In this recent study, Amitay et al. (JAMA Pediatr 2015: 169(6) ) conducted a meta-analysis of published studies. This means they carefully searched every published study that examined the association between breast feeding and leukaemia (which represents 30% of all childhood cancer) and analysed them to find the overall (average) result. They found 18 separate studies: these showed that compared with no breast feeding or shorter breast feeding, breast feeding for 6 months or more was associated with a 19% lower risk of leukaemia. This is a large study – in total 10292 leukaemia cases and 17517 control cases were examined. That’s quite a lot and far more than you could ever study in a single country. In a further analysis in just 15 studies they showed that ever breast fed, compared with never breast fed, was associated with a 11% reduction.
Observational studies, and meta-analyses (like this), can never prove direct causation and there are methodological flaws in every study. No study gives you the perfect correct answer. But the size of this reduction for such a serious condition is important - maybe prospective mothers need to know. http://archpedi.jamanetwork.com/journal.aspx