Is eating vegetables in the Allium family (garlic, onion, leeks, spring onions, garlic stalks) protective in regards to colon cancer? A recent study from China suggests that eating higher amounts of these vegetables is associated with a lower incidence of colorectal cancer in both men and women. Interestingly, the researchers only looked at these 5 vegetables, which are commonly eaten in China, but not other Allium vegetables that are commonly eaten elsewhere in the world - such as chives, scallions, and shallots. All Allium vegetables are rich in flavonols and organosulfur compounds, which have properties that inhibit tumors (anti-tumor) in laboratory studies. High intake of Allium vegetables is thought to be protective for a variety of cancers, e.g. prostate cancer.
The researchers mention that other studies examining this issue had mixed results - with some finding a protective effect of Allium vegetables, but not others. The researchers suggested that the high intake of these vegetables in the groups they studied and also cooking methods (which vary among different regions of China, as well as different countries) could explain the differences. For example, slicing and crushing fresh garlic releases beneficial compounds, but boiling onions leads to an approximately 30% loss of beneficial substances. After reviewing a number of studies that looked at Allium vegetable intake and cancer, it appears that while eating them cooked in any way is good, the most beneficial effects seem to be from raw Allium vegetables.
How much of the Allium vegetables did they eat? The healthy (non-colorectal cancer) group ate about 2 ounces or 1/4 cup of Allium vegetables per day (or 47 pounds annually), versus the colorectal cancer group ate about 1.5 ounces per day (or 15.92 kg or 35 lbs annually). Other differences between the groups were that the colorectal cancer group had higher intakes of alcohol and red meat, but less milk, other vegetables, and fruit (as compared to the healthy group). But both groups had the same intake of fiber. Bottom line: eat a diet rich in fruits and vegetables, including Allium vegetables (onions, garlic, leeks, spring onions, chives, scallions, shallots).