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Is 100 grams of protein per day for someone who weighs 140 pounds enough for me to build muscle and lose fat?
The general axiom is 1g per pound, but that is with a safety margin and is really overkill. It's not that extra protein is harmful, so it's fine to shoot for that - some people, however, who don't like protein will do find taking in less and not stressing out over achieving it.
Protein intake has been highly researched. One of the most prominent and well-respected researches is Dr. Peter Lemon. His findings were that the people who required the most protein were actually professional soccer athletes, and they benefitted from 1.6 grams per kilogram of body weight. To convert to pounds, that is 0.73 grams per pound of body weight, so actually less than 1 pound.
Most people shoot for this, which is interesting, because very few people I know on these boards actually train as strenuously or intensely as professional soccer players, so it stands to reason their demands will actually be less.
So, at 140 pounds you wouldn't need more than 102 grams of protein. Going over is fine and many people do so for an insurance policy, and since protein is the hardest for the body to use for energy, it is the safest macronutrient to go over on. However, the notion that you need tons of protein is something that was instilled by .... you guessed it ... the supplement industry which is selling protein powders. "You can't get enough from natural foods so buy our shake."
I'll be honest. In my experience, having higher protein intakes can accelerate muscle gain. I gained the most muscle in the shortest period of time when I had a really high protein intake (most of that was from natural foods - i.e. 12 oz steaks, for example). However, I can also attest to the fact you can gain muscle with as little as half of your body weight in protein intake, and certainly maintain it.
So it really becomes a question of goals. For me, I don't feel healthy or energetic when I shove in high quantities of protein. About 100 - 150 grams is fine for me, so that's what I do. I'm less concerned with winning a race to put on as much mass in as short a period of time, and more interested in maintaining a healthy body.