Georgina: Very seriously, long-term malnourished mothers may find their milk is marginally short of iron, and this might mean their babies might need to start iron-containing solids a little sooner than six months, according to the research. But these are (often) very young mothers in places like the slums of Mexico City (where one paper was done) and they begin their pregnancies severely anaemic, the anaemia is untreated, so the babies are born with smaller iron stores. It's a judgement call, too, as stopping breastfeeding means the mothers' periods return sooner and this means a greater risk of anaemia....
This is emphatically not a problem in the West.
There is no evidence at all - and it has been looked for - that breastmilk risks being short of 'nutrients' and treating breastmilk supposed inadequacies with formula supplementation is just daft.
Sometimes, the quantity of breastmilk is insufficient, often as a result of factors surrounding the birth, or the early days afterwards, or misunderstandings about normal feediing patterns etc etc. In severe cases, a baby might need formula rather than the problem persisting untreated as it could get worse...and the baby meanwhile needs feeding somehow.