Q. Are store-bought pre-ground flaxseeds as nutritionally effective as buying whole seeds and grinding yourself?

A. “Probably not,” says Barry Swanson, Ph.D., a fellow at the Institute of Food Technologists and professor of Food Science and Human Nutrition at Washington State University. (We went straight to a food-tech pro for this answer.) While pre-ground flaxseeds offer similar amounts of protein, total fat, fiber, minerals and vitamins as those you buy whole and grind yourself, they likely contain lower levels of heart-healthy polyunsaturated fats, including omega-3 fatty acids and conjugated linoleic acid (CLA), says Swanson. Pre-ground flaxseed has been exposed to oxygen longer; oxygen causes polyunsaturated fats to break down. (So do heat and light.) Tell-tale sign that this has occurred: an off smell and/or flavor you might call “rancid.”

To retain the polyunsaturated fatty acids, store your flaxseed—whole or ground—in an airtight container in the fridge or freezer. If you do buy pre-ground, be sure your storage container is dark—and not made of metal, which contains minerals that contribute to fat breakdown. Best bet: Buy whole. Grind only what you need. Relish the freshness. (Note: Whole flaxseeds are very hard and will pass through your body undigested if you don’t grind them before sprinkling them on your cereal or salad. A coffee grinder or small food processor does a fabulous job of “freeing” the nutrients of your flax so that you can absorb them.)