What I found is that it's not necessarily the fact that you started losing weight,maybe it's just that you are growing up. I also loved going out and having wild parties with friends, but since about last year when I turned 28 I just don't feel like doing that anymore. I married very young (22) and have 2 babies . I used to long for the days of partying,but since last year I actually prefer to stay at home and have quiet time with my family. I don't think your friendships are doomed and on it's way to ending, you and your friends will find each other again,seeing that you came a long way.