Why might fish oil supplements be having this effect?
Fish oil is naturally anti-inflammatory, and some research suggests that prolonged inflammation may increase the risk of cells becoming cancerous.
"Anti-inflammatory supplements reduce the events within a cell that lead to inflammation," White says. "Specifically, by reducing inflammation, they reduce cell turnover."
Reducing this turnover is important becuase the more cells replicate, the higher the chance is that they will accumulate genetic errors -- a precursor to a cell becoming cancerous.
Thus, it is possible that reducing inflammation can lower the risk of cells becoming cancerous, White notes, though this is not yet proven.
How you get your fish oil may be an important element of the risk-reduction equation as well.
The fact that past studies failed to find a connection between dietary fish oils and breast cancer prevention may have been an issue of dose, Brenna says.
Taking fish oil in concentrated supplement form could offer a higher, more regular dose of the beneficial compounds in the oil, DHA and EPA.
"Most American diets provide only small amounts [of EPA and DHA]. It may well be that the supplements are supplying them to women whose diets are not naturally adequate in omega-3."