I was extrememly active (martial arts, skating, lifting) until about a year ago, when I started feeling run down and tired all the time. I chalked it up to "getting older" and my weight steadily creeped up from 135 to 150.
While I'm not "fat" the extra weight did bother me, because my clothes started fitting tighter and I just didn't feel good. I t to the doctor, who did some tests, and I found out I am allergic to a bunch of foods- wheat, soy, corn, peanuts, and to a lesser extent milk. And since all those ingredients are in every day food (who knew there was corn in Rice Krispies?) I was basicaly gorging myself on allergens, which makes one's systems very tired. So I eliminated all that stuff from my diet and felt a little better, but still tired, and the weight stayed the same.
More tests. Now I've found I have hypothyroidism, which shed light on many things. I've started taking replacement thyroid hormones but haven't gotten to the point where my thyroid functions correctly yet. It's a slow process.
I read a book called "The Mood Cure" and it suggested I try "tyrosene", which is an amino acid. I've only been on it a week and already I feel a difference. I don't feel like napping at work, and my appetite has decreased slightly. I am hoping that when I've taken it for a little while my energy will be back up to my previous levels and I will want to hit the gym more often.
So the moral of this story is: if you previously were active and are now feeling sluggish, I would recommend asking your doctor for a thyroid function blood test. Hypothyroidism is quite common but often undiagnosed.