The body needs amino acids from the diet to replace proteins that are lost when cells from our skin and those lining our gastrointestinal tract are shed. Dietary proteins are also needed to allow for the accumulation of additional body protein mass that occurs with growth, pregnancy, increasing muscle mass, and to support the growth of hair and nails, as well as for wound healing.

It is important to emphasize that proteins are synthesized as needed to support necessary body functions, so that consuming protein in excess of need will not increase the amount of proteins made. In other words, excess amino acids are not stored in our body as proteins. Excess amino acids are used as an energy source or stored as fat.

Proteins in the body are constantly being broken down and reassembled in a process called protein turnover. In fact, many of the amino acids used to make proteins don’t come from the food we eat each day, but are drawn from a pool of amino acids obtained from the breakdown of the body’s own proteins. Although we consume about 70 grams to 100 grams of protein daily, approximately 300 grams of proteins in cells and fluids throughout the body are broken down and resynthesized each day. Most of the amino acids released by the breakdown of body proteins are reused in the production of new proteins, but some amino acids are metabolized (chemically altered), which prevents them from being used for protein synthesis. These modified amino acids must be replaced with dietary proteins to provide sufficient amino acids to remake all the body proteins that were broken down. (INFOGRAPHIC 8.7)

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Question 8.7

In many cases, the metabolism of amino acids requires that they first be stripped of their amino group, which leaves a carbon skeleton. This carbon skeleton is used primarily to synthesize glucose to maintain blood glucose levels when carbohydrate intake is low, or to a lesser degree, as a direct source of energy. In times of energy abundance, the body may also convert amino acids to fatty acids that are then stored in adipose tissue as triglycerides. (INFOGRAPHIC 8.8)

Question 8.8

When amino acids are used for energy, or to synthesize glucose or fatty acids, the amino group that was stripped off must be disposed of; otherwise it would accumulate in the body as ammonia, which is toxic. To prevent this, the liver converts ammonia to a less toxic substance called urea. Urea is then released into the blood, filtered by the kidneys, and excreted in urine.

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Scientists can measure urea in urine to study protein turnover in what is called a nitrogen balance study. As the name implies, nitrogen balance reflects if a body is gaining, losing, or maintaining protein. Protein intake is measured, as well as nitrogen output in urine and feces. Nitrogen losses from less significant sources (sweat, skin, hair, nails, breath, saliva, mucus, and other secretions) are estimated. As nongrowing, weight-stable adults, we have approximately the same amount of total protein in our body from day to day, so we are in nitrogen balance, such that the amount of nitrogen we consume (N_in) is equal to the nitrogen we excrete (N_out), or N_in = N_out. In contrast, a growing child, a pregnant woman, or someone who is just starting a resistance-training (weight-lifting) program will be increasing their mass of total body proteins, so they must excrete less nitrogen than they consume, or N_in is greater than N_out. (INFOGRAPHIC 8.9)

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Question 8.9

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Instead of lab-based nitrogen balance tests, researchers have typically measured protein requirements for competitive athletes in their normal environment, says Linda Lamont, PhD, professor of kinesiology at the University of Rhode Island in Kingston. They may give athletes some packaged meals, or ask them to write down everything they are eating, and give them a container to collect their urine. Based on these experiments, some researchers have concluded that highly active athletes might need more than the 0.8 g/kg/d RDA for protein. Instead, they might recommend that, for performance advantage, athletes get between 1.2 and 1.7 g/kg/d to help with endurance and strength, respectively. For the average 150-pound adult (approximately 68 kg), that’s a shift from about 50 to more than 100 g/kg/d. However, most athletes do not need to make significant alterations in their diet to meet this level of protein intake as the average intake of many American’s falls within this range. Furthermore, the increased calorie intake of athletes naturally leads to an increase in protein intake as well.

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This methodology has Lamont concerned. For one, when researchers repeat the experiments in the laboratory on the same athletes, they often get different results. That’s not a surprise, she notes—a field experiment performed outside a controlled laboratory setting can’t possibly measure all of the protein athletes consume and excrete. Furthermore, protein metabolism is affected by sex, age, total calorie intake, and factors related to exercise, such as how long and hard you work out, she adds. There is even evidence that athletes may need less of a protein-rich diet than the average person, since regular training renders the body more efficient at using amino acids.

However, many researchers and nutrition professionals believe extra amounts of protein can help highly competitive athletes who need to get maximum performance out of their bodies. How much protein athletes should eat is really two questions, says Stuart Phillips, PhD, professor of kinesiology at McMaster University in Ontario—how much they have to eat to replace what they lose during exercise and normal metabolism throughout the day, and how much will boost their performance. “Athletes don’t need any more protein than the average person. What an athlete can benefit from,” he says, “is a higher protein intake than the average person—which I call an optimal protein intake.”