Gutshot can probably clear this up, he seems to know more about nutrients than me.

Before I start, protien is a nutrient used by the body for cell and tissue growth and repair. it has other functions like in starvation it is used as an energy source but it barely contains any calories compared to fat and carbohydrates.

When people say 'eat protien, gain weight' they are talking about eating a diet high in protien so that your body can build as much muscle from the stress caused by training. This also requires alot of energy though, usually from carbohydrates and some fat.

there are all sorts of 'high protien' weight loss diets around at the moment, the weight loss doesn't come from eating a high amount of protien but from changing the balance of the diet so that protien is the main energy source and carbohydrates and fat are only secondary (such as the Atkins diet).

It just comes from people chcuknig half arsed explinations around, alot of protien by itself won't build muscle and alot of protien by itself won't make you loose 'weight'. its more to do with the balance of other energy sources in your diet. Personally I don't think that High Protien diets are in the slightest bit health for you.