It shouldn't be controversial to state that muscle and fat are two sides of the same coin, but people these days are so pro-muscle and so anti-fat that it may cause some cognitive dissonance.
Why? Because they are both forms of food storage. They both require a lot of food to produce. To say that they both result from over eating is obvious, but if you are going into a long Winter without much food, over eating is a very good idea. Likewise, if you are being sponsored by an Indian Raj to wrestle against other sponsored champions, by all means bulk up!
Muscle and fat are normal adaptive responses to eating too much food. In our topsy turvy world people sometimes over eat because they are over working at (stressful?) mental activity-- that tends to become fat. I suspect if those people got enough sleep, the excess fat would just burn off while they were asleep. After all, we can't eat and sleep at the same time! I've never really believed the "just laziness" argument, you have to be lazy and over eat and not get enough sleep. But it also seems that at some point the system can get so taxed it spirals out of control.
People also over eat and then "need" to exercise. Meaning they want to interrupt the efficient fat storing mechanism and replace it with the adaptation to stress response which makes food into muscle.
There is a whole conversation about the chemistry of the endocrine system I'm not going to have here, except to say that people like me tend to put on muscle very easily, it's genetic. But that muscle doesn't happen unless I over eat.
One of the reasons internal martial arts are "hard" to learn is that active people like me who have the discipline to practice also tend to put on muscle too easily. That muscle is conditioned strength which obstructs spontaneous whole body integration and whole body liquid unity thus reducing power efficiency and momentum mass transfer. (Yes, that sentence was a summary of the last 40 blog posts.)
Why are we over eating? There are a lot of good theories out there. Coming back from 10 quiet days in the mountains by myself, I was overwhelmed by how much intensely delicious food there was in my refrigerator! My mind has all sorts of automatic functions which are normally outside of my control. For instance, when I was living with goats earlier this Summer, I became an expert on what goats like to eat and what they don't like to eat. It was like I had a pre-loaded observation program in my head that had an intense desire to know and catalog what goats are eating! Weird right? My point is that there is a heck of a lot about human motivations we don't understand.
There is also the theory that food corporations have gotten so good at figuring out what we love to eat and selling it to us cheaply that we have become helpless automatons. No doubt, but we are also helpless against the urge to make fresh plum juice from the trees in my yard, or going out to see the latest Bat Man movie for that matter. Paradise sucks.
Well, as you can see by now, this post isn't going anywhere. But it wouldn't be complete unless I offered the theory that a significant part of the population may be over eating because they are exercising too much.