"An apple a day keeps the doctor away", the saying goes, implying that apples are somehow especially good for the immune system.
But does this idiom have any validity to it? Are apples high in certain nutrients that are key to immune function? Do they have a high nutrient-to-calorie ratio (which I assume is good for immune function?) Does their high fiber content contribute to this effect?
Or, are they really no better for the immune system than any other fruit/vegetable/nutritious food?
There are a number of things wrong with the general idea that apples are healthy. [1]
One of those is that "health" is not a one-dimensional thing, and nutrients are not always needed. If you have a balanced diet you won't have any shortages and an apple will just be a tasty bite of calories and fibers, keeping you feeling full for a reasonable amount of time, and most of the nutrients it contains will be excreted or metabolized.
If you're short on some vitamins or minerals that are in an apple, eating an apple might be good for you. However, not all fruits are created equal, and "nutrients" is are not a single entity. If you are low on potassium you should eat a banana, if you're low on vitamin C you should eat a bell pepper.
Another thing is that obesity is the #1 food-related health risk for almost all people. [2] While an apple is more healthy than the same amount of sugar in a candy bar, fruits contain more calories than vegetables (in general), so for most people it would be better to eat some vegetables than some fruit.
Apples have more vitamin-C than, say, pears. So if it is correct that vitamin-C is "good for the immune system", it would be better than a pear, for instance. Vitamin-C is not the only thing said to be needed by the immune system, though. Frequently zinc and iron are mentioned. Some berries have lots of iron and vitamin-C. does vitamin-c really boost your immune system?
Though, the quote you gave is very old (?) and I doubt the "immune system" was a concept when that saying originated. The immune system is not the only part of the body which needs nutrition.
Apples are probably one of the most widely known and available types of fruit, hence the choice for that old saying you quoted. It is common knowledge that they used to have barrels with apples on ships hundreds of years ago, to prevent the crews from getting scurvy.
Their nutrient vs. calorie ratio is certainly better, in average, than what the e.g. victorian poor folk apparently ate: bread, butter, potatoes, beer, and tea. (Potatoes contain more of certain minerals, though). what the poor ate
Btw.: Apples, and some berries, too, are rather aggressive on the teeth due to their high acid content, though. I wouldn't eat them all day - as a matter of fact, I used to, and it did harm my enamel. It was copious amounts over some years, though. That famous "five a day" suggestion would not come near what I ate.
