Why should vegetables not be overcooked?

Many vegetables are best eaten raw. Some need to be cooked to be digested better (a potato for example). Some of the nutritional value of vegetables will be lost by cooking, vitamin C, for instance is destroyed by heat, so the longer you cook, especially at high temperatures, the less vitamin C remains. Eating overcooked food of any sort, especially when burnt, is not good for you. The DNA or protein in foods must be broken down by your digestive system to be absorbed, so in normal cooking of foods the change that happens with DNA and protein typically wont effect its nutritional value.