Some of the other answers here have been causing controversy for not addressing the question. In keeping with the StackExchange principles, this is an attempt to combine the best parts to make one full answer. I thank @jwenting, @manojlds, @joe and @Sklivvz for much of the preliminary research.
The OP asks several different questions, which I will paraphrase as:
- Is there justification for the common saying?
- Are apples healthier than other fruit?
- Is eating a lot of apples risky?
Let's look at each one of those:
Q: Is there justification for the common saying?
A: It depends whether you speak English or Old English.
The phrase is thought to come from Old English, where apple used to mean any round fruit.
Phrase Finder attribute the origins of the phrase to a Welsh book from 1866, and note:
Apples may be good for us but it
wasn't their precise medicinal
properties that were being exalted
when this phrase was coined. In Old
English the word apple was used to
describe any round fruit that grew on
Etymonline has a similar view of the original meaning of apple:
O.E. æppel "apple, any kind of fruit; fruit in general," from P.Gmc. *ap(a)laz (cf. O.S., O.Fris., Du. appel, O.N. eple, O.H.G. apful, Ger. Apfel), from PIE *ab(e)l "apple" (cf. Gaul. avallo "fruit;" O.Ir. ubull, Lith. obuolys, O.C.S. jabloko "apple"), but the exact relation and original sense of these is uncertain (cf. melon). In M.E. and as late as 17c., it was a generic term for all fruit other than berries but including nuts (e.g. O.E. fingeræppla “dates,” lit. “finger-apples;” M.E. appel of paradis "banana," c.1400).
If you believe this etymology (and the understanding that fruit, in general, is considered healthful, you may decide you do not care whether the (modern) apple is special.
- there are other proposed etymologies for the saying,
- the dates (M.E and late 17th versus 1866) don't exactly line up,
- people still say the expression today,
- there does seem to be some early belief that apple apples (i.e. Malus domestica) did have some health benefits (see below)
so let's continue.
Q: Are apples healthier than other fruit?
A: It is impossible to make a blanket statement, but in some respects, yes.
Some features of the apples are special. No doubt, advocates of the other fruits could make alternative (and correct) health claims about their fruits as well.
The efficacy of a diet of raw apples in the treatment of diarrheal conditions in children has long been known in many parts of Germany by the populace. Occasionally brief mention of the value of raw apple has been made by medical authorities. It was first used systematically in the Kindersanatorium at Koenigfield by Sister Frieda Klimsch. Within the last three years this form of therapy has been advocated a great deal in Germany by the physicians, and several articles have appeared in the literature. All have reported favorable results.
(Alas, I haven't read enough to know whether this report actually agrees with earlier ones!)
In mice, it has been associated with staving off dementia. (Ref:
Chan A, Graves V, Shea TB, A (Aug 2006). "Apple juice concentrate maintains acetylcholine levels following dietary compromise". Journal of Alzheimer's Disease 9 (3): 287–291. ISSN 1387-2877. PMID 16914839.)
In rats, it may help with cholesterol. (Ref)
Apple juice is less harsh on tooth enamel than orange juice.
If you are thinking this evidence is pretty weak at putting apples on a pedestal above other fruit, I agree with you. Get some other fruit and veg into you, too.
Q: Is eating a lot of apples risky?
A: Yes, it can be.
The same study that rates apple juice better than OJ & cola at not stripping enamel, rates diet cola even higher. (Water, having a higher pH, will rate even better, I suspect.)
An apple is 10% sugar (Ref), which should be considered on a calorie-controlled diet. (You are talking about eating lots of them.)
Some people are allergic to apples. (Presumably, this applies to small as well as large quantities.)
Also consider what other essential foods you will be missing out on if you eat too much fruit, in general.