LONDON, Oct. 5 -- Drinking black tea reduces measures of stress-related hormones and harmful cardiovascular changes, reported researchers here. In short, it seems to calm jangled nerves.

In a double-blind placebo-controlled study, tea drinkers had lower levels of cortisol following a stress challenge, felt more relaxed, and had reduced platelet activation than placebo drinkers, reported Andrew Steptoe, D.Sc.., of University College London, and colleagues, in the online edition of Psychopharmacology.

"Drinking tea has traditionally been associated with stress relief, and many people believe that drinking tea helps them relax after facing the stresses of everyday life," said Dr. Steptoe. "However, scientific evidence for the relaxing properties of tea is quite limited."

To determine what effect drinking black tea might have on stress, the authors conducted a parallel group, double-blind randomized trial. Their goal was to subjectively assess the effects of tea consumption of cardiovascular, cortisol, and platelet responses to acute stress.

They recruited 75 healthy non-smoking men who agreed to be withdrawn from tea, coffee, and other caffeinated beverages during a four-week washout period. During that time, the participants were given a fruit-flavored caffeinated placebo tea. The volunteers also refrained from using NSAIDs or taking dietary or caffeine supplements, and restricted their diet to avoid fruits rich in antioxidant flavonoids that could have skewed or masked the association of tea with stress reduction.

At the end of the washout period, the volunteers were subjected to a psychophysiological evaluation, and were then randomized to either six weeks of a fruit flavored-black tea preparation or a fake-tea placebo.

Both the tea and the placebo were given to the participants in the form of fruit-flavored powders of a different flavor (apple or lemon) from the one they consumed during the washout phase. The purpose of this switch was to mask any sensory changes that the group assigned to tea drinking might otherwise have detected, the authors noted.

The tea preparation was based on the components of an average cup of black tea, and included theobromine, epicatechin, catechin, epigallocatechin, flavonoids, and other compounds. The sachets provided contained 6.4% flavonols, and were equivalent to four cups of strong black tea per day.

In all, 38 patients were assigned to placebo, and 38 to active tea. Patients were evaluated at baseline, during the study, and afterward with cardiovascular measures, and both cortisol and platelet activation were assessed before and after stress-inducing tasks. The tasks were designed to produce increases blood pressure, heart rate, and subjective stress ratings.

The authors found no significant between-group differences in response to stress at baseline. But when they looked again after six weeks, they found that the tea drinkers had lower levels of platelet activation, assessed by flow cytometry, both before and after being subjected to stress than did placebo drinkers (P<0.005).

Post-stress cortisol levels were also lower among the tea drinkers vs. controls (P=0.032). Less than one hour (50 minutes) after the stress-inducing task, cortisol levels had dropped by an average of 47% in the tea-drinking group compared with 27% of the placebo-drinking group.

Tea sippers also reported being significantly more laid back on subjective assessment of relaxation than placebo imbibers.

Their findings suggest that tea may help the beverage's drinkers to return more quickly to baseline levels of stress hormones and platelet activation than placebo drinkers, the authors suggested.

"Evidence that tea enhances post-stress recovery was provided by two measures," they wrote. "First, ratings of relaxation during the recovery period were more positive in the tea than in the placebo conditions. As there were no differences in subjective stress responses to the tasks or in cognitive appraisal of tasks, this finding suggests that tea drinking had a specific effect on subjective recovery from stress. Second, cortisol showed a greater decline following stress in the tea condition."

The authors noted that the study was not designed to tease out the individual components of tea that might contribute to the stress reduction mechanisms they observed.

"Tea is chemically very complex, with many different ingredients," Dr. Steptoe said. "Ingredients such as catechins, polyphenols, flavonoids and amino acids have been found to have effects on neurotransmitters in the brain, but we cannot tell from this research which ones produced the differences.

"Nevertheless," he continued "our study suggests that drinking black tea may speed up our recovery from the daily stresses in life. Although it does not appear to reduce the actual levels of stress we experience, tea does seem to have a greater effect in bringing stress hormone levels back to normal. This has important health implications, because slow recovery following acute stress has been associated with a greater risk of chronic illnesses such as coronary heart disease."

The limitations of the study, as noted by the authors, "include the fact that participants were young, apparently healthy men, so we do not know whether other groups would respond similarly. Platelet stress responses were small, and there was no absolute increase in cortisol level. Future studies would benefit from a non-stress control condition."

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Black tea is a product made from the Camellia sinesis plant. The aged leaves and stems are used to make medicine. Green tea, which is made from fresh leaves of the same plant, has some different properties.

Black tea is used for improving mental alertness as well as learning, memory and information processing skills. It is also used for treating headache and low blood pressure; preventing heart disease, including “hardening of the arteries” (atherosclerosis) and heart attack; preventing Parkinson's disease; and reducing the risk of stomach and colon cancer, lung cancer, ovarian cancer, and breast cancer. It is also used for type 2 diabetes, stomach disorders, vomiting, diarrhea, and as a diuretic to increase urine flow. Some people use black tea for preventing tooth decay and kidney stones. In combination with various other products, black tea is used for weight loss.