More apples, less lung cancer
The most common type of fruit in supermarkets reduces the risk of lung cancer, and may reduce the risk of colon and breast cancer as well. According to a meta-study that Italian researchers at the University of Perugia published in Public Health Nutrition in 2016, apples may protect against some forms of cancer.Study

The researchers collected previously published epidemiological studies in which researchers determined the effect of eating apples on various forms of cancer. This included both case-control studies and cohort studies.In case-control studies, researchers often compare a group with a specific condition to a control group, while in cohort studies, researchers track a group of study participants over time. Case-control studies are known to frequently report relationships that do not appear to exist in subsequent studies.
Results
The strongest association the researchers found was between eating apples and lung cancer. In both the case-control studies and the cohort studies, the study participants with the highest intake of apples were less likely to develop lung cancer than the study participants with the lowest intake of apples.

In each study used, the highest intake was different. But if you eat an apple a day, according to most studies you were in that category.
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The above results are in accordance with the results of animal studies, in which quercetin and other phenols in apples tend to accumulate in the lungs.
The researchers also found associations between the consumption of apples on the one hand and the risk of breast cancer [[Figure] and colorectal cancer [Figure] on the other. These associations were significant in the case control studies, not in the cohort studies.
Conclusion
"The present meta-analysis indicates that consumption of apples is associated with a reduced risk of cancer in different anatomical sites", summarize the Italians.

Source:
Public Health Nutr. 2016 Oct;19(14):2603-17.