Overcooking vegetables is one of the easiest ways to lose nutrients. Heat-sensitive vitamins like vitamin C, folate, and B vitamins degrade when exposed to high temperatures for too long. For example, broccoli and spinach lose their vitamin C content when boiled for too long. To retain the most nutrients, opt for steaming, microwaving, or quick stir-frying, as these methods help preserve the vitamins and minerals found in vegetables.