Friday February 15 2008
A second glass is damaging?
One glass of red wine can be good for you, but a second may not, reports the Daily Mail. The first drink relaxes the blood vessels and “reduces the amount of work the heart has to do”, the newspaper says, but the second “countered any health benefits – increasing the risk of high blood pressure, coronary artery disease and heart failure”.
The story is based on a small study that looked at the direct effects of red wine, plain alcohol, and water on people, using complex measurements of the heart, blood vessels and nervous system. This is the latest addition to the continuing debate of what may be a safe or even optimal amount of alcohol to drink for health. Red wine in particular has repeatedly been hailed as a protection against heart disease.
Although this study found a difference in the results when one drink was given compared with two, it is difficult to see how the experiment relates to real-life drinking patterns, as the trial was conducted in only 13 people, and each of the drinks given on only one occasion. Studies that examined the effects of different alcohol doses over longer periods in more people might produce useful results. At the moment, it seems sensible to follow standard recommendations for limits to alcohol consumption.
Where did the story come from?
Dr Jonas Spaak and colleagues of University of Toronto and Liquor Control Board of Ontario, Canada, carried out this research. The study was supported by Operating Grants from the Heart and Stroke Foundation of Ontario and the Canadian Institutes of Health Research. It was published in the (peer-reviewed): American Journal Physiology of Heart.
What kind of scientific study was this?
This was a small randomised crossover trial designed to investigate the potential benefits of alcohol consumption on blood circulation, blood vessel function and its related nervous system supply, and whether these effects are dose-dependent (how they change with the quantity consumed) or affected by whether red wine or diluted pure alcohol is consumed.
The researchers selected a group of 13 healthy non-Asians (seven men and six women), who normally drank moderate quantities of alcohol. On three separate occasions during the trial period each of the participants were randomly assigned to drink either water, red wine or ethanol (diluted pure alcohol). During each of the test sessions, the volunteers were seated and linked to an ECG (heart monitor), blood pressure machine. They had an electrode placed in one of the nerves of the leg, and a hand-held ultrasound machine was used to detect heart output and blood flow in the arm in response to changes in pressure change of the blood pressure cuff. Blood was also drawn from the arm for measurements of chemical levels in the blood.
After the first set of measurements were taken, the volunteer drank the alcohol, wine or placebo over five minutes. The measurements were repeated when the peak blood alcohol had reached a target level (using a breathalyser). The measurements were repeated for a third time after the second drink had been given and the peak blood alcohol had reached a higher target level. At the end of the test, a urine sample was taken. There was a two-week period between each of the three test sessions.
A named brand of red wine was used which is known to have high levels of resveratrol and catechin – the chemicals believed to have heart-protective and anti-oxidant properties. For the other alcoholic drink, 95% ethanol was diluted with Perrier water to an equivalent concentration. Perrier water was used as the control.
What were the results of the study?
The circulatory output of the heart fell slightly after water and after one drink of red wine or ethanol, but increased after two drinks of either alcohol compared with water. Also, one drink did not alter sympathetic nerve activity (nerve impulses that are not consciously controlled, e.g. the fight or flight response), but two drinks of either alcohol significantly increased nerve activity compared with water.
There was significant increase in the diameter of the main artery in the arm after both one and two drinks of alcohol compared with water. There was no effect of any of the drinks upon the artery diameter (after conducting a test to see how much the artery would dilate when increasing the pressure in the blood pressure cuff to a level that would cause the arm to go red). Ethanol and red wine had no effects overall on either blood pressure of heart rate (although two glasses of wine slightly raised heart rate). Red wine significantly raised blood levels of resveratrol and catechin.
What interpretations did the researchers draw from these results?
The authors conclude that one alcoholic drink (either red wine or ethanol) causes blood vessel dilation without an increase in heart rate or sympathetic nerve activity. When compared with water, two drinks increases heart circulatory output, sympathetic nerve activity, and heart rate (red wine only), with no increase in blood pressure. Despite wine causing higher blood levels of resveratrol and catechin – the chemicals believed to have heart-protective and anti-oxidant properties – no differences in measurements were found compared with plain alcohol.
What does the NHS Knowledge Service make of this study?
This is a complex and well-conducted scientific experiment. However, there are several points to note when interpreting the results:
- This is a small study of only 13 volunteers who received each drink once only. It is unclear how this experimental situation relates to any real-life drinking pattern. Studies examining the effects of different alcohol doses over a longer period of time (e.g. one glass per day compared with two) and in a much greater number of people would be valuable.
- Overall, there were no differences found in the effects of two glasses of wine compared with two glasses of plain alcohol; therefore the findings are not limited to wine drinking as the newspaper headlines may imply.
- Importantly, there is nothing to suggest from this study that “a second glass is bad for the heart”. It is unclear how the small changes in sympathetic nerve activity, blood vessel diameter and heart output after two glasses of wine or alcohol on a single tested occasion in this study would relate to health.
- The size and alcohol content of the “drink” given in this study may not be comparable to the real life situation. The researchers calculated the alcohol concentration and volume that they believed would be needed to reach the target blood level in a particular individual. The earlier part of the report mentions the American Heart Association Nutrition Committee defining one drink as 120ml glass of wine or a 44ml shot of spirit; however, in the study different quantities of alcohol were taken by each of the volunteers depending on bodyweight.
- The second drink of alcohol was consumed after the breathalyser test had fallen to a defined alcohol level. However, it is not possible to say for certain whether any affects on the heart, blood vessels, or nervous system from the first drink may have continued after this time and could be clouding the effects of the second drink. Likewise, the effects of two drinks were not followed over an extended period.
- Although the study is reported as being single-blind (i.e. either the investigators or the participants did not know which of the study drinks were given) no further details of this are given. Presumably, the investigators would have been unaware of the drinks, as the participants would be able to tell which of the drinks they were consuming. However, even if this were the case, as they were checking for a rise in blood alcohol levels it would seem they would be able to tell when water had been taken. This may have led to some bias in result reporting; although as all measurements were objective this is uncertain.
- This trial is only conducted in non-Asian populations and findings may not be transferable to these groups.
Many questions have been raised by this research and further study would be needed to try and provide some of the answers. At the current time, it seems sensible to follow standard recommendations for alcohol consumption.