Why might onions, garlic lower cancer risk?

Study co-author Jo Freudenheim, Ph.D., chair of epidemiology and environmental health at UB, also comments on the uniqueness of the study, saying, "There is very little research on breast cancer in Puerto Rico."
"This study was a collaboration between my colleagues here at UB and at the University of Puerto Rico to help us understand why rates there are lower than in the rest of the U.S., and why rates there are continuing to increase, while they are decreasing in the rest of the [U.S.]."
Although the study was observational and cannot explain the mechanisms behind the findings, the researchers suspect that the flavonols and organosulfur compounds abundant in onions and garlic may be responsible for the anticancer effects.
In particular, the S-allylcysteine, diallyl disulfide, and diallyl sulfide in garlic and the alk(en)yl cysteine sulphoxides in onions have shown "anticarcinogenic properties in humans, as well as in experimental animal studies," says Dr. Lina Mu, an associate professor of epidemiology and environmental health at UB and the study's senior author.

Some study limitations

However, the authors also recognize some limitations to their research. For instance, they write, "The size of the study was small, particularly for analyses stratified on menopausal status."
Secondly, the number of people who had never consumed garlic or onion was too small to use as a reference group. Therefore, the low exposure group still consumed some onion and garlic.
For this reason, say the researchers, the results "may have underestimated the true association between garlic and onion intake and breast cancer."
"Furthermore, there is not a standardized recipe for sofrito, a condiment which is frequently homemade and therefore varies at least somewhat in recipe from person to person," add the authors. "Hence, we were also not able to estimate the amount of onion and garlic in sofrito."
"Although the recipe for sofrito varies to some extent, other ingredients, such as bell peppers, tomatoes, cilantro, and black pepper are usually added. Since we did not adjust our models for these ingredients, we cannot be sure that our results were due to the sole effect of onions and garlic."
sources:medicalnewstoday