No proof red wine makes you slim or is good for your gut
"Lose weight and still drink wine! It's 'good for your gut and keeps you slim'," reports the Sun.
Like many "too good to be true" headlines, the story is more complicated than that.
Researchers looked at the self-reported drinking habits of 916 female twins in the UK, and cross-checked their findings in similar groups from the US and Belgium.
They also assessed the micro-organisms, such as bacteria, living in the women's guts. A more diverse population of micro-organisms in the gut has been linked to better gut health.
They found women who drank red wine had more diverse gut micro-organisms.
They also noted that women who drank red wine tended to have a lower body mass index (BMI), which their analyses suggested might be related to the effect on gut micro-organisms.
The researchers speculated that chemicals called polyphenols in red wine may create conditions that favour more diverse gut micro-organisms.
But because of the type of study this is, we cannot say whether red wine was the cause of either gut diversity or a lower BMI.
Other factors may have been involved, such as women's overall lifestyle. The researchers did try to adjust for the impact of some factors, but it's difficult to remove them completely.
Drinking red wine is not recommended as a way to lose weight. And this study also did not prove that doing so can help with your gut health.
As the researchers made clear, any potential beneficial effects on red wine intake could possibly be achieved by drinking just 1 glass of red wine every 2 weeks.
Regularly drinking more than the recommended limits (14 units of alcohol a week) can put you at a risk of a range of long-term health conditions, such as liver disease and cancer.
Find out more about the risks of drinking too much alcohol
Where did the story come from?
The researchers who carried out the study were from King's College London and the Rega Institute for Medical Research in Belgium.
The study was published in the peer-reviewed journal Gastroenterology, although at the time of writing it had not been made available on their website.
We reviewed a version made available to the media, which may differ from the final published version, and did not include supplementary materials.
Predictably, some of the UK media jumped on the link between red wine and weight.
Despite the fact the study does not show that red wine causes a lower BMI, the headlines state that red wine "helps drinkers avoid obesity" (Mail Online), "is healthier for gut and figure" (The Times) or "can help you stay slim" (the Sun).
BBC News' reporting was more balanced.
What kind of research was this?
This was a cross-sectional study involving women who'd previously been involved in twin research.
Twins are often recruited for studies that aim to assess the relative impact of genetics and environment on people's health.
Cross-sectional studies give a snapshot in time of different factors, such as alcohol consumption, gut micro-organism composition and BMI.
But they do not show how these may change over time, or how they're related.
What did the research involve?
The researchers analysed information on women who'd been involved with 3 separate twin studies previously.
In these studies, the women reported their alcohol consumption in food frequency questionnaires.
They stated how many glasses they drank in an average month of:
- beer or cider
- red wine
- white wine
- spirits
- any alcohol (total)
Researchers calculated the biodiversity of the women's population of gut micro-organisms, presumably from stool samples, although this information was not reported in the main study paper.
The researchers looked at whether consumption of different types of alcohol was related to gut micro-organism diversity.
They adjusted the figures to take account of women's:
- age
- BMI
- healthy eating (an index derived from answers to the food frequency questionnaires)
- education
- family structure
They also looked at whether gut micro-organism diversity might be influencing the relationship between alcohol consumption and women's BMI, blood fasting glucose, insulin, total cholesterol, LDL ("bad") and HDL ("good") cholesterol.
The researchers initially looked at data for 916 women from the UK.
For analyses where they found a link, they then repeated them in 2 other groups of women from the US (904 women) and Belgium (1,104 women).
What were the basic results?
Drinking red wine was linked to gut micro-organism diversity, with more frequent consumption linked to greater diversity.
Even those who rarely drank red wine appeared to have greater diversity of gut bacteria.
Red wine drinkers had more diversity of gut bacteria than those who did not drink red wine.
This was seen in all 3 groups of women (those from the UK, the US and Belgium).
White wine (which contains lower levels of chemicals called polyphenols) was also linked to gut micro-organism diversity, but less strongly than red wine.
Other types of alcohol were not linked to gut micro-organism diversity.
But the researchers said "frequency of red wine drinking accounted for only a modest proportion" of diversity between individuals.
The link was mainly seen for 3 types of micro-organism.
The researchers said drinking red wine was linked to lower BMI, and statistical tests suggested this could partly be explained by gut micro-organism diversity in the UK and US groups, but not the Belgian group.
But these figures were not included in the version of the paper we received, so we cannot comment on them in detail.
For example, we do not know how much difference there was between the BMI of women who drank red wine compared with those who did not.
How did the researchers interpret the results?
The researchers said that, "We demonstrated that red wine consumption is positively associated" with gut micro-organism diversity, and that "our results suggest that even rare consumption may be sufficient" to increase diversity.
They added: "We also showed that this may contribute to some but not all of the highly debated health benefits of moderate red wine consumption, such as improvement of cholesterol metabolism or reduced adiposity [body fat]."
Conclusion
When a headline sounds too good to be true, it usually is. There's nothing in this study to suggest that people should start drinking red wine to lose weight.
The potential effect of diet on the micro-organisms in the gut is a new and interesting field of science.
This study provides new evidence about a possible effect of substances found in red wine on the growth of micro-organisms in the gut, and suggests that this may affect the way the body works.
But the study has several limitations. Because it's cross-sectional, it shows us only a snapshot in time.
We do not know how the women's gut micro-organisms, BMI or red wine consumption changed over time.
This means we cannot say whether 1 of these factors may have been directly influencing the other.
Because it was an observational study, we do not know whether red wine was the cause of differences in BMI or gut micro-organisms.
Other factors may have been involved, such as women's overall lifestyles.
The researchers did try to adjust for the impact of some factors, but it's difficult to remove them completely.
Also, the study relied on women's reports of how much alcohol they drank. People often underestimate how much alcohol they drink.
We know there's a big downside to drinking alcohol, especially in excess. There's no "safe" level of alcohol consumption, but drinking less than 14 units of alcohol a week is considered low risk.
Regularly drinking more than this increases the risk of several types of cancer, stroke, heart disease, liver disease, brain damage and nervous system problems.
For people who enjoy an occasional glass of red wine and drink less than 14 units a week, this study suggests they may have more diverse gut flora.
But there's no reason to start drinking red wine in the hope of achieving improved gut health or losing weight. The study does not provide enough evidence of this.
Find out more about alcohol units and how to drink within recommended limits
