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.
When you’re buying roasted nuts, look for those that are stored in cans or otherwise protected from light and air, and smell them to be sure they’re not rancid before you eat them or add them to other foods.
I buy mostly raw, unsalted nuts and store them in the refrigerator until I need them. I prefer to eat raw nuts, but I know some people who find roasted ones easier to digest. You can roast your own by stirring raw nuts in a dry skillet over medium heat or spreading them on a baking sheet placed in a 350 degree oven and tossing them occasionally until they are done to your liking. Use them up quickly.
In addition to their healthy fat profile, nuts provide you with vitamin E, trace minerals, fiber, and in the case of walnuts, alpha-linolenic acid (ALA), a fatty acid that is similar to the heart-healthy omega-3 fatty acids found in fish. (Remember that peanuts are legumes, not nuts, and have a less desirable fatty acid profile.)
Brazil nuts are a good source of selenium, and one ounce of pistachios contains more fiber than a half-cup of spinach. These nuts also are good sources of vitamin B-6, thiamin, copper, phosphorus, and magnesium.
Despite their beneficial nutritional profile, bear in mind that nuts are relatively high in calories, so enjoy them in moderation. I usually eat a handful per day - my favorites are cashews, almonds and walnuts.
Andrew Weil, M.D.