I wanted to post about what would look like a contradiction: in this post, I said to eat when hungry but in this post I talked about how I will have to eat more in order to gain muscle.
After thinking about it more, what I really meant to say is that I needed to eat more such that thermodynamically I'm taking in more than I expend, but that this surplus should be driven by hunger. This takes a little more explanation. Here's how Taubes presented the first law of thermodynamics in his book:
Change in energy stores = Energy intake - Energy expenditure
And here's a corresponding quote about growth in children from the book: "Children do not grow because they eat voraciously; rather, they eat voraciously because they are growing. They require the excess calories to satisfy the requirements of growth; the result is positive energy balance. The growth is induced by hormones and in particular, by growth hormone."
So if a person is lifting weights and not gaining weight/muscle, then here's some points of speculation as to what's happening:
1) The workouts are not properly releasing growth hormone. Maybe there's not enough intensity or volume, or perhaps overall it was too taxing.
2) A person is doing too much overall endurance exercise. This is suspected to lower growth hormone (and testosterone) levels.
3) The growth hormone and corresponding hunger after the workout is blocked. For example, if you eat simple carbs after a workout this supposedly shuts down growth hormone relase. I'm not 100% sure this is true yet, but let's assume this is true for now. If simple carbs are eaten after a workout and growth hormone is blocked, then the corresponding hunger is blocked, hunger which would have led to increased protein and fat consumption. So without the post-workout carbs, maybe you have raging hunger and feasting on protein and fat. With post-workout carbs, you shut down this process.
4) Overall eating patterns are incorrect. We know that intermittent fasting increases growth hormone. So if there are no fasting periods in a person's normal eating pattern, they may be missing these growth hormone bursts, which may help to increase hunger.
5) Food composition. Let's say a hunter-gatherer was eating meat. If the meat was 30% fat instead of 20% fat, would he necessarily eat any less meat? In the modern world with all the low-fat preaching, people's tastes may not be used to high-fat meat. So fat consumption may be indirectly limited just by traditions and cultural forces.
These are all just educated guesses on my part, but I do think they are valid points. I do not think a person should have to force-feed food in order to build muscle. It's probably just a matter of setting up things correctly, allowing the growth hormone to drive hunger, then consuming the proper foods, which will naturally lead to a calorie surplus and some new muscle.