People often ask why any adult would need a diet providing more than 5 to 6 percent of calories from protein. I had several questions about this after I posted my recent article about protein in vegan diets. After all, human breast milk is around 6 percent protein and it supports health during the fastest period of growth of the entire lifecycle. How could adults need a more protein dense diet than an infant?  
Babies certainly have high protein needs for their size. They require almost 0.7 grams of protein for every pound of body weight. Vegan adults need far less—around 0.4 to 0.45 grams of protein per pound of ideal body weight. But when it comes to getting enough protein, infants have a distinct advantage: They are little eating machines. A 13-pound baby can consume as much as 500 calories per day. Although their food has low protein density, babies get plenty of protein simply because they eat so much of that food. (An additional advantage is that the protein in breast milk has high bioavailability.)
Unless you’re on a mission to gain weight, you can’t eat quite as enthusiastically as the average one-month-old. A young infant needs around 9 grams of protein per day and 500 calories. A 135-pound vegan woman may need about 6 times more protein than an infant, but only around 4 to 4 ½ times more calories. Obviously, her diet needs to be more protein dense than the baby’s diet.
A person who is very physically active and eating a high calorie diet will meet protein needs on a diet that is less protein dense. In contrast, if you are eating a low-calorie diet for weight control, you’ll need to pack more protein into fewer calories, which means you need a more protein dense diet.   
As a result, protein density of optimal diets varies quite a bit among individuals. Depending on calorie intake, vegan adults generally need diets that are between 9 and 12 percent protein to meet recommended intakes. The number may be somewhat higher for those who are dieting.
It’s not the least bit difficult to meet protein needs on a vegan diet. Vegans who are consuming enough calories and eating a variety of foods including legumes will meet requirements with ease.  But for adults, diets providing only 6 percent protein are likely to fall short.