When it comes to protein, Mary Kate Keyes can’t overemphasize how crucial it is to human survival.

“It’s what makes us,” says Keyes, a registered dietitian nutritionist “Our organs, bones, muscles and systems all rely on protein to function and stay healthy.”

Keyes, director of nutrition and wellness at Mind First Health & Fitness (mindfirsthealth.com), explains that the body uses protein by breaking down the food we eat into amino acids — 20 kinds, in total — which serve as basic building blocks. The body calls upon those amino acids when it needs to do something such as produce skin cells.

“If we’re not getting enough protein, any one of these systems can start to break down,” Keyes says. “We will feel fatigued, weak, can be more prone to infection, and can even start having swelling in our feet and ankles.”

Harvard’s T.H. Chan School of Public Health describes protein as an essential macronutrient found throughout the body. The enzymes that power many chemical reactions and hemoglobin, which carries the oxygen in blood, are made of protein.

Proteins are an essential part of blood clotting, healing damaged tissue and producing enzymatic reactions, such as using food for fuel, according to clinical dietitian Kristian Morey.

Where to Get It

Protein can be found in most whole foods, with the exception of fruit. Nuts, whole-grain breads, yogurt, broccoli, eggs, meat and fish are all good sources. The best source is whatever people most enjoy eating, Keyes says. Omnivores can benefit from both plant and animal sources of protein, she adds.

Eggs and chicken contain what nutritionists refer to as complete proteins because they contain all 20 amino acids. By contrast, plant proteins are incomplete, lacking a few of those amino acids, Keyes explains.

Those missing pieces vary from source to source, so mixing sources — such as the combination of rice and beans — can provide that complete protein picture. That approach comes with the added bonus of protein sources such as Brussels sprouts, pasta, lentils and edamame providing fiber, vitamins, minerals and phytonutrients, Keyes says.

“It used to be thought that animal proteins were better than plant proteins because they were more accessible or bioavailable to the body,” Keyes adds. “This theory makes sense when you consider that we’re animals, too, so we do well with foods similar to our own tissue.

“However, this idea that animal protein sources are better does not matter much to healthy individuals eating a balanced diet. Some plant sources are missing some essential amino acids, but when you’re getting variety by eating a veggie and whole grain at every meal, the pros outweigh the cons.

“There is some evidence that the fiber in plant protein may make these foods a little less digestible, but variety matters more than choosing foods solely based on their amino acid availability.”

Harvard notes that eating healthy protein sources such as beans, nuts, fish or poultry instead of red and processed meats can lower the risk of several diseases — including heart disease, diabetes and cancer — and premature death.

Environmental Impact

It is impossible to ignore the environmental impact of producing animal protein sources, Keyes says. As worldwide population growth continues and personal incomes rise, people are consuming more animal proteins. But production of meat — feeding, watering, housing and managing animal waste — is a large contributor of methane, which contributes to global warming, she explains.

Harvard adds that food production places “an enormous demand” upon natural resources, with agriculture contributing to deforestation, species extinction, and freshwater depletion and contamination.

“One way to reduce your meat consumption for the health of our planet is to try to cut back red meat consumption by one time per week,” Keyes says. “Then, look for other ways to reduce animal consumption. It all adds up.”

Daily Quota

Keyes says there are complex calculations that can determine how much protein people need daily, with the National Academy of Medicine noting anywhere from 10 percent to 35 percent of calories from protein daily are acceptable. But the simple math to the minimum amount needed per day is to divide one’s weight in half, then aim for that number in grams of protein each day. Someone who weighs 150 pounds should try to get at least 75 grams of protein daily, according to Keyes.

“This is an estimate of average daily intake, so some days you may fall a little low, some days a little high, and that’s totally normal,” she adds.

Athletes may need more protein, especially when starting a training program. The needs of men and women are the same, proportionally, though men are generally large, meaning guys tend to need more, Keyes says. Protein consumption needs to increase around age 65 and older. The halving math still mostly works, but senior citizens should add one or two extra protein-rich foods per day, she adds.

“We need this protein as we age because it’s especially critical for bone health,” she says.

Pregnancy and lactation also require more protein, as the body is creating and feeding a new human, Keyes says. Infants need even more protein per pound, she adds.

“We never grow so fast as we do in our first year of life,” Keyes says. “In fact, an infant’s needs are close to double per pound what an adult needs, but they’re so tiny that the actual grams of protein per day is still a small number.

“This trend for more protein per pound stays high, but slows down throughout childhood and adolescence until we reach adulthood, then holds steady for a few decades.”

The body generally excretes any excess protein by breaking down amino acid molecules and sending them to the kidneys, which send out extra metabolites as waste, Keyes explains. There are potential downsides to consuming too much protein.

To get rid of the extra protein via urination requires a lot of extra water. The body prioritizes eliminating the waste, and that water use can lead to dehydration, Keyes says.

“Dehydration is dangerous because it can damage the kidneys over the long term, and in the short term it just makes you feel crummy — think dry and headachy,” she adds.

Too much protein can also lead to weight gain, she says. That may be great for people intentionally stressing their muscles through resistance training to increase mass, but those who eat more protein than can be incorporated into the body will see the excess converted into fat, Keyes says.

Harvard notes people diagnosed with certain diseases, such as those of the kidneys and liver, should monitor their protein intake under a doctor’s guidelines.

Harvard adds that millions of people worldwide, especially young children, are not getting enough protein because of food insecurity. The effects of protein deficiency and malnutrition include growth failure, loss of muscle mass, decreased immunity, weakening of the heart and respiratory system, and death.

Healthy adults in the United States and many other developed countries rarely have a deficiency of protein, according to the university.

Morey adds that inadequate protein intake over a period of time in children can turn into marasmus, or present with skin rash and a swollen belly, a condition known as kwashiorkor.

“These conditions are dangerous because it can cause growth to stop, including brain development, as well as reduced ability to regulate body temperature and absorb nutrients,” Morey says. “If this is not treated properly and quickly it can lead to death.”