How long do you need to breastfeed to get anti-cancer benefits?—Anonymous
The background on this question is that although many of the claimed benefits of breastfeeding are overstated (as I argue in much, much more detail in Cribsheet), one that does have support in the data is the link between breastfeeding and lower breast cancer risk in mom. The overall effects are fairly large — a 20% to 30% reduction in breast cancer risk — and there is a plausible mechanism (breastfeeding prompts cell changes that could contribute to the lower risk).
This question is the obvious follow-up: Does it matter how much you breastfeed? As usual, getting more detailed answers is harder. But to the extent that we can evaluate, it does seem like the reductions are larger for longer breastfeeding lengths. One review looks at this directly, comparing longer with shorter nursing durations and finding bigger reductions for the longer duration. A related set of evidence compares exclusive with partial breastfeeding — related, though not exactly the question above — and finds that the reductions are larger in magnitude for women who breastfeed exclusively, but the differences are not statistically robust.
The direct answer to this question, then, is that some benefits are probably delivered even by a relatively short duration, and longer durations deliver more.