Whether you’re trying to lose weight, gain muscle or stay strong and healthy as you age, protein is vital.
But how much protein is enough versus too much? The current recommended daily allowance for adults is 0.8 grams per kilogram of an individual’s body mass (or 0.36 grams per pound of body weight). However, over the past decade, researchers found that increasing protein intake is even better, says Chad Kerksick, director of the Exercise and Performance Nutrition Laboratory at Lindenwood University in St. Charles, Missouri.
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By 2015, a comprehensive review in Applied Physiology, Nutrition, and Metabolism concluded that consuming 25 to 35 grams of protein per meal was sufficient for most adults. However, some people, especially those trying to lose weight or gain muscle, as well as older adults, likely needed more protein. The review found that for people trying to lose a substantial amount of weight, diets that included roughly 25 percent of their daily calories from protein helped prevent muscle loss and weight regain. For a person eating 1,600 calories per day, that would equal 100 daily grams of protein. The latest protein review, published in 2018 in the Journal of the International Society of Sports Nutrition, concluded that, at a minimum, people need double the recommended daily allowance to effectively build muscle.
Meanwhile, other experts have warned that consuming more than 20 grams of protein in a single sitting would turn protein to fat. Even worse, some experts have warned that excess protein intake can cause serious and potentially fatal damage to the kidneys.
How Much Protein Your Body Needs
Your body will use “however much [protein] you eat — whether that’s 50 grams or 250 grams,” says Donald K. Layman, professor emeritus in the department of food science and human nutrition at the University of Illinois.
The only difference is how your body uses protein, Layman says. The body can only use so many amino acids (the building blocks of protein) for muscle protein synthesis, the process by which muscle cells repair and grow, he says. And muscle health and growth is the primary reason that people consciously increase their protein intake.
Exactly how much protein the body can use toward muscle-building depends on a multitude of factors, including body size, physical activity level, age, total calorie intake and health, explains board-certified sports dietitian and registered dietitian Marie Spano, co-editor of the National Strength and Conditioning Association’s “Guide to Sport and Exercise Nutrition.” People who are larger and more active have greater protein requirements, as do those who are in a caloric deficit and want to prevent the body from breaking down existing muscle tissue for energy. Plus, after about the first 25 to 30 years of life, people naturally become less efficient at using the amino acids from protein to build muscle, meaning they need to eat more to get the same effect, Layman says.
To determine an individual’s ideal protein intake, the efficiency at which the body uses protein is an important consideration. Why? When it comes to absorbing and using protein to build tissue, muscle cells don’t operate like an on/off switch. Instead, they act more like a dimmer, where, at a certain point, they gradually become less efficient at using each amino acid to build muscle tissue.
“You need a certain amount for protein to be effective at triggering muscle protein synthesis, and there is an increasing effect up to a certain point, but then the effect of each additional gram starts decreasing,” he says. “So, for example, you might get 90 percent of maximum efficiency with 25 to 30 grams, but then start to have decreasing efficiency after 50 grams.”
A 2018 review published in the Journal of the International Society of Sports Nutrition concluded that in order to maximize muscle-building, people should eat between 0.4 and 0.55 grams per kilogram of their body mass at least four times per day. For a 180-pound adult, that recommendation works out to four meals of 33 to 45 grams of protein at each meal.
[See: 5 Unintended Consequences of Eating Too Much Protein.]
What Happens to Protein That Isn’t Used to Build Muscle
“We mostly talk about dietary protein in regard to muscle protein synthesis, but there are trillions of cells in the body that require protein,” Kersick says. “It is needed for the healthy cell turnover and recovery in the organs, tissues — all of the cells within the immune system require protein.” Basically, muscle aside, protein serves as a primary structural component throughout the body, allowing even the most basic of physiological functions.
That means that whenever you eat protein, some amino acids are used for muscle protein synthesis, while others are used to build proteins throughout the cardiovascular, neurological and other systems of the body, he says. Then, when more protein is consumed at a meal than is needed at that exact moment, the protein can be temporarily stored in the stomach tissue and released later to build other tissues — both muscle and otherwise, Spano explains.
Meanwhile, the body can also use the calories in protein (each gram of protein contains four calories) as an energy source. This typically occurs when the total daily calories or calories from carbohydrates and fat are in short supply. The body can also process protein into glucose, a simple sugar, to deliver to the brain and other parts of the body that rely on a steady stream of glucose for healthy functioning, Kersick says.
It’s worth noting that for people who are following a ketogenic diet, in which the body uses fat rather than carbohydrates for energy, this is why it’s important to consume only low to moderate amounts of protein. The high levels of protein can prohibit the body from entering ketosis, Layman explains.
So, can the body store protein as fat? Yes, this can happen when consuming excess caloric intake from any macronutrient, such as a carbohydrate, protein or fat, but it is actually less likely that any excess calories stored as fat will be from protein, Layman says. “Fat is the easiest macronutrient for the body to put in the fat cells,” he says. “It’s much more convenient, and next is carbohydrates. Storing protein as fat is biologically more complicated. Saying that if you eat too much protein, it will go to fat is a stretch of the metabolic truth.”
How Protein Intake Impacts Kidney Function
“[For] forever, people have tried to make it about kidney function,” Layman says. Researchers have debunked the myth that in healthy adults, high protein intake negatively affects the kidneys, he explains.
Every time you eat protein, the liver converts its nitrogen into the compound urea, which the kidneys then process and allow you to excrete from your body through urine, he says. “Urea production is very healthy and normal,” Kersick says. “It’s a natural byproduct of nitrogen metabolism. When you consume more protein, you just produce a little more of it.”
However, in those with urea cycle disorder, the liver doesn’t properly process the nitrogen, allowing it to build up in the blood as ammonia. Heightened levels of ammonia in the blood can contribute to everything from fatigue to death. According to the National Urea Cycle Disorders Foundation, one in 8,500 babies are born with urea cycle disorder, and it, along with other metabolic disorders, may account for up to 20 percent of sudden infant death syndrome, so it is an extremely rare condition among adults.
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“In people who have some form of compromise [in metabolic function], elevated intakes of protein are not recommended. That’s a clinical discussion that should happen with their doctor,” Kersick says. “There is no evidence that in an individual who is otherwise healthy, there is any concern that protein harms the kidneys or is dangerous,” he says. “That needs to be shouted from the rooftops.”
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