I’m taking out my Merriam-Webster Concise Dictionary of English Usage - The Essentials of Clear Expression, and here’s what it says:
In the Saturday Review of 17 Mar. 1979 Thomas H. Middleton wrote, "The sad truth is that healthy has so often been used to mean ‘healthful’ that any dictionary worth its flyleaf just has to list ‘healthful’ as one of healthy’s meanings." This and a notice in a column by James J. Kikpatrick (28 June 1985) attest to the flourishing condition of the old issue of the dictinction between healthful and healthy. Here is the gist of the prescription: healthful means "conducive to health" (it does) and healthy means "enjoying or evincing health" (it does), and never the twain shall change places. The trouble is that healthy is used for the sense of healthful just given. How long has this sloppy confusion of distinct words been going on? Since the middle of the 16th century - in other words, for more than four hundred years.
The distinction itself was invented by Alfred Ayres only in 1881. It has certainly been repeated many times since, right up into the 1990s. But it should surprise no one that the distinction has often not been observed: there never was a distinction in the first place.
Burchfield 1996 notes that healthful has little use in British English.
Healthy, since its introduction in the middle of the 16th century, has been used much more than healthful in both senses. So if you observe the distinction between healthful and healthy you are absolutely correct, and in the minority. If you ignore the distinction you are absolutely correct, and in the majority.
Here are some examples of each word in its main acceptations:
... they demand more food and beer than the natives consider either decent or healthful - Anthony Burgess, Saturday Rev., 22 July 1978
He felt incapapable of looking into the girl’s pretty, healthful face - Saul Bellow, Herzog, 1964
... would almost certainly result in a healthy marine back to normal duty within a week - Thomas C. Butler, Johns Hopkins Mag., Summer 1971
... to achieve more genteel or healthier living habits - J. M. Richards, Times Literary Supp., 27 Nov. 1981
... even when they are poorer and eat a less healthy diet - Jim Hold, N. Y. Times Bookd Rev., 16 Apr. 2000
Bernstein 1965, 1971 disapproves healthy meaning "considerable", thinking it slangy. You can see from these examples that it is not:
Count Frontenac, who had a healthy respect for the Iroquois - Samuel eliot Morison, Oxford History of the American People, 1965
Signed manuscripts by living novelists generally drew the healthiest prices - Richard R. Lingeman, N. Y. Times Book Rev., 21 May 1978
Brazilian dude
And they lived healthily and healthfully ever after.
