Is your salad healthy or healthful?
This Google Ngram chart compares the phrases healthy diet and healthful diet in published books, and you’ll see that this battle was lost around 1940 and about 35 years ago healthy diet took a dominant lead over healthful diet and just kept widening the gap.
Today, although The Chicago Manual of Style and the AP Stylebook do make a nod to that old traditional thinking, they both say that it is fine to use healthy to describe something that is good for you.
It is fine to use healthy to describe something that is good for you.
If you want to feign an “old-timey” air or you’re writing a novel set in the 1800s, you might want to use healthful though. For example, here’s a line from The Simpsons in which Homer is pretending to be Sir Walter Raleigh trying to court Queen Elizabeth. He said, “Your Majesty, I come from the New World with a gift: a healthful and slimming herb, tobacco.”
Here’s a line from the 1997 video game The Curse of Monkey Island: “Oh gosh, yes! It's a very healthful drink! Even better for you than placing leeches on your tongue.”
If you want to sound even more archaic, you can use healthsome, which people started using a few years before healthy—around 1530—and is now labeled “rare” and “obscure” in the Oxford English Dictionary. Some words win and some words lose. Some were born to sing the blues.*
You can get more tips like this in my book 101 Troublesome Words. It’s the last book I wrote, and I saved all the tricky questions—like this one—for last.
*I couldn't resist the Journey reference.
Cooke, M.C. Social Life, Or, The Manners and Customs of Polite Society. Matthews-Northrup Company. Buffalo, NY. 1896. http://j.mp/1FgrWHt (Accessed May 10, 2015).
Patterson, C. Advanced Grammar and Elements of Rhetoric, Book 2. Sheldon & Company, New York. 1887. http://j.mp/1zTPaR5 (Accessed May 10, 2015).
Phelps, A. and Frink, H.A. Rhetoric: Its Theory and Practice. English Style and Public Discourse. Charles Scribner's Sons, New York. 1895. http://j.mp/1E2xPRm (Accessed May 10, 2015).
“Ask the Editor.” AP Stylebook, online edition. http://www.apstylebook.com/online/?do=ask_editor&id=11757 (Subscription required. Accessed May 10, 2015).
“healthy, adj.” Oxford English Dictionary, online edition. (Subscription required. Accessed May 10, 2015)
“healthy; healthful.” The Chicago Manual of Style, 16th edition, online. 5.220 http://chicagomanualofstyle.org. (Subscription required. Accessed May 10, 2015).
“healthy, adj.” Dictionary.com. http://dictionary.reference.com/browse/healthy?s=t (Accessed May 10, 2015).
“healthy.” American Heritage Dictionary, online edition. https://www.ahdictionary.com/word/search.html?q=healthy&submit.x=48&submit.y=17 (Accessed May 10, 2015).