Does vitamin b12 increase body weight or fat?

No

No. Vitamin B12 is one of the essential nutrients and you need to take about 10 micrograms per day to stay healthy. Taking more will not make health better, nor will it make you fat. For healthy diet: http://www.choosemyplate.gov/., exercise 30 minutes/day, drink enough water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs.