Wine is essentially a grape juice in which fruit sugars have been fermented to alcohol. This results in a complex liquid containing a rich selection of antioxidant polyphenols derived mainly from the grape skins and pips. Red wine contains higher polyphenol levels than white wine, as its production involves leaving the juice in contact with grape skin pigments for longer, to soak up more of the red pigments.
On their own, antioxidants normally have protective effects on the circulation by improving cholesterol balance, promoting blood vessel dilation, and discouraging the formation of unwanted blood clots. So what difference does the presence of alcohol make?
In the short-term, alcohol has a relaxing effect that causes arteries and veins to dilate so blood pressure falls. This seems at odds with strong evidence that long-term alcohol consumption increases the risk of developing hypertension.
Blood pressure increases by approximately 1 mmHg for each 10g alcohol (1 drink) consumed per week, and drinking two or more alcoholic drinks per day increases the risk of developing hypertension by at least 16%. This effect is largely reversible within two to four weeks of abstinence.
Researchers are only just beginning to unravel why short-term intakes of alcohol lower blood pressure yet long-term intakes cause blood pressure to rise.
Red wine, alcohol and blood pressure in healthy males
One study that sheds light on the paradox involved 25 middle-aged men who were slightly overweight but otherwise healthy, with normal blood pressures, cholesterol and glucose levels. Each volunteer was asked to drink either half a bottle of red wine (375 ml supplying 41 g of alcohol), or 375ml de-alcoholized red wine, or 375ml water with a light meal (bagel with cream cheese) on three occasions, to compare the effects.
Their blood pressures were recorded over the following 24 hour periods with an ambulatory monitor. This showed their blood pressure fell by an average of 4.7/3.9 mmHg during the first four hours after drinking red wine, compared with the alcohol-free wine or water. During the following 24 hours after drinking red wine, their blood pressures were also lower, overall, by an average of 2.1/1.4 mmHg. However, during the last 4 hours, their systolic blood pressure (upper figure) increased significantly by 1.8 mmHg.
This initial fall and subsequent rise in blood pressure appeared to be linked with their levels of a powerful blood vessel constrictor called 20-HETE (20-hydroxyeicosatrienoic acid). Levels of 20-HETE fell in the 2 hours after drinking all the beverages (possibly a dilution effect) but was then relatively higher 24 hours after drinking red wine. As this substance causes arteries to constrict, it could help to explain why long-term drinking is associated with an elevated blood pressure.
As there were no significant changes in blood pressure, or 20-HETE levels, when they drank the de-alcoholised red wine or water, the researchers concluded that the effects of red wine were due to the alcohol rather than the polyphenols.
However, this study was in people with normal blood pressure control, and those with high blood pressure appear to respond differently.
Red wine and alcohol in men at risk of heart disease
Another study tested the blood pressure effects of red wine against de-alcoholized red wine and gin, in 67 older men (aged 55-75 years) who were at high cardiovascular risk due to having diabetes, or at least 3 heart disease risk factors such as hypertension, high cholesterol, smoking or obesity.
During 3 separate treatment periods of 4 weeks, each volunteer drank either red wine (272ml providing 30g alcohol) every day, or the equivalent amount of de-alcoholized red wine, or gin (100ml containing 30g alcohol).
On the day after each treatment period, their blood pressure was measured 3 times, at 5-minute intervals after sitting for 15 minutes at rest.
In this study, involving men at high risk of heart attack or stroke, the month of drinking de-alcoholized red wine lowered blood pressure by 5.8/2.3 mmHg, significantly more than the red wine (a fall of 2.3/1.0 mmHg) or the gin (no significant change).
In a few people, blood pressure readings went up, but the overall change was a reduction in blood pressure readings with the alcohol-free wine.
Blood tests showed these changes were associated with increases in a powerful blood vessel dilating substance called nitric oxide.
The researchers concluded that the daily consumption of de-alcoholized red wine could be useful for the prevention of low to moderate hypertension in men at risk of cardiovascular disease.
Although these blood pressure reductions may seem modest, data from one million adults shows that a decrease of 4/2 mm Hg in blood pressure is associated with a 14% reduction in coronary heart disease risk, and a 20% reduction in stroke risk, which are clinically meaningful benefits.
Red wine extracts and blood pressure
Red wine extracts containing concentrated grape polyphenols are also available which provide the antioxidant benefits without having to drink the wine. Some supplements contain as much resveratrol as 300 or more glasses of Chianti!
In ten studies assessing the effects of mixed grape polyphenols on blood pressure, five showed a significant reduction in systolic blood pressure, while five showed no significant changes. When all the results were analysed, the overall reduction remained significant, but small, with grape polyphenols lowering systolic blood pressure, on average, by 1.48 mmHg compared to placebo.
When supplements consist of concentrated resveratrol rather than mixed grape polyphenols, however, the effects are more pronounced. Six studies, involving 247 people with high blood pressure showed that taking at least 150 mg resveratrol per day lowered systolic blood pressure (upper reading) by an impressive 11.90 mmHg compared with placebo.
There were no significant changes in diastolic blood pressure (lower figure) and lower doses of resveratrol did not show a significant effect.
Should you drink red wine?
If you have high blood pressure, best advice is to drink no more than one small glass (125ml) of red wine daily. Different varieties of grape contain different levels of antioxidants, and the most concentrated polyphenols are found in red wines made from the Argentina malbec grape, the Italian sangiovese grape, and wine made from the tannat grape in the Madiran reagion, France. Typical values for resveratrol in red wine are as follows:
Richest resveratrol wines
Resveratrol per 100 mls
|Red wine made with Californian muscadine grapes||3 mg resveratrol|
|Red wine: Merlot||1.4 mg resveratrol|
|Red wine: Pinot Noir||1.2 mg resveratrol|
|Red wine: Cabernet Sauvignon||0.10 mg resveratrol|
|Rose wine||0.12 mg resveratrol|
|White wine||0.04 mg resveratrol|
If you want to obtain the benefits of red wine without the alcohol, then alcohol-free red wine appears to have a beneficial effect on blood pressure for those with hypertension. Alternatively, you could take a resveratrol supplement.
If your blood pressure is raised, self-monitoring is key to maintaining good control.
Click here for advice on choosing a blood pressure monitor to use at home.
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