"Garlic can slash the risk of cancer, heart disease and type 2 diabetes," is the premature claim made in the Mail Online.
Researchers looked at a selection of existing studies about the chemical compounds in garlic, and how these might interact with chemical signals between cells in the body.
They were particularly interested in the fact that certain compounds can release certain gases, such as hydrogen sulphide, into the body's tissue.
They wanted to investigate whether these compounds and associated gases had an impact on health.
They mostly focused on studies that explored these biological questions in a laboratory setting, rather than in a real-world situation involving humans.
As such, they could only speculate as to whether the effects observed could lead to definite changes in people's long-term health.
Although the article is an interesting introduction to the topic, on its own it doesn't help us draw firm conclusions about the health benefits of eating garlic.
Where did the story come from?
The story was based on an article by researchers from the University of Nottingham, Macau University of Science and Technology, and the National University of Singapore.
It was published in the peer-reviewed journal Trends in Pharmacological Sciences.
This journal does not publish new research. Instead it invites researchers to write summaries of the most relevant recent literature and suggest what future research is needed on topics of interest.
The authors did not receive any specific funding for their article.
The UK media somewhat overstated the importance of this research.
The Independent failed to clearly point out that this was not a new piece of research, but rather a discussion of existing research.
The Mail Online was more balanced, noting that the article reviewed existing literature and there are a lot of unknowns.
The Mail's headline, however, was rather overoptimistic in its claims about the benefits of garlic, saying scientists have concluded that, "Garlic can slash the risk of cancer, heart disease and type 2 diabetes".
What kind of research was this?
This was a narrative review, where researchers aimed to highlight the latest research looking at how garlic might have an impact on human health.
They looked at a selection of previous studies on this subject and summarised the findings.
Importantly, they did not aim to carry out a systematic review on the health effects of garlic.
In a systematic review, researchers aim to give a definitive answer to a question by methodically looking for everything on that question, assessing its quality, and analysing what answers the best-quality research gives them.
Narrative reviews can be more limited in the evidence that they discuss.
While they're interesting and helpful to read for an introduction to a topic, they're not intended to be reliable assessments of evidence on which to base health and medical decisions and advice.
What topics did the article cover?
The researchers note that garlic has been used by different populations for centuries for improving health.
They also say that more recently, research studies have found associations between eating garlic and other alliums, such as onions, and improved health outcomes.
For example, some studies have suggested that alliums could reduce risk of cancers of the digestive tract.
The authors did not go into these studies in detail, instead focusing on how garlic might have these effects.
They were particularly interested in compounds in garlic that contain sulphur, which may be released in the form of gases as garlic is broken down.
These compounds help to provide garlic with its flavour and smell, but might also act in useful ways in the body.
Cells in the body "talk" to each other using certain chemicals called signalling molecules. Some of these signalling molecules are gases such as oxygen or carbon dioxide.
The researchers were interested in whether garlic compounds can affect gaseous signalling molecules in the body.
They discussed whether this might be helpful in reducing inflammation, which may in turn reduce the risk of some health conditions where inflammation plays a role, such as heart disease.
The authors did not go into detail about whether it's possible to improve one's health by eating more garlic, or whether taking supplements is any better or worse than eating fresh garlic.
How did the researchers interpret the studies they discussed?
The researchers noted that the biology involved in breaking down the various compounds contained in garlic is more complicated than previously thought.
They discussed how, although we're starting to understand how things work at a chemical level, we do not yet understand what impact these processes have on people's health and illness.
They listed many unanswered questions, including how cooking affects the compounds in the garlic we eat, how much we would need to eat to cause actual changes in the body, and whether drugs containing garlic-derived compounds could be developed that deliver those compounds directly to specific tissues in the body.
What do other studies say about the effects of garlic?
Pinpointing or singling out the exact impact of individual foods on our health is difficult.
The evidence about the impacts of eating garlic specifically on risk of cancer or other diseases is not yet conclusive.
In the past, researchers have carried out various systematic reviews of the evidence on the health effects of garlic.
For example, in 2000 a good-quality systematic review concluded that trials of garlic supplements had "promising, but modest and short‐term effects" on outcomes such as cholesterol levels, and reducing some aspects of blood clotting.
But whether garlic reduced important related outcomes such as stroke had not been shown.
A limited amount of data was available looking at whether eating garlic affected cancer risk.
The evidence did suggest that a high intake of garlic was associated with a reduced risk of some cancers, such as stomach and bowel cancers.
But these studies were mostly case-control studies, which may not be reliable, as they require people to remember how much garlic they ate in the past.
There have been other systematic reviews on the effects of garlic, but conclusive evidence of an effect on outcomes such as cancer, stroke or heart attacks still appears to be lacking.
Garlic may well have health benefits, but this research does not provide us with conclusive evidence.
Ideally, an up-to-date systematic review would be needed to summarise the most recent research.
This would give us a better idea of what all the current research says, and whether there are any areas where more studies are needed.
What we do know is that eating more fruit and vegetables does reduce our risk of cancers, such as bowel cancer.
As such, garlic and other members of the allium family can form part of a varied and healthy balanced diet.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
Mail Online, 27 April 2018
The Independent, 27 April 2018
Links to the science
Trends in Pharmacological Studies. Published online April 26 2018