Just about everyone who works through the question of "How do I lose my belly fat?" gets the bright idea that all they need to do is to cut out the carbs and the fat and focus on protein. There is just one major problem with that plan.
If you don't eat carbs, your muscles can't use protein.
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That is because muscle tissue gives priority to having the energy it needs for future activity. Muscles absorb glucose from the bloodstream and convert it into glycogen for future use. If they don't have the store of glycogen for future use, they don't repair their tissues. You have to have the carbs for your muscles to be "interested" in protein. Your body also needs certain essential fatty acids to make growth hormone, testosterone (which is also made in women's bodies), estrogen (which is also made in guys' bodies), cortisol, just about any hormone or regulatory substance it needs. You have to get protein, carbohydrate, and fat.
You just don't have to get an excess of any one as long as your basic calorie needs are met.
The human body can't store protein, but it doesn't need a lot of protein. As little as 22.5 g (that's about 3/4 of an ounce) a day is adequate for a 150-pound (68 kilo) man who doesn't work out, and even a bodybuilder may be able to get along on 100-125 g of protein a day. You have to get the full range of all the different amino acids the body needs to make its tissues, and it takes a lot less food to do that if you eat meat. You could however, get all the protein you need from a spinach salad nicely decorated with slices of clementines grown on an organic collective farm on Maui-but most fitness fans eat meat.
Similarly, you absolutely have to get some carbohydrate. If you don't, your body will make its own-from your muscles. And no matter how fat you eat, if you don't consume at least a few grams of the essential fatty acids-most easily obtained from fish oil, which lowers blood sugars and reduces inflammation-you aren't going to the growth hormone and sex hormones your muscles need to maintain themselves, much less bulk out.
So, no, you can't just eat protein. In fact, you don't even need a whole lot of protein, as long as its complete protein. Get your protein from high-quality sources like whey, fish, and lean meats, as little 160 g (6 oz) a day, and you're set.