From a Wall Street Journal article by Heidi Mitchell:

Unlike carbohydrates or fats, proteins are the only nutrients that can be used to build new cells that can form tissue, said Dr. Walter, a registered dietitian. 

“These have to be supplied by food, and the best source of them is what we call a complete protein, which includes meat, chicken, fish, milk or eggs,” she said. A total of eight ounces, or about 45 grams of protein, is all an adult needs each day, she said, and the type of complete protein it comes from doesn’t matter in a balanced diet that includes fruits, vegetables and grains.

To read more click on the following link: https://www.wsj.com/articles/how-much-protein-should-you-eat-each-day-11563374327