When we talk about what to eat, we must first realize who, or rather what, is eating. In fact, we, ourselves, are not really doing the eating. It is our cells that eat. When we put food in our mouth, that is just a continuation of the transport of food from the farms to the grocery store then into our mouth; the food is then transported to our cells by our bloodstream. It is our cells that really do the eating and that need the fuel and the parts to regenerate themselves. And cells can only eat two kinds of food for fuel. They can eat sugar or they can eat fat, and their health and your health will be determined by the primary fuel that they burn.
I have been asked to summarize in a single sentence what would best promote health. It is this: Health and lifespan is determined by the proportion of fat versus sugar people burn throughout their lifetime. The more fat that one burns as fuel, the healthier the person will be, and the more likely they will live a long time. The more sugar a person burns, the more disease ridden and the shorter a lifespan a person is likely to have.
Becoming a Fat Burner
How does one learn to, and how does one become a fat burner? How does one change their primary fuel from sugar to fat? One gets good at most anything by doing it frequently. You can become a good tennis player by playing tennis frequently and a good golf player by golfing frequently. Likewise, your body becomes adapted to burning fat by burning fat frequently. However, most people become very adapted at burning sugar; your body continues to want to "keep playing" sugar, to burn more sugar, even when you are not eating. When you're sleeping at night, your body then prefers to burn sugar and it gets that sugar by breaking down proteins in your body, which means lean body mass, which includes muscle and bone. I call that metabolic momentum.
Your body continues to like to do what it has become accustomed to doing. If you have burned sugar throughout the day, you prefer to burn sugar at night even when you are not eating. Your body does not store very much sugar and prefers to hold onto much of it and, therefore, you'll continue to manufacture sugar by a process called gluconeogenesis from lean body mass. You store fat -- and, in many people, lots of it -- in your "cupboard" and not very much sugar, because fat is the fuel that your body would prefer to store and later to burn to stay healthy.
However, when you eat sugar and fat together, your body will burn sugar first. I believe that it burns the sugar off because that is one way to get rid of it. Sugar causes damage by glycosylation and having it around too long is extremely damaging and accelerates aging. Therefore, your body might get rid of sugar to minimize the damage caused by keeping it around. You'll have to burn off almost all the sugar that you eat before you can start burning fat and, in most cases, that means that the fat you have eaten with sugar gets stored. Your body continues to become adapted to burning sugar and not fat. People get fat not so much because they eat fat, but because they have forgotten how to burn it, and because of poor hormonal communication.