• Green tea, cocoa, and many fruits contain high levels of flavanols, which improve blood vessel function and lower blood pressure.
  • In the first study of its kind, researchers found that drinking cocoa may protect males’ blood vessels against the harmful effects of psychological stress.
  • The finding may help explain why diets rich in fruits and vegetables reduce cardiovascular disease and mortality risk.

Stressful events — such as sitting an exam, giving a presentation, or attending a job interview — temporarily increase heart rate and blood pressure and dilate arteries.

This is a normal part of the body’s “fight-or-flight” response, but the lining of blood vessels, known as the endothelium, can take up to 90 minutes to recover after this kind of stress.

This may help to explain why a single episode of stress temporarily increases the risk of an acute cardiovascular event, such as a heart attack.

On the other hand, experts know that eating a diet rich in fruits and vegetables reduces a person’s risk of cardiovascular disease, heart attacks, and cardiovascular mortality.

Among the healthy nutrients found in fruits and vegetables are polyphenolic compounds known as flavanols. In the long term, these improve endothelial function and lower blood pressure.

The effects of flavanols during stress and in its immediate aftermath are unknown, however.

Scientists at the University of Birmingham in the United Kingdom have conducted the first study to investigate this question.

Their research suggests that drinking cocoa, which is rich in flavanols, in the hours before a stressful event can improve blood flow during the experience and help restore endothelial function in males aged under 45 years old afterward.

“Our findings are significant for everyday diet, given that the daily dosage administered could be achieved by consuming a variety of foods rich in flavanols — particularly apples, black grapes, blackberries, cherries, raspberries, pears, pulses, green tea, and unprocessed cocoa,” says senior author Dr. Catarina Rendeiro, Ph.D., of the university’s School of Sport, Exercise and Rehabilitation Sciences.

“This has important implications for measures to protect the blood vessels of those individuals who are more vulnerable to the effects of mental stress,” she adds.

The research, conducted by postgraduate student Rosalind Baynham, appears in the journal Nutrients.

Stress tests

The scientists recruited 30 healthy males between 18 and 45 years who underwent tests at the lab on two separate occasions at least 7 days apart.

The research involved them fasting for 12 hours before each session and abstaining from alcohol, vigorous exercise, and polyphenol-rich food and drink for 24 hours beforehand.

At the start of each visit, while the participants were relaxed, researchers assessed their “brachial flow-mediated dilatation,” which is a measure of endothelial function.

They also measured their blood pressure, heart rate, and blood flow.

On one of these visits, the participants drank regular cocoa, which is rich in flavanols, and on the other visit, they drank a special cocoa drink that was low in flavanols.

About 90 minutes later, they took a competitive test of mental arithmetic designed to elicit stress.

For example, during the test, the researchers filmed the participants, and they had to watch their performance on a screen. The researchers told the participants that “body language assessors” would assess them.

The arithmetic task got progressively faster, and the participants lost points whenever they got an answer wrong.

An experimenter in the room marked their responses and sounded an alarm when they made a mistake, or once every 10 answers if they did not.

At 30 and 90 minutes after the stress test, the researchers remeasured the participants’ endothelial function and blood pressure.

The results showed that 30 minutes after the stress test, all participants had impaired endothelial function. This measure remained significantly higher 90 minutes after the stress test.

However, participants who drank the high-flavanol cocoa saw smaller declines in endothelial function than those who consumed the low-flavanol cocoa.

In addition, high-flavanol cocoa increased blood flow during and after the test compared with low-flavanol cocoa.

Other cardiovascular and blood pressure measures were similar for both conditions.

Future investigations

The researchers note that the main limitation of their study was that it excluded females.

They report that, for the sake of consistency, they wanted to avoid the impact of hormonal fluctuations during the female menstrual cycle on the vascular changes under investigation.

This means that it is not possible to apply the outcomes of this study to females. Researchers remain unclear on the impact of flavanols on stress response in females.

However, existing evidence shows there are gender differences in how a body responds to stress. So, future studies should focus on females.

The researchers also write that a more in-depth investigation would include molecular measures of cardiovascular function. These should consist of nitric oxide, which is a signaling molecule that dilates blood vessels, and the stress hormone cortisol.

Finally, they write that future research should explore the effect of flavanol intake on stress responses in individuals at higher risk of cardiovascular diseases, such as high blood pressure, and people who experience mental stress, such as care workers.

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