I agree with lcatennessee: excess calories make you fat.  This whole debate about whether fats or carbs make you fat is pretty silly to me- I'm a cell biologist studying to take my MCAT right now for med school and just covered the chapter on digestion… let me share!  Put simply- everything breaks down into their original building blocks through a process called digestion. 

The enzymatic breakdown of carbs/starch/sugar starts in your mouth (ever redipped a spoon that was in your mouth back into a yogurt container and waited a day– notice it was all watery and broken down?) and continues through your stomach and, for the most part, ends in your small intestine.  They are all broken down into simple sugars (monosaccharides) which diffuse into your bloodstream as those sugars or glucose.  Eventually they are all converted to glucose.   

Proteins are broken down starting in your stomach.  They are broken down to polypedtide fragments and then eventially into individual amino acids (which is how they enter the blood)! 

Fats/lipids are broken down in the small intestine with the help of bile!  Bile is like soap- it's wate-rsoluble on one end and water-insoluble on the other… this forms a "micelle" or bubble around the fat, allowing it to be emulsifie…