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

“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.”