I'd use "healthier" in most cases. I suppose one could say, for instance, that something is "more healthy than it is unhealthy."

#0: BNC finds some solutions for 'more healthy'

BNC = British National Corpus?

There has been a steady increase in comparatives using "more", especially in two-syllabled words. This has been going on (and commented on) for years. I would not be surprised to see either form in the case of "healthy", and they mean the same.

We had a discussion on this a while back, but no idea what the thread name was. In any case, I agree with escoville. My daughter recently got back a class test in English in which the teacher marked several things like this wrong even though native speakers mix and match all the time. Had to tell her to try to remember the "rules" she learns in class and not just write what came naturally to her :-S

There may even have been more than one past discussion; I think it might have been Amy-MiMi who commented on it, but perhaps others as well. I just had a quick search but didn't find anything.

I hadn't noticed the trend until whoever it was brought it up, but now I hear unnecessary two-word comparatives more and more, even in the news, and it grates on me. My sense is that teachers are right to encourage students to use regular comparatives and superlatives with -er and -est whenever possible; I only wish people would teach that now to native speakers as well.

Thank you all for shedding some light on the subject!
But I guess we all have to adapt to developing languages, no matter if it's English or German.
And yes, BNC = British National Corpus

OT: No, actually we all have the freedom, and, indeed, the responsibility, to resist so-called 'developments' that we consider undesirable, whether on grammatical or stylistic grounds. Not every change is for the better.

#7
But we can also welcome those situations where two possibilities exist, rather than trying to suppress one of them.

Having two different forms can allow the development of subtle differences in meaning and permit choices. We may, for instance want to use a parallel construction with more and less or two mores, or the prosody of one form may be better ("Thou art more lovely and more temperate"), etc.

Gibt es da evtl. einen Bedeutungsunterschied? Healthy im Sinne von "bei guter Gesundheit", und im Sinne von "gut für die Gesundheit"? M.W. ist nur das erstere korrekt, aber das zweite gerade bei Essen zunehmend gebräuchlich...setzt sich der "neue" Plural evtl. in diesem Sinne durch?

I wish I could find the old threads too. I suspect they make the point I wish to make which is that there are some combinations of more+ a one or two-syllable adjective that are easier on the ears than the grammatically correct form.

A couple of examples are "terse" and "complete". Terser and completer sound odd IMO. Googling suggests they are more commonly used in the more+ form.

That may be, in a few cases, but surely that's not the case for the vast majority, including 'healthy'? What's wrong with 'healthier'?

In fact, could it be partly just that our ears aren't accustomed to the comparative, because there are some adjectives that tend to appear less frequently in comparisons, even though the form is perfectly okay? 'Sicker,' for example, may not sound particularly elegant, but surely 'more sick' sounds even worse?

I'm not bothered by the occasional deliberate variation, like the Shakespeare one MikeE cites or other ones that aid poetry or parallelism or whatever. But far too many instances seem to be just people who somehow aren't actually thinking when they're talking or writing, or at least not thinking more than one word ahead. I have the impression that they may think only as far as 'more ...,' and then they decide a split second later which word they're actually going to use in the comparison. But how hard can it be to think of the word before you say it?

One of the things the teacher marked wrong was actually my daughter's "go slower": she said it had to be "more slowly". I think the other one was similar.

Here's one of the threads: related discussion: more free?
I made the point that in cases like "easy to use" ("healthy to eat?") people sometimes seem to see the whole phrase as being compared, not just the first word, so make it "more [easy to use]".

There was another more detailed thread, though, wasn't there?

CM2DD:
"go slower" is another matter. That's an adverb. We can argue about that too, but it's a different argument.

Personally, I agree with hm -- us: (in principle) I don't like these "more" forms. But I find myself using them.