There are some foods (primarily fruits and vegetables) that lose vitamins and fiber when they’re cooked…especially boiled. Roasting, grilling and sauteeing at least concentrate flavors if not vitamins. All cooking methods will break down fiber. Spinach is an exception…I seem to remember an article that stated cooking spinach releases certain vitamins not as readily available when consumed raw.
When we cook most fruit and vegetables we destroy a lot of the beneficial nutrients they contain as well as the flavor. This includes grilling, boiling, frying and roasting.
Some vegetables are healthier cooked than raw as it releases the vitamins and minerals they contain and makes them easier to digest. It may also destroy harmful chemicals some vegetables contain. Foods to cook include squash, tomatoes, carrots, asparagus, artichokes, red bell peppers and mushrooms. Spinach and Kale are one of those leafy greens which offer different levels of nutrition when eaten cooked or raw. Never eat raw beans. They are toxic.
Generally, the healthiest way to cook most vegetables is to lightly steam them as they are cooked at a minimum temperature. Carrots and tomatoes release more antioxidants when cooked with a healthy fat (eg. coconut oil) Olive oil is nutritious when not heated.
Avoid the use of a microwave oven to cook or reheat food. Apart from destroying valuable nutrients, it changes the molecular structure of the food (affecting absorption) and radiates your body when you turn the microwave oven on. This has been shown to cause a multitude of health problems you may not be aware of.