Better Biomarkers of Tea’s Effects on Oxidative Stress Needed

WASHINGTON—Can the humble cup of tea ward off chronic diseases like cancer? Hints of the potential health benefits of tea are intriguing, but the emphasis remains on the “potential,” said Jeffrey B. Blumberg, PhD, chief of the Antioxidants Research Laboratory, Tufts University. He spoke at the Second International Scientific Symposium on Tea and Human Health.

Tea Alone Not the Answer

“Tea’s ability to counter oxidants, hypertension, cholesterol, and platelet aggregation can and should be part of a wider dietary approach to disease prevention,” Dr. Blumberg said at the tea symposium. “We should be moving from avoiding nutrient deficiency diseases to promoting optimal health, maximizing physiological function, reducing chronic disease, and resisting environmental toxins,” he said.

“We won’t find just one antioxidant, or one vitamin, or one polyphenolic compound that protects against disease. We’ll need a complex antioxidant network, both enzymatic and nonenzymatic.”

Research into tea’s efficacy is thus far less than conclusive, he said. Analysis of 16 case-control studies of black tea and gastrointestinal cancer showed no effect in 12 studies, an inverse correlation in one, and a positive correlation in three (which may have been confounded by other factors, like cigarette smoking). Nonetheless, he said, even these hints of benefit could have an impact on the 155,000 new cases of colorectal cancer diagnosed each year.

To demonstrate the difficulty in teasing out useful information on tea’s beneficial effects, Dr. Blumberg recounted his own trial of the effects of black tea on biomarkers of oxidative stress status in humans. The randomized, double-blind, placebo-controlled study included 24 healthy, nonsmoking, older adults (average age, 65 years) with no history of cancer. The subjects drank four cups of black tea or placebo every day for 9 weeks. Blood and stool samples were taken twice before and twice at the end of the trial. Other dietary patterns were unchanged.

Plasma measurements of tea polyphenols taken after a 14-hour fast were consistently low, indicating that these antioxidants had cleared from plasma prior to measurement. Plasma levels of vitamins C and E, beta-carotene, lipid hydroperoxides, and antioxidant capacity were unchanged in the tea drinkers, compared with those on placebo. Tea polyphenols did appear in exfoliated colonocytes, but there was no change in cancer biomarkers: CEA, secretory component, or DNA hypomethylation. Urinary epicatechin increased after drinking black tea but vanished after 24 hours.

This lack of effect may have been due to the choice of subject population. While “tea-naïve,” they had been eating well, probably in the upper quartile of consumers of fruits and vegetables. “We may need to look at people of higher risk in order to record any effect,” he said.

In general, he said, research into polyphenolic phytochemicals has to address bioavailability, metabolism, tissue levels, specificity of action, and interactions with other phytochemicals and drugs. “We need to use all approaches,” he said. “Clinical observation is a good start, but it is only suggestive. Epidemiology is often inexact and confounded. Clinical trials may be exact, but too often are expensive, of short duration, and use limited doses. We need better biomarkers.”