The liver then breaks down or metabolizes aspartame to its toxic components - phenylalanine, aspartic acid and methanol. This process requires a lot of energy from the liver which means there will be less energy remaining in the liver cells. This means the liver cells will have less energy for fat burning and metabolism, which will result in fat storing. Excess fat may build up inside the liver cells causing "fatty liver" and when this starts to occur it is extremely difficult to lose weight. In my vast experience any time that you overload the liver you will increase the tendency to gain weight easily.
Aspartame also causes weight gain by other mechanisms ---Causes unstable blood sugar levels, which increases the appetite and causes cravings for sweets/sugar. Thus it is particularly toxic for those with diabetes or epilepsy. Causes fluid retention giving the body a puffy and bloated appearance. This makes people look fatter than they are and increases cellulite.