I have heard that when you toast whole grain bread, you lose most of the nutrition. I'm wondering if this is true, and if so, do you lose the nutrients when toasting nuts?
Toasting only effects the thin outside layer, and most nutrients cannot be modified by the low heat used in typical domestic cooking, so the amount much "lost" (modified) should be very small
As with many foods, cooking actually increases the availability of some nutrients to the human body's digestive system, and improves the flavour so you will eat the food. This is probably more important then the small loss, if any, from toasting
One of the worst cooking nutrition losses is water soluble vitamins when cooking them in hot water, and the water is then not returned into the finished dish/meal. For general vegetables you would expect a 25% loss of vitamin C with 5 minutes in hot water. Not great, but not a disaster