Plenty of non-meat foods provide protein, including quinoa. (Lexey Swall/For The Washington Post)

One of my son’s eighth-grade friends recently became a vegetarian. He joins the approximately 4 percent of youths in this country (up from 2 percent 10 years ago) who eat meatless. As much as my boys respect his choice and recognize his passion for the environment that spurred the decision, neither of them truly understands it.

Although my sons eat plenty of vegetables, their most requested dinners include sausage, pork or ground beef. In fact, their favorite meal is grilled pork tenderloin with bacon corn relish. We call it “pig on pig.” We eat it with loads of green vegetables and rotate fish, chicken and quinoa on other nights. But I won’t lie: I make it a lot.

The boys asked how their friend could put on enough muscle, possess enough energy or be such a good athlete without meat. I told them that meat can be very good for growing boys and athletes, as its protein helps to build muscle, repair tissue, provide energy and balance mood — but it is by no means necessary. If he’s eating enough vegetarian sources of protein, iron and B vitamins, their friend will perform just as well. In fact, the Academy of Nutrition and Dietetics’ position on vegetarian diets is that “well-planned vegetarian diets are appropriate for individuals during all stages of the life cycle, including pregnancy, lactation, infancy, childhood, and adolescence, and for athletes.”

There are many athletes who have risen to the top ranks of their sports while being meat-free, including tennis legend Martina Navratilova, football hero Joe Namath, 1998 Heisman Trophy winner Ricky Williams, Olympic track star Carl Lewis, baseball slugger Prince Fielder and tennis icon Venus Williams. In 2011, Venus Williams was diagnosed with an autoimmune disease that caused her to step off the court for a period. She attributes her strong return to tennis to her mostly vegan diet and better lifestyle choices such as rest days. Ricky Williams told Men’s Journal that going meat-free “changed my game, and it changed my body. I had tons of energy.”

There is no doubt that meat provides protein, but so do beans, eggs, nuts, yogurt and even broccoli. The following non-meat foods contain plenty of protein:

●Nuts and seeds (4-10 grams per 1 ounce serving): walnuts, cashews, pumpkin seeds, pistachios, sunflower seeds, almond butter, hemp, chia and flax seeds.

●Beans and legumes (7-10 grams per half-cup): black beans, white beans, lentils, chickpeas, hummus and green peas.

●Grains (5-8 grams per cup): quinoa, brown rice, oats, millet and barley.

●Soy (9-16 grams per ½ cup): tofu, edamame and tempeh.

●Fruits and veggies such as avocado (4 grams per cup), dark leafy greens (about 5 grams per cup) and broccoli (4 grams per cup).

●Dairy (milk, yogurt, cheese) and eggs provide 6-9 grams of protein per serving.

●Here’s a surprise: Raw cacao nibs provide 4 grams of protein (plus antioxidants, vitamins and minerals) per 1-ounce serving.

According to the Institute of Medicine, we should all consume between 10 percent and 35 percent of our daily calories from protein. This really is not that much and can be easily achieved with the non-meat foods listed above.

Babies: 10 grams a day.

School-age kids: 19-34 grams a day.

Teenage boys: 52 grams a day.

Teenage girls: 46 grams a day.

Adult men: 56 grams a day.

Adult women: 46 grams a day (more if pregnant or breast-feeding).

There are many benefits to eating a well-balanced vegetarian diet including cost savings, loads of fiber (which aids in digestion), less saturated fat (good for the heart), and a wider variety of vitamins and minerals proven to reduce diabetes, cancer and heart disease.

Just last month, New York City Mayor Bill de Blasio (D) announced a pilot program to bring meatless Mondays to 15 public schools. His isn’t the first city to take this approach; Baltimore, Vancouver, Pittsburgh, Sacramento and many other towns have launched similar programs in an effort to reduce obesity and halt climate change.

De Blasio is onto something: People can reduce their carbon footprints by cutting back on meat just once a week. According to the United Nations, the meat industry generates nearly a fifth of the man-made greenhouse gas emissions that are accelerating climate change worldwide. And approximately 1,800 to 2,500 gallons of water go into making a pound of beef, far surpassing the amount needed for vegetables and grains.

I don’t eat a lot of meat; in fact, my 13-year-old son loves to tease me for eating so many vegetables. If I say I’m hungry, he encourages me to roast the potted plant in our hallway. I am doing just fine, even though I eat more vegetarian proteins than “pig on pig,” and so will his newly vegetarian friend.