Of course people think that they know this question.  I certainly thought so.  I would eat just Tuna and rice and run five miles a day.  I would lose weight to an extent but it was hard to keep such a life style going for long.  I started putting on extra weight when I was 26 or so.  I was not sure why.  I was even confused by it and thought that maybe my frame was getting bigger!  It just kind of sneaks up on you. What happened?  I ate what I considered healthy.  I did not eat fattening foods and never ate fast food, drank soda, or ate chips.    I never had anything close to a sweet tooth so what happened?   Well, it turns out that modern science is completely wrong when it comes nutrition.  What people are told is that fat makes you fat.  Only one problem, the science was never really done on this.  In fact before the 1970s people it was common knowledge that starchy foods and sugar made people fat.   This was well understood by not only the general public but to health professionals as well.   Then everything began to change and the all of health science was flipped on its head for the worst.