Ok, this is how it goes, as far as I understand it. I may not have all the facts 100% accurate but you get the gist.
100 years ago, it was normal for babies to be breastfed, either by their mothers or by wet nurses. Solids were introduced at 6-9 months and bfing continued into toddlerhood.
Around 50-70 years ago, childrearing became more 'medicalised' and it was believed that formula (early formula was very different to what we have today) was superior to breastmilk, and women were encouraged to bottle feed.
There was also a social element to it, breastfeeding was for poor people who couldn't afford formula.
The idea of 4 hourly feeding routines etc also became common around this time, which also meant that breastfeeding was often unsuccessful, as the breasts weren't stimulated frequently enough to produce enough milk.
The formula available then wasn't as nutritious as it was today, so babies began to lose weight.
To counteract this, babies were given 'solids' earlier. But 3-4 month old babies can't manage solid food, so purees were used, as young babies could swallow these.
In the mid 70's (? - maybe a bit later, can't remember), research showed that early introduction to solids could cause allergies and digestive problems so the DoH introduced recommendations that solids should be introduced between 4-6 months and not before.
More research was done, and in the early 2000's, the World Health Organisation recommended that babies had no nutritional need for solid food before 6 months, and the DoH in this country adopted this guideline.
Research has also shown that babies still get the bulk of their nutrition from milk until they're one year old. Food is a secondary source of nutrition until then, so while it is important that it is introduced, it's nutritional value to the baby is limited.
The Baby Led weaning movement has followed on from these guideline when people realised that if you wean babies at 6 months rather than 4, they are capable of putting food into their mouths, and learning to chew and swallow it, without the need for purees.
Habits and culture takes rather longer to change, despite the research and guidelines having been around for several years now, and many people are uncomfortable with the thought of of 6,7,8 month old babies eating relatively small amounts of solids as they explore food and learn at their own pace.
The advertising from baby food manufacturers, plus the sometimes unrealistic expectations of sleep patterns of babies of this age, ensure that some people will continue to feel more comfortable feeding babies larger quantities of pureed food, even though it's not nutritionally necessary.
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