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It's a common enough concern. However, much higher temperatures are needed to break down the nutrients in food. Generally temperatures in excess of 600 degrees would be needed at sustained levels to cause complete breakdown of nutrients.
Naturally, if you cooked everything you ate it probably wouldn't be a good idea. Which is why we have so many raw food movements. Fruits are for the most part consumed raw and many vegetables are consumed raw as well.
May I still help you with this question? Please let me know if you have any other questions.