Sir: Jerome Burne offers a catalogue of evidence to show that humans are designed to breast-feed for some years ("Should we breast-feed till they're six?" 7 March). He does not, however, include compelling data about human milk itself.
Some animals are born almost mature - for instance, the guinea pig. Its mother's milk is high in protein and designed to last only a few days. Of all mammals, the human infant is the least mature at birth and human milk is correspondingly low in protein.
Dairy scientists consider humans to be long-lactating animals, a theory which is borne out by the feeding spans of traditional peoples around the world. Not every mother will wish to sustain breast-feeding until her child is in school uniform. But the often fraught transition to solids might be a lot more relaxed if mothers weren't under such pressure to wean.