If you cook food, does that change the macronutrient values in the food?

For example, if I am counting calories, and I use 1 serving of olive oil and cook lets say 1 serving of onions, is it safe just to add the nutrition values up, or does the chemical reaction with the heat has any significant effect? If yes, is it for the better or worse? (For someone on a fat loss diet)

I used onions as an example because they taste sweeter than raw, so I wonder what happens to the carb values when the onions are caramelized.

If anyone knows the answer or a related article I would appreciate it.

Thanks,

this may help

http://forums.johnstonefitness.com/showthread.php?t=394

cheers