Well, I have this consensus paper in German. The paper is very breastfeeding friendly, mentions the WHO recommendation, and then discusses why this paper departs from the WHO rec, (European babies aren't in danger of starving) and goes on to discuss why solids should be starrted between 4-6 months. Among other reasons, it is mentioned that waiting 6 months to introduce solids does not prevent allergies. And introducing glutens early reduces the risk for certain diseases like Diabetes mellitus Type 1 and even reduces the risk of developing celiac by 50% as long as the baby is still nursing. Really wish this paper was translated into Englisch. It's a good read.
Haha that's funny, my SIL presented me with an article about this research recently (we are also in Germany), and I sort of got annoyed and snippy, because she was coming at me saying "Yeah PJ, this says breastmilk is not enough after 3 or 4 months and you should start with purees now, etc". I was annoyed because I like to believe, quite firmly, that exclusively BFing is the very best for all babies for the first six months, and sometimes beyond if the baby hesitates in starting solids right at six months. I voted for the third option, because it's what we've done. And I agree with Cecelia's Mama that you can find research to support ANY opinion these days (I mean, the CIO folks have their research too). We are also doing BLW so I didn't even want to get into it with SIL about why we're skipping purees because this research is not compatible with that.
Aaaanyway, I don't think it's going to cause any major harm to give a baby under 6 months an apple to gnaw on. I didn't do it, but I try not to be too uptight about this stuff and once my hubby gave DS some bread at 5 months and he of course put it right in his mouth...and this was before we started solids and our plan is to give only very limited grains at first....but I didn't really mind because, well, ya gotta choose your battles so to speak and this just didn't strike me as such a big deal.