There are raw-food proponents who argue that cooking a food destroys all the nutritional value. This is not true. To lightly steam or saute a food does destroy about 10% of the nutrition, but that remaining 90% is then unlocked and available. Whatever you put in your stomach that is raw and cold, you have to heat and cook. This steals your energy and slows your metabolism. We get all of our energy from our digestive system; we don't want to waste too much of that energy in cooking our food. The less efficient our digestion is, the more food our bodies will ask for.
Some people assert that we want to have inefficient digestion. That by giving ourselves harder-to-digest foods, then that will make our digestive systems work harder and go faster. That's like arguing that "I want to make my car go faster, so I'll put a 2000 pound trailer on the back of it". The digestive system is not a muscle, it is a system. The harder we make it for it to do its job, the slower it will work.
We all know people who are trying to lose weight, who eat a big salad every day, and are not losing weight. This is because salad is too difficult to digest. It dampens the digestive fire. In China they have virtually no raw food and do not suffer from nutritional deficiencies as a result. Thew few raw-food vegans that I know are some of the most unhealthy-looking people I know. They are gaunt and pale. They are not getting enough nutrition out of their food.
Calories and energy are separate concepts. We want to get all the energy out of the food so we can be active and animated enough to burn all the calories. But as I have written in previous posts, calories really don't matter anyway. Caloric science is flawed.
Cooking has been a part of every recorded culture. It was not long after people harnessed fire that we figured out to use it on food. This is because we used to pay attention to how foods affected us. And we noticed that things digest better when then are cooked. This lightens the load on our digestive system and then we can just serve as the filters - send the good stuff to the tissues, the bad stuff to the tissue paper.
Remember, balance and moderation are the keys to good health. So I am not recommending that you never have anything cold or raw. Just that the majority of your food should be slightly cooked. However you change your diet or lifestyle, you should do slow slowely and gently. That's how it'll be sustainable. Read more about healthy eating in my book "The Asian Diet: Simple secrets for eating right, losing weight, and being well."