Sam Feltham is a personal trainer who likes to do experiments. On himself.
On a series of experiments he decided to overeat first fat, then carbs and then a vegan diet to see what would happen. Each experiment lasted three weeks, during which time he carefully monitored his calories in, his weight and his body fat.
As far as the physics goes and many people believe, a calorie is a calorie is a calorie. If so, the weight gain should be pretty uniform over each of the diets. Yet, we only start to obey the laws of physics once we die. Up till then we follow the laws of biochemistry. And the results are very different depending on where those calories come from.
Click on this link to see what happened: http://live.smashthefat.com/category/self-experiment-conclusions/