and several randomized controlled trials in humans have
shown a benefit from green tea catechins in reducing
features of metabolic syndrome, including waist circum-
ference, LDL cholesterol, blood pressure, and insulin
resistance. However, other trials haven’t observed those
benefits. This may be simply due to variation in study
lengths and participant characteristics, but clearly more
well-controlled, long-term studies are needed.2 ,18Cognitive Health
Oxidative stress and generation of free radicals play
a role in neurodegenerative diseases, just as they do in
cancer and CVD. Both clinical and epidemiologic stud-
ies have shown that green tea consumption may have
a protective effect against neurodegenerative diseases
and cognitive impairment, but, again, at consumption
levels far above average.^2
Several observational studies have found an
association between tea intake and lower rates of
Parkinson’s disease, although a few have found no
effect. On the other hand, studies focusing on Alzheimer’s
disease largely have failed to find a significant benefit
from green tea intake, but results are more promising
when looking at cognitive impairment.^19 Most of the cross-
sectional studies conducted in the last dozen years found
an association between increased green tea consumption
and lower prevalence of cognitive impairment, and
longitudinal studies have found similar results. However,
randomized controlled trials have been inconclusive.^20Inflammatory Bowel Disease
Catechins’ anti-inflammatory and antioxidant proper-
ties also make them a potential tool for managing inflam-
matory bowel disease, a chronic inflammatory disease
of the intestine, most frequently manifesting as ulcer-
ative colitis or Crohn’s disease. It’s thought that catechins
also may inhibit the abnormal activity of immune-related
cells, prevent gastrointestinal lesions from worsening,
and improve the functioning of the tight junctions between
cells lining the intestine. However, at this time there are
few animal or human studies to support these theories.^21Viral Illnesses
In in vitro studies, catechins minimize the propensity
of the influenza virus to infect and proliferate, in part by
inhibiting its ability to interact with the cell membrane
when it invades a cell. Epidemiologic studies and ran-
domized controlled trials have found a significantly lower
incidence of influenza in individuals with higher con-
sumption of green tea or, in the case of the clinical trials,
green tea capsules.22,23 It’s possible that these antiviral
properties may help inhibit other viruses, including HIV,
hepatitis B and C, herpes simplex, Ebola, and Zika.^24How to Counsel Clients
Researchers have observed beneficial effects on
weight, metabolic syndrome, and type 2 diabetes with
intake of three to four 8-oz cups of green tea per day,
which supplies about 600 to 900 mg of catechins, but
lower levels of intake would still add to the total intake
of polyphenols in the diet.^1 Taking enough green tea
polyphenol products to equal the amount of EGCG in
eight to 16 cups of green tea each day may overcome
the issues with poor bioavailabilty,^7 as encapsulation
can make the catechins more bioavailable.^10 However,
using high doses may not be ideal due to some evidence
of toxicity.^7 Although green tea is considered safe and
has a long history of use, taking high levels of green
tea extract–derived dietary supplements has caused
liver toxicity in some individuals, especially those with
preexisting conditions or medications that already
impact the liver.1,25,26
“I think for the people taking catechins for weight
reduction, liver toxicity could be a concern,” Yang
says, adding that there are about 30 to 40 case reports
about liver toxicity in individuals using catechin
supplements for weight reduction. He says that when
catechin intake stops, people generally recover.^26
However, if excess consumption continues, the toxic
effect could be serious.
In addition, catechins’ ability to bind to minerals,
proteins, lipids, and other biomolecules—while
conferring some health benefits—could contribute
to nutrient deficiencies and other problems when
ingested in levels far above what could be reasonably
consumed through drinking tea.^1 Furthermore, the
tannin content of green tea can interfere with intestinal
absorption of some nutrients and drugs, although
there’s little research in this area.^3
Yang has been researching green tea for 30 years,
and he says the research has become more mature,
perhaps tempering the highs of early optimism. “Those
are not miracle molecules. Green tea happens to have
some beneficial health effects. It’s just one of the many
things we can consume that may benefit health, and
it’s better than many other beverages we might drink.”
His bottom-line advice? “Tea is a healthful beverage,
especially if you like it and enjoy drinking it.”— Carrie Dennett, MPH, RDN, CD, is the nutrition
columnist for The Seattle Times, owner of Nutrition
By Carrie, and author of Healthy for Your Life: A
Holistic Guide to Optimal Wellness.For references, view this article on
our website at http://www.TodaysDietitian.com.22 today’s dietitian august 2019