I heard a snippet of this Science Friday program about how exercise can change your DNA and I had to go download the podcast. I listened to it two or three times because it went by so quickly that I wanted to make sure I understood it.
The gist, in non-scientific terms: Our DNA can become littered with tags that change the way it is read by our cells. Scientists found that unexercised muscle cells had tags that kept them from metabolizing fat and sugar. After strenuous exercise (35 minutes at 80% of maximum effort), the research subjects' muscles showed that these tags had been removed, which would improve fat and sugar metabolism. Exercise at lower intensities did not seem to have the same effect. Interestingly, the tags are even more pronounced in people with insulin resistance associated with Type II diabetes, which would suggest that regular strenuous exercise might blunt some of the disease's effects. You can read the whole research paper if you like, but the Science Friday segment summed it up nicely.
This would suggest that if you are struggling with weight loss, you might want to decrease the duration of exercise and really ratchet up the intensity. This may explain why strength training is so effective for weight loss even if it doesn't burn a lot of calories -- it might be more effective in making the changes to DNA expression if it is strenuous enough. It might also explain why once you reach a certain level of fitness it seems easier to get even fitter -- you are more able to reach the level of exertion necessary to make DNA changes. It also explains why I didn't have so much luck losing weight when I was training for a half marathon -- I couldn't run at a high enough effort level to increase fat and sugar metabolism. Spin class is an especially effective exercise mode for me, because I can really crank out the effort on the bike and work myself hard for 45 minutes.
It's actually time for me to get changed for Spin class. I have DNA to change.