
studies are simply not sensitive enough to eliminate confounding factors,"
says ARS chemist Joseph T. Judd, who is with DHPL. In the case of tea
studies, it could be as simple as a volunteer's getting the same flavonoids
that are in tea from other foods consumed during the study.
Blumberg notes that flavonoid concentrations differ in tea beverages,
depending on whether the preparation was blended, decaffeinated, brewed,
or iced. Milk protein, for example, when added to tea, had previously
been reported to possibly bind to, and therefore reduce, the flavonoid
"There has been only one study that showed that adding milk decreased
the bioavailability of catechins in tea," says Blumberg. "Those
results were not replicated in any of several subsequent studies."
Many factors can affect the way tea compounds are absorbed, metabolized,
and excreted, according to Blumberg.
A Low-Down on Lipids
Judd, who is with the DHPL, is the lead author of a recent study that
found that drinking tea lowered cholesterol and, therefore, could possibly
reduce the risk of coronary heart disease (CHD). Judd points out that
while several epidemiological studies found that green and black tea
consumption is associated with reduced risk of CHD, experimental studies
had not confirmed this. "The experimental studies did not control
the background diet of the volunteers," says Judd. "Other
foods or nutrients consumed during the studies could very well have
affected the risk factors."
Judd's recent study assessed the effects of black tea consumption on
blood lipid and lipoprotein concentrations in adults with mildly high
cholesterol. He carefully controlled the volunteers' diet and weight.
Seven men and eight women were given five servings of black tea a day
for 3 weeks and a tea-flavored water for another 3-week period. In a
third study period, caffeine was added to the tea-flavored water in
an amount similar to that found in the tea.
"Overall, we found a 6 to 10 percent lowering of blood lipids
in drinkers of black tea in just 3 weeks," says Judd. What's more,
the study showed no effect on high-density lipoprotein, or "good,"
The study's authors concluded that drinking black teaalong with
following a prudent diet moderately low in fat, cholesterol, and saturated
fatty acidsreduces total and LDL cholesterol by significant amounts
and may, therefore, reduce the risk of CHD. The study is slated to appear
in an upcoming issue of the Journal of Nutrition that will feature
the tea symposium proceedings and an introduction by Blumberg.
Judd is now conducting a study on the antioxidant effects of tea phytonutrients
Enhancing Insulin Activity
ARS chemist Richard Anderson found that regularly brewed tea, when
added to the fat cells of laboratory rats, increased insulin activity
by more than 15 times. Anderson is with the BHNRC's Nutrient Requirements
and Functions Laboratory. He noted that this increased insulin activity
was found with green, black, and oolong teas, regardless of whether
caffeinated or decaffeinated.
Further, his research showed that in green and oolong teas, the catechin
EGCG was largely responsible for the results. In black tea, active ingredients
included tannins and theaflavins, in addition to EGCG. "The amount
we tested was comparatively very small, considering the effect we observed,"
says Anderson. Confirmational studies in humans are required before
the results can be applied to people.
Ernst J. Schaefer, director of the HNRCA's Lipid Metabolism Laboratory,
recently completed a pilot study during which 8 volunteers with type
II diabetes had lower blood sugar levels by 15 to 20 percent after drinking
6 cups of tea per day for 8 weeks. Schaefer and Blumberg have since
launched a 24-week, randomized, double-blind study involving 40 male
and female volunteers with type II diabetes, not taking insulin. "We
want to examine the effect that green and black teas have on the glucose
levels of the volunteers," says Schaefer.
Topping the Flavonoid Charts
Since nearly 95 percent of tea's polyphenol compounds are flavonoids,
tea ranks among plants with the highest total flavonoid content. Green
tea contains more simple flavonoids, called catechins, while black tea
contains more complex varieties, called thearubigins and theaflavins.
Some polyphenols have recently been determinedin test tube studiesto
be more potent antioxidants than the well-known vitamins A, C, and E.
But results from such test tube, or in vitro, tests cannot be applied
to humans because they do not account for factors such as bioavailability,
metabolism, and excretion, says Blumberg.
"We know that these flavonoids are not as bioavailable as vitamin
C on a per-milligram basis," says Paul E. Milbury, a scientist
with the Boston HNRCA's Antioxidants Research Laboratory. Still, one
6-ounce cup (about 173 grams) of green tea has about 235 milligrams
of catechins, whereas a medium-large (178 grams) apple has 16 milligrams
of catechins and 10 milligrams of vitamin C. This data is available
from the BHNRC's Flavonoid and National Nutrient databases.
Up to 90 percent of tea consumed in the United States is black. But
green tea consumption has more than doubled recently. "Over the
last 4 years, green tea consumption increased tremendously, going from
3 to 4 percent of total tea consumed in the United States to about 9
percent today," says Joe Simrany, president of the New York City-based
Tea Council of the U.S.A. Simrany says the council is seeking a standardized
system for measuring and labeling commercial teas' antioxidants.
Blumberg says consuming a variety of tea types and preparations adds
nutritional benefits to the diet. "The beauty of tea is that it
can be enjoyed in so many ways, depending on individual tastes and preferences,"
he says. "My hope is that future studies will be designed to accurately
assess tea's polyphenol levels and to measure tea's role in lowering
the risk of chronic diseases."By Rosalie Marion Bliss,
Agricultural Research Service Information Staff.
This research is part of Human Nutrition, an ARS National Program
(#107) described on the World Wide Web at www.nps.ars.usda.gov.
To reach scientists mentioned in this article, contact Rosalie
Bliss, USDA-ARS Information
Staff, 5601 Sunnyside Ave., Beltsville, MD 20705-5129; phone (301)
504-4318, fax (301) 504-1641.
"Brewing Up the Latest Tea Research" was published
in the September
2003 issue of Agricultural Research magazine.