Sugar. For me, at least, it seems to have a significant bearing on my weight.
Early in 2015 I started going to the gym. I went 5 days a week for an hour a day for almost 6 months and lost around 8 lbs. So not only did I not enjoy going to the gym, but I wasn't getting the desired weight loss. My trainer at the gym had suggested that sugar may be a part of the problem, but I didn't give it much thought as I didn't think I was taking in too much sugar. Once my job changed I ended up not having the motivation to keep going to the gym.