State of the Science
Cancer
Because of its low bioavailability, it’s unclear
whether the anticancer effects of EGCG observed in
vitro can be extrapolated to animals and humans.9,10
Observed mechanisms from in vitro studies
include apoptosis, interruption of the cell cycle
and cell signaling pathways, and suppression of
proinflammatory molecules.^2 However, there’s limited
understanding about the mechanisms by which
green tea catechins might have cancer-preventive
effects in humans. It’s likely that green tea catechins
have multiple mechanisms that act synergistically
to prevent cancer, and they may work in tandem with
chemotherapy drugs.3,5
Recent systematic reviews and meta-analyses have
found that green tea consumption was associated
with reduced risk of breast and ovarian cancers,11,12
and both in vitro and in vivo studies have established
chemoprotective effects, although with some
inconsistencies between studies.13,14 Catechins also
appear to act synergistically with tamoxifen, the most
commonly prescribed breast cancer drug therapy.^15
Kumar says some of the clinical trials evaluating
green tea catechins for prostate cancer prevention are
promising, but more research is needed. She says this
research needs to be systematic, the way it would be
in the drug discovery model—sometimes referred to
as “from bench to patient”—which requires developing
green tea compounds based on data from population
studies, using in vitro studies to understand the
mechanisms and early effectiveness in target tissues,
then evaluating safety and effective doses in vivo with
phase I and II trials in humans before embarking
on phase III trials. “This systematic process was
lacking in earlier studies, as most people thought that
these were just natural supplements, so it was OK to
proceed to phase III without systematic evaluation,”
she says. “Our group is changing this mindset.”
Although trials for cancer prevention have been
inconsistent, possibly because of the challenges of
running long-term studies, Kumar says interest in green
tea research isn’t waning.^1 Her group just received a
$3.1 million grant from the National Cancer Institute to
evaluate the safety, efficacy, and mechanisms by which
green tea catechins prevent progression of prostate
cancer in a randomized, double-blinded clinical trial.
“Green tea catechins influence several hallmarks of
carcinogenesis, including prostate carcinogenesis, with
an acceptable safety profile, making them attractive
candidates for chemoprevention,” she says.^16
Of note, Yang says green tea shouldn’t be drunk too
hot, as frequent consumption of tea hotter than 140 ̊ F
is associated with a significantly increased risk of
esophageal cancer.^17Cardiometabolic Disease
So what avenues of research likely hold the most
promise? “I think in terms of practical applications, still
the prevention of obesity, diabetes, and cardiometabolic
disease,” Yang says. “Tea consumption is associated
with lower death rates from cardiovascular disease.”
Indeed, large cohort studies in Japan have found an
association between increased green tea consumption
and decreased risk of CVD-related death, although
consumption may have to meet or exceed seven cups
per day.^2 In human intervention studies, the strongest
evidence is for reducing metabolic syndrome.^1
Catechins may inhibit specific enzymes and modu-
late signaling pathways, helping to prevent type 2 dia-
betes and metabolic syndrome.1,8 Animal studies have
demonstrated that EGCG can improve endothelial func-
tion and insulin sensitivity and reduce blood pressure,august 2019 http://www.todaysdietitian.com 21