shown that all the catechins are required to provide
the cancer-preventive effects seen with green tea
catechins,” she says. “If EGCG is given only as a
single catechin, the cancer prevention effects are not
observed—proving once again the synergistic effects of
these natural substances and the need to provide these
supplements as they appear in nature.”
Overall, the strongest evidence for the beneficial
effects of tea catechins comes from laboratory
and animal studies, with epidemiologic studies and
randomized controlled trials being less conclusive.^1
Because EGCG behaves differently in cell culture
studies (in vitro) than it does in animals and humans
(in vivo), in part because the bioavailability of green
tea catechins is fairly low, in vitro studies need to be
interpreted carefully.1,2 The amount of EGCG needed to
have a biological effect in many in vitro studies is higher
than the levels typically detected in blood or tissue from
human and animal studies.^2
“In the laboratory studies, you can select the
optimal conditions to observe an effect,” Yang says.
“In animal studies, the concentration might be higher
than in human studies, and all of the animals in a study
are the same, except the group taking the catechins. In
humans, many factors can interfere—genetic factors,
dietary factors, other lifestyle factors.” For example,
he says that even though there’s very strong evidence
in laboratory studies, some epidemiologic studies
show a cancer-protective effect, while others don’t.
Human intervention studies, however, are a different
story. “Usually we choose high-risk populations, and
we study them for six months to two years, maybe five
at most,” he says. “That’s not a long-term study.” He
points out that these studies also miss the early part
of participants’ life histories—including previous tea
consumption or lack of it.How Catechins Work
The proposed mechanisms of green tea catechins
include far more than antioxidant activity. For example,
catechins can bind to specific regulatory proteins and
enzymes, modulating specific signaling and metabolic
pathways. Both of these mechanisms can help prevent
metabolic syndrome, type 2 diabetes, CVD, and cancer.^1
Bioavailability of catechins is low, largely because
they’re unstable under physiological conditions. This
makes it difficult to attain a therapeutic dose, even
when administered intravenously, and getting them to
reach target tissues remains a challenge.^7 However,
catechins that don’t end up in the blood still may have
effects in the gastrointestinal tract. Catechins’ ability
to bind to lipids and proteins—including digestive
enzymes—decreases the lipids’ and proteins’absorption, which may help prevent weight gain and
lower cholesterol. Intestinal microbes can degrade
catechins, yet the resulting metabolites may have
biological activity—an area that needs more research.1,8
Although catechins have antioxidant properties,
they also can become pro-oxidant, generating reactive
oxygen species (ROS), which include free radicals,
although this may be more likely in an isolated cell
culture than in living humans.^1 Nevertheless, ROS
can work against us by contributing to cell death
(apoptosis), advanced aging, and diseases such
as atherosclerosis and cancer, or work for us by
activating internal defense systems and regulating
signaling between cells. In other words, it appears
that the proposed benefits of catechins may come
from their ability to both generate and scavenge
ROS.^2 Catechins also have secondary antioxidant
effects by inducing the body’s natural antioxidant
enzymes, inhibiting pro-oxidant enzymes and
producing enzymes needed for phase II of the body’s
detoxification process.^420 today’s dietitian august 2019