That's the first law of thermodynamics, not the second (although you can't violate the second either). In general, through my life, I've found the best way to

That's the first law of thermodynamics, not the second (although you can't violate the second either). In general, through my life, I've found the best way to think about it is exercise for fitness, diet for weight maintenance. What happens to you once you have responsibilities to others (wife, kids, etc.) unless you are a professional-level athlete is that you really don't have the time and energy to burn enough calories. It's hard to burn 400 kcalories, but it's easy to not eat that piece of pie (or to have a small piece and no bread with your meal). There are exceptions, and most of us who are somewhat active have probably gone through that phase of life where we do 5000 kcal workouts 5 days a week and so on, but it's a very time-and-energy consuming proposition. I have known some men who have made it a point to continue that lifestyle into middle age, but they are exceptional and almost to a person never married (gay I think) or several times divorced. In other words, it takes total self absorption and those of us with normal family and community responsibilities have to let it go eventually. That's also the failure I see with people and their New Years' Resolutions to get fit: quit drinking, go to the gym twice a day, etc. It's not sustainable.