The answer is that among other nutrients they contain lignans. So what are Lignans? Well, plant lignans are phytoestrogens (mimic some of the effects of estrogen), which include lariciresinol, matairesinol, pinoresinol, and secoisolariciresinol.
Diets rich in lignans, especially flaxseeds, are associated with reduced risk for cardiovascular disease. In addition, there are other nutrients in foods rich in lignans that may also contribute to their beneficial effects.
The richest sources of lignans are flaxseeds and sesame seeds. Other good sources of lignans are pumpkin and sunflower seeds, berries, vegetables, whole grains, and bran.