In the end its low calories BUT the amount of energy that it takes to metabolise fats means that carbs are way ahead (9cals/gm for fat and 3.5-4.0 cals/gm for carbs). BUT the satiety factor of fats is important. That means by adding a bit of fat to your diet you can feel not hungry. On very low fat diets you feel hungry all the time. Plus you can't absorb enough fat soluble vitamins. Plus you have to remember that many of your hormone are made from essentially fat molecules and that some of those hormones are paradoxically fat burning hormones. So, in the end the answer is a boring balanced diet! :-) Try not to go under 20% fats by calories. Others will say 30%. Find what works for you. In the end everyone is different and you need to fnd what works for you. Good luck.