I've have had an apple watch for about a week now and one of the things it does is track how many calories you use in the course of a day, it takes gender, age, height and weight and factors them into the equation. According to the data I burn far fewer calories than I had figured. I eat and diet figuring that I burn 2500+ calories a day, on days when I work both of my jobs. My watch tells me that I only burn about 1500 calories when I just work my full time job, about 2000 on days that I work both, my full time and part time job. So now I am going to have to re-evalute my diet and now hopefully with better data my losses and gains will be better.