I'm trying to lose fat and gain muscle (but mainly lose fat and just tone up overall), so I got some 100% whey protein powder. This powder is 80 calories per serving, with 20 grams protein, 1 gram carbs, and zero grams fat. So why not use this for weight reduction? Isn't this as filling a breakfast or snack you can get for that amount of calories (since its so high in protein and almost no carbs/fat)? If I use this product, will I gain weight?

Well if you exercise and drink protein shakes you will gain weight from muscle gain. Weight gain isn't bad unless it's fat (Or if you don't want muscles.)

You wont gain weight from protein shakes alone, nor from protein shakes and working out. You're total calories consumed must be greater than calories expended. Whey is a protein suppement and is designed to assist your diet in getting the proper amount of protein in your diet. Protein powders can be used when losing or gaining weight.

What Tony said. Calories are Calories. Whether they are from protein, carbs, or fats. Just eat fewer calories than you expend in a day and you wont gain weight.

You should be eating close to 1g of protein per pound of body weight. Use whey shakes to help you acheive that. But try to get as much of that protein from real foods as you can, and use shakes to supplement the rest.

As far as why it says that on your shakes, they are most likely saying that the shake should not be used as a meal replacement.

That warning is more for "meal replacement". Meaning don't substitute the powder for a meal, as you will lose overall nutrition.

Yeah, some people think they can survive off 3 whey shakes a day, which is less than 500cals, and NOT healthy.

That warning kind of stems from the early 80's when liquid protein diets were the fad, but people started dying because that was all they were eating.

Over on MH, there was a guy doing four muscle milk shakes a day. He was getting his proper number of calories, but I'll put money down he was pooping liquid or severly constipated a lot because of the lack of fiber.

Been there. Done that. Sucks REALLY freakin bad too, especially on squat day