When protein is metabolized, the nitrogen is released and flushed out by the kidneys. Comparing nitrogen in (protein intake) with nitrogen out (in urine) is the standard method of determining whether a person is getting the right amount of protein. For adults, nitrogen-in should match nitrogen-out. If you are losing less nitrogen than you are consuming in food, you are retaining protein. Retaining protein is normal for kids who are growing or for someone who is building muscle through strength training. But, losing more nitrogen than you consume means protein-rich tissue (mostly muscle), is breaking down. There are many potential causes for this, such as illness, stress, or chronic inactivity. The current recommendation for protein intake for healthy adults is 0.8 g protein per kilogram of body weight per day. This number was determined using studies on healthy young adults. But consider that body composition changes as people get older.
Once adults pass the physical prime of their teens and 20's, they lose an average of 10 ounces of lean body mass a year. This is mostly in the form of muscle tissue. Few people actually lose 10 ounces of weight a year. Instead, most gain about a pound a year, so the loss of lean tissue is masked.
Another way to look at this is the average person gains about 1 pound and 10 ounces of body fat per year. This phenomenon, called sarcopenia, is derived from Greek words for "vanishing flesh." This change in body composition contributes to impaired wound healing, loss of skin elasticity, and an inability to fight infection.
Louis Bryant
Once adults pass the physical prime of their teens and 20's, they lose an average of 10 ounces of lean body mass a year. This is mostly in the form of muscle tissue. Few people actually lose 10 ounces of weight a year. Instead, most gain about a pound a year, so the loss of lean tissue is masked.
Another way to look at this is the average person gains about 1 pound and 10 ounces of body fat per year. This phenomenon, called sarcopenia, is derived from Greek words for "vanishing flesh." This change in body composition contributes to impaired wound healing, loss of skin elasticity, and an inability to fight infection.
Louis Bryant