Lithium can definitely cause weight gain. I gained about 30 pounds in a little less than a year when I first was diagnosed and started on Depakote. I have since discontinued it but I was still too overweight. I went to 1 session with a nutritionist which was affordable and not a real committment and I am now 50 pounds lighter with strictly dietary changes. I am on 6 meds right now, one of them being Lithium 1200mg and I have not gained any weight. If I gain or lose a little bit I know how to adjust my diet one way or another to maintain where I'm at. Weight gain is such a problem and it does suck to have to choose between mental and physical health, but you're on the right track with the personal trainer. Maybe the gym where you belong has a nutritionist you could meet with, or maybe you can find one in the community. Remember calories in must be less than calories out to lose weight. Good luck with this... it's such a hassle but you can do it!!!