So, why do many athletes and coaches maintain that much higher protein intakes are desirable for increased muscle mass? The answer probably lies in the nitrogen balance method. Athletes desiring increased muscle mass should be in positive nitrogen balance, thus the conclusion is that protein intakes above 1.6g/kgBW/d are considered necessary. Furthermore, when nitrogen balance is measured in athletes, the greater the protein intake, the greater the nitrogen balance(2), thus athletes conclude that the more protein they eat, the bigger they’ll get.
Unfortunately, the increase in muscle mass that would be associated with these very high nitrogen balances is simply not possible (see box 1), even if anabolic drugs were administered. If this level of nitrogen were actually incorporated into muscle, these athletes would gain around 100kg of muscle mass in a year(1,7)! Clearly, there are problems with this method for determining nitrogen balance, especially at high levels of protein intake.
Box 1: Why positive nitrogen balance is not the same as muscle gain
Suppose an athlete eats 2.5g protein/kg/d; this would result in a positive nitrogen balance of around 15g of nitrogen per day;
But nitrogen constitutes only around 16% of protein by weight, therefore actual protein gain = 15g N x 1g protein/0.16g N = around 94g of protein per day;
Since muscle is 75% water, amount of muscle accretion per day = 94g protein x 25% protein in muscle (75% water) = 282g muscle per day;
Per year = 282g protein per day x 365 days = 102930g/yr or 103kg of muscle gained in one year!
More direct evidence that high positive nitrogen balance is disassociated from gains in lean mass has accumulated. Several studies demonstrate that athletes may have a very high nitrogen balance, but lean body mass does not increase(1,7). Thus, the basis for very high levels of protein intake is unsound.
