A: We have no clear evidence that flaxseed or any other particular food or nutrient can prevent a disease all on its own. But there's reason to believe that regularly eating flaxseed can potentially help lower the risk of colon cancer—and a number of other diseases.
The seed is a highly concentrated source of cancer-fighting, heart-healthy omega-3 fatty acids; a single tablespoon contains more than 2 grams. But your body converts only a small amount of flaxseed's omega-3 fats (called ALAs—they're also found in certain vegetable oils, walnuts, and some green vegetables, like spinach and kale) into the kind found in fish. And it's mostly the fish-based omega-3s that, in studies, have been shown to deliver health benefits. However, recent findings suggest that ALAs may work similarly. For example, ALAs seem to ease inflammation in the same way that fish oils can. That's important because chronic inflammation in organs or joints, for example, has been linked to the development of cancer.
In addition to ALAs, flaxseed delivers a concentrated dose of fiber; a tablespoon has nearly 3 grams. Because fiber adds indigestible bulk to your diet, it can help you lose weight by making you feel full with fewer calories; research also shows that fiber can lower cholesterol. The studies on fiber's ability to ward off colon cancer are mixed, but I find the evidence of a benefit persuasive. In countries where people typically eat a high-fiber diet, colon cancer is much less common.
Finally, flaxseed contains generous amounts of a number of disease-fighting nutrients: potassium, magnesium, antioxidants, and even plant compounds that mimic estrogen and may help protect against cancer.
To get the full benefit of flaxseed, grind it just before you use it. People typically put it on cereal, as you note, but you can also sprinkle flaxseed on salads or bake it into bread or muffins.