Fruits, vegetables, grains and other foods that come from plants contain fiber, the part of a plant that you can't digest. Fiber plays an important role in your health by relieving constipation, reducing the chances of other digestive problems and improving cardiovascular health. Cooking foods that have fiber softens the fiber, but doesn't destroy it.

Plants contain cellulose and pectin, two types of fiber. When you cook plants, the heat softens these fibers. A cooked carrot is softer than a raw carrot, which is crunchy. The longer you cook food, the more its fiber is broken down and the softer it becomes. For example, cooking lentils or beans for too long makes them mushy.

Softening the fiber in fruits, vegetables and grains means that your body gets slightly less fiber, but this is often beneficial. For example, it's easier to chew fruits and vegetables that are firm and tender instead of crunchy. Some people, such as the elderly or people with dental problems, may not be able to eat very crunchy or stringy fruits and veggies. In addition, many foods taste better when they've been cooked. Finally, cooking may mitigate the digestive problems some people experience from eating fiber, such as bloating and gas.

Avoid fiber loss by cooking most fruits, vegetables and grains until they are tender and firm, not mushy. Mushy food has lost too much of its cellulose and pectin, and it's usually not as appetizing, either. However, some vegetables, such as squash, are best eaten soft. If you're cooking grains, such as quinoa or rice, don't use too much water, as pouring off extra water removes fiber and nutrients. Eat raw fruits and vegetables to get the most fiber.

Don't add alkaline ingredients, such as baking soda, to dishes; alkalis turn fruits and vegetables to mush and destroy nutrients. Acids make the fibers in foods firmer, so you may need to extend the cooking time if you add vinegar, lemon juice, tomato sauce or other acidic ingredients to a dish.