I think how you metabolize carbs has a lot to do with it. When I was 19, I got up to 240 pounds and decided to do something about it. I actually did a pretty good job sticking to a low carb diet for a period of 8 or 9 months. When I got under 200 pounds, I started jogging for the first time in my life. I couldn't jog a mile on the first day. I kept it up and went a little farther and got a little faster every day. Eventually, I became a real runner. I felt compelled to do it every day. I loved it. Continuing to eat low carb/high protein - I eventually dropped down to 169 pounds, was < 10% body fat, and ran 6 to 8 miles most every day. I did a 1/2 marathon a couple of times just to see I if I could do it. I could. It was not a problem.

Anyway - goal achieved, I did what most people do and went back to eating the Standard American Diet. I thought I could maintain my weight by keeping junk food out of my apartment and by continuing a daily jogging routine. It worked for a few years (while I was still in college) but it was not as easy and I thought it would be. I'd put 10 pounds back on and have to go LC again to get them off. I did that a couple of times. I wish I knew then what I know now. I don't metabolize carbs very well. When I was doing LC, I loved to run. When I was eating carbs, I had to force myself to run. I ran less often and shorter distances. I thought it was just lack of will. Now I know it was a result of elevated insulin from all the pizza, burgers, fries and soda that I was consuming.

Fast forward 30 years and 200+ pounds... I'm feeling the same thing now. Keeping my blood sugar low (and insulin low) I am compelled to get my butt moving and I have. In fact, every time I've done a low carb diet for any length of time, I end up exercising again. When I go back to eating carbs, I stop. When my metabolism is in balance, I have energy to burn and enjoy doing it. I'm not running 1/2 marathons -- but I am working out almost everyday. How I eat makes all the difference.

I met a skinny runner back in the old days and he is still skinny and still running regularly. Even before he started running, he could eat a large pizza and a pitcher of coke at every meal and never gain an ounce. He tried protein powder, lifting weights, eating health food, you name it. He just could not gain weight. If he ate a lot, he was totally hyper. He just had to start running to do something with all that energy. I'm friends with him on Facebook -- and I see he still runs 1/2 marathons and 10Ks every chance he gets. His FB posts are all about running. He metabolizes carbs very well. For him, carbs = energy. For me, carbs = high BG, high insulin, fat storage, and my brain thinks I'm starving. We are polar opposites the way I see it. But I can be more like him if I consistently eat in a way that keeps my insulin down. He couldn't be like me if he tried.

Interesting point. Those of us hanging around here just have too little sympathy for hopelessly skinny people.

My first husband was the Shrimp all through high school. Grew to 6'2 in the Army, and weight-lifted his way to a he-man upper body. He always had those skinny legs though, rather like a barrel on stilts. We were Jack And Mrs. Sprat, sort of.

Ken, I never thought about low-carb eating as actually promoting the desire to exercise, but I experience both energy and results when I do both. Maybe there's a link for me too.