Of course six months is a 'construct'.....no one is saying that all babies wake up on the day they are six months and need something other than breastmilk. But from the evidence we have from all over the world, plus the knowledge we have of how the gut works, and matures, somewhere around the middle of the first year is when babies start to need other tastes, other textures and other nutrients.
It's not really calories they need, as breastmilk would easily continue to provide all of the necessary calories to grow, as long as they had enough of it, frequently enough. This is why we don't say 'oh, breastmilk isn't enough' when a baby of, say, six weeks starts to feed more often than he was doing the previous week because we know that simply feeding more often will do the trick. In fact, though, sometimes this is when formula is advised because there is little trust in breastmilk to do the job of feeding and growing. It is the same later on. Babies, generally speaking, are fine, calorie wise, on breastmilk for a long time. But somewhere around 6 mths, generally speaking, they will start to benefit from other foods. There are exceptions to this, of course, but as a general policy, mothers should be enabled to breastfeed exclusively for 6 mths if this is what they wish to do, and there is a strong public health argument for this being a national policy.
Blistering - you see one study on teething you think does not marry with your opinion, and that is why you disregard all evidence that in general, babies are healthier and better off without solids until about 6 mths .
It is not wishy washy to point out, as I did, the lack of studies showing benefits of solids earlier than this - a public health policy supporting solids earlier than about 6 mths is an intervention to the physiological norm, and any intervention should be shown to have benefits. And in this case, there is nothing to show that it does have benefits, and a consierable body of evidence that it does harm.