Apples are known to promote health as they contain vitamin C, which aids the immune system, and phenols, which reduce cholesterol. Now, this idiom has been known to send out the message that if you eat apples (or any fruit), it will keep you healthy. According to several studies have found an association between apple consumption and reduced risk of cancer, especially lung cancer. This could explain literally why this line has been so powerful: https://www.pritikin.com/your-health/healthy-living/eating-right/829-an-apple-a-day-really-does-keep-the-doctor-away.html