Garlic, onions, leeks, chives, and shallots are classed as allium vegetables.
They are grown throughout much of the world and form the bedrock of family meals far and wide.
Earlier studies have shown that certain compounds in allium vegetables — including flavanols and organosulfur compounds — are bioactive.
Some have been shown to hinder the development of cancer.
Scientists from the First Hospital of China Medical University recently set out to understand whether consuming greater amounts of these vegetables might prevent people from developing colorectal cancer. They recently published their results in the Asia-Pacific Journal of Clinical Oncology.
Not counting skin cancers, colorectal cancer, also called bowel cancer, is the third most common cancer diagnosed in men and women in the United States.
Researchers are aware of certain dietary risk factors, such as consuming high levels of red or processed meats. However, they know less about foods that might protect against bowel cancer.
Unpeeling the onion
Scientists have already investigated whether allium vegetables could reduce bowel cancer risk. Although some have identified a significant effect, others have found either a small interaction or none at all.
The authors of the latest study believe that the variation in results is partly due to how data was collected. For instance, some studies combined all allium vegetables into one group for analysis, and others did not include data from some, less common, types of allium vegetable.
With this in mind, the researchers designed a study that would more faithfully capture the impact of allium vegetables on colorectal cancer risk.
To investigate, they matched 833 individuals with colorectal cancer with 833 control participants without it, who were similar in age and sex and who lived in similar locations.
Each participant was interviewed, and their dietary habits were recorded using a validated food frequency questionnaire.
The benefits of allium vegetables
The researchers found that, as theorized, there was a significant relationship between the level of allium vegetables that an individual consumes and their risk of colorectal cancer.
Specifically, in adults who consumed the highest levels of allium vegetables, the risk of developing colorectal cancer was 79 percent lower than those who consumed the lowest levels.
“It is worth noting that in our research there seems to be a trend: the greater the amount of allium vegetables, the better the protection.”
Senior author Dr. Zhi Li
The inverse relationship was seen in the overall consumption of allium vegetables, as well as in specific types, namely garlic, garlic stalks, leeks, onions, and spring onions.
The correlation was also significant in both men and women. This is interesting because, in some earlier studies, differences were seen between the sexes. For instance, one study found a weakly protective effect in women and a slight increase in colorectal cancer risk in men.
As previously mentioned, past research into allium vegetables and colorectal cancer risk produced conflicting results. However, evidence in favor of the relationship is now growing.
For instance, a study with Southern European participants found “an inverse association between the frequency of use of allium vegetables and the risk of several common cancers.”
Similarly, a meta-analysis assessed links between allium vegetable intake and the presence of adenomatous polyps, which are precursors of colorectal cancer.
The authors concluded that “[h]igh intake of total allium vegetables may be associated with a risk reduction of colorectal adenomatous polyps.”
A simple dietary change
Dr. Li believes that this group of vegetables provides a simple lifestyle change that could help reduce the risk of colorectal cancer.
Of course, consuming these vegetables alone will not reduce risk in a meaningful way, but used in conjunction with other dietary changes, they might make a difference.
The debate is likely to continue for some time; there is a dizzying array of reasons why allium-rich diets may seem (or not seem) to impact cancer risk. For example, how a vegetable is cooked can significantly alter its chemical composition.
This might help explain why studies conducted among different global populations had differing results; future studies would do well to take this into account.
Conclusions cannot yet be drawn, but, if the results are replicated, adding extra onions and garlic to our dishes may be a tasty way to reduce colorectal cancer risk.
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