by Craig Weatherby
A team of American and South Korean researchers conducted cell studies that shed new light on the relative anti-cancer properties of green and black tea.
The evidence collected to date suggested that tea's proven anti-cancer powers stem primarily from its powerfully antioxidant flavonoid-type polyphenols.
Hold the Milk, Help Your Heart
A small German study in postmenopausal women found that, compared to water, black tea increased their arteries’ ability to relax and expand. But this benefit disappeared when when milk was added to the tea.
This findings echo studies comparing the arterial effects of dark and milk chocolate, in which milky bars fail to match the benefits of the darker stuff.
Lorenz M et al. Addition of milk prevents vascular protective effects of tea. Eur Heart J. 2007 Jan 9; [Epub ahead of print].
Thus, it’s been assumed that green and white tea must be the most powerfully anti-cancer forms of the ancient beverage, since they contains more flavonoid-type polyphenols than black tea does.
While black tea (green tea oxidized by fermentation) contains between 3 and 10 percent water-soluble polyphenols by volume, green tea boasts ten times that concentration.
However, as we will see, polyphenols make up only part of the solids in tea: and they may not be the sole or sufficient explanation for tea’s well-documented anti-cancer properties.
And green and black teas also contain different types and proportions of polyphenols.
When tea leaves ferment long enough to turn them black via oxidation, the majority of their catechin-class flavonoid polyphenols change into tannin-like flavonoids called theaflavins and theanine.
However, some population studies have suggested that black tea is as or more protective against certain cancers (including certain breast tumors) as green tea.
Tea flavonoids do five things that work together to stop cancer in its early stages:
- Cause programmed “suicide” (apoptosis) among cancer cells
- Block P450 enzymes, which activate pro-cancer compounds
- Stop tumor-promoting chemical signals from cancer cells
- Disable damaged, cancer-promoting DNA
- Block growth of new blood vessels in the tumor (angiogenesis)
Given this context, the general thrust of what the USDA team found came as no big surprise.
But their unexpected findings vis a vis the effects of tea flavonoids on human cancer cells is sure to send scientists back to the bench for further exploration.
Black and green tea work equally well; Benefits not tied tightly to flavonoid content
The joint USDA/South Korea team examined the ability of nine green tea catechins, three black tea theaflavins, and theanine from black tea to induce cell death (apoptosis) in isolated human cancer cells (Friedman M et al 2006).
Tea, berries, and other polyphenol-rich plants are considered key anti-cancer agents because they undermine cancer when it is most vulnerable: during its early, so-called “promotion” stage.
The researchers reported that most of the flavanol-type flavonoids in both green and black tea—catechins, theaflavins, and theanine—cut the numbers of cancerous human breast, colon, liver, and prostate cells, without big differences among them.
However the efficacy of each different extract of black or green tea tested depended not on its flavonoid content, but simply on the sheer amount of dissolved solids per volume of liquid, regardless of their flavonoid content.
This suggests that something about the non-flavonoid compounds play key roles, and that therefore, black tea may offer anti-cancer benefits closer to those of green and white tea than thought.
They also found that ethanol/water extracts of tea possess more flavonoids and are more potently anti-cancer, suggesting that tea supplements (mostly made this way) may have a bit of an edge over water extracts, such as plain cups of tea. But any hypothetical anti-tumor advantage supplemental tea capsules might offer is likely to be quite expensive, compared with enjoying tea by the cup.
As the USDA/Korea team said (Friedman M et al 2006), their findings extend our knowledge of the anti-cancer potential of tea, and suggest, most significantly, “…that consumers may benefit more by drinking both green and black teas.”
Stay tuned… Next issue, we’ll report on a recent finding that attributes some of tea’s anti-cancer and weight benefits to an very different, rather unexpected realm.
- Friedman M et al. Structure-Activity Relationships of Tea Compounds against Human Cancer Cells. J Agric Food Chem. Published on-line ahead of print, December 16, 2006: ASAP Article doi: 10.1021/jf062276h S0021-8561(06)02276-X