No, nuts do not lose their heart-healthy monounsaturated fat during the roasting process. However, roasting may alter and damage the polyunsaturated fats that nuts also contain and that are more vulnerable to oxidation. Oxidized fats account for rancidity, giving nuts, and other foods, an “off” taste and a bad odor reminiscent of oil paint. Rancid oils are pro-inflammatory and carcinogenic. The more the surface of the nut is exposed to air, the more likely it will be to oxidize. Roasted, chopped, and ground nuts go rancid more quickly than whole raw ones.