I'm out in the elements where the heat indices approach 110 degrees working all day. I've started working out at the gym with my son. And I actually gained weight. In my twenties, I never had any trouble with weight. I subscribed to the simple notion that weight was all about the simple equation:
calories in = calories out
To lose weight, just eat less and exercise more. I'm realizing first hand that's a simplistic biological notion. Your body is not a like your checking account. It's as if your checking account could grow or shrink irrespective of how much you make or spend. So I've realized that when I started working out, my appetite increased voraciously to compensate. And I didn't necessarily eat bad things either. Likewise, I've read of multiple studies where starving oneself leads to a slowing of metabolism which is why diets are yo-yo in nature. It's not only the weight loss that's difficult. It's the maintenance that's equally hard but just as important. And I'm not sure the medical community has come up with a satisfactory strategy for that yet.