I have always thought and told parents that whenever "solids" are introduced to baby that intent isn't for the nutrition at first.  Too many parents think they have to get "3 square meals" in the kiddo from the start.  The idea of breast milk being "insufficient" only reinforces this.  Instead, I like to think of the first weeks of solids as saying to the baby,  "Guess what!  food comes in other flavors and textures than mommy's milk."  The baby is learning this and exploring the new foods rather than concentrating on getting just the right amount and combination of foods while breast milk is still the primary source.  Some take to it right away while others take some time to accept and like this new source of nutrition.,  By changing the expression from "4-6 months" to "around the middle of the first year" we should be making the whole process more relaxed without a lot of rules.  
Of course the time comes when babies need more that what they are getting in mom's milk.  It is NOT because the milk is inadequate, but because the baby's stores of some nutrients is reduced and their needs for some nutrients is increasing.  I have seen some babies that do just fine without anything but mom until closer to one year of age (usually because of concerns about strong family history of allergies) and I have seen a couple cases where earlier introduction of solids was warranted.  (One was the 7th baby of a mom who had nursed her first 6 "a la La Leche League" with no problems but this one wasn't gaining.  As soon as she added banana at about 3.5 months, the baby started gaining again and continued to nurse well into toddlerhood.)
WATCH THE BABY, NOT THE CALENDAR!!!!

Winnie

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