"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," said lead researcher Professor Andrew Steptoe from University College London.
The researchers, from UCL and Unilever Research Colworth, found that drinking four cups of black tea every day for six weeks reduced blood levels of the stress hormone cortisol by 20 per cent more than the placebo group, backing up the 'old wives tale' that tea helps people relax.
"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 Steptoe. "However, scientific evidence for the relaxing properties of tea is quite limited. This is one of the first studies to assess tea in a double-blind placebo controlled design."
The study, published on-line ahead of print in the journal Psychopharmacology (doi 10.1007/s00213-006-0573-2), recruited 75 healthy young male volunteers (average age 33). The men were put through a four-week wash-out period before the study, withdrawing consumption of tea, coffee, other caffeinated drinks, aspirin, ibuprofen, dietary supplements, and various fruit and vegetables rich in flavonoids.
They were then randomly assigned to the black tea group (37 men, four cups of black tea per day) or the placebo group (38 men, a caffeinated drink identical in taste, but devoid of the active tea ingredients) for six weeks.
Both groups were subjected to challenging tasks, while their cortisol, blood pressure, blood platelet and self-rated levels of stress were measured. In one task, for example, the subjects were asked to verbally respond in front of camera and argue their case after being exposed to one of three stressful situations (threat of unemployment, a shop lifting accusation or an incident in a nursing home).
The tasks triggered significant increases in blood pressure, heart rate and subjective stress ratings in both of the groups, and no difference was observed between the groups concerning blood pressure or heart rate. However, 50 minutes after the task, cortisol levels had dropped by an average of 47 per cent in the tea drinking group compared with 27 per cent in the fake tea group.
The researchers also found that blood platelet activation - linked to blood clotting and the risk of heart attacks - was lower in the tea drinkers, and that this group reported a greater degree of relaxation in the recovery period after the task.
"Tea, therefore, appears to influence the effectiveness of post-stress recovery, rather than the magnitude of stress responses themselves," wrote the reviewers.
The news adds to a growing list of benefits that has linked tea and tea extracts, particularly the catechin, epigallocatechin gallate (EGCG) to reducing the risk of Alzheimer's, certain cancers, as well as having a role in weight loss.
Indeed, European demand for tea extracts is currently surging, having reached 500 metric tonnes by 2003.
Steptoe and his fellow researchers however could not identify which of the compounds in tea, either independently or synergistically, were behind the benefits observed in this clinical trial.
"Tea is chemically very complex, with many different ingredients," he 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."
The researchers do reference previous results from both animal and human studies that report that tea flavonoids have an effect on the sympathetic nervous system of rats, the amino acid theanine found in tea increased brain wave activity in specific areas of the human brain, and EGCG has been reported to have a sedative effect and reduce responses to separation stress.
"Nevertheless, 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."
The research was funded by UK's BBSRC, Unilever Research and the British Heart Foundation.
The global tea market is worth about €790m (£540m, $941m). Green tea accounts for about 20 per cent of total global production, while black tea (green tea that has been oxidized by fermentation) accounts for about 78 per cent.