SANFORD, N.C. — Marshall Reid, 12, a sixth-grader from Sanford, N.C., has a know-it-all quality that can drive some teachers crazy. As he does prep work for a Cuban black-bean stew for his family’s supper, he leans over a cutting board with a self-assured smile and a dramatically furrowed brow. Quickly, he dices red and green peppers for the pot, then slices limes and avocados and chops cilantro, scooping them into garnish bowls.

Next, his mother, Alexandra Reid, hands him scallions.

“I forget,” he says hesitantly. “Which part do I chop?”

A minor lapse, especially since not even two years ago, Marshall thought dinner usually meant, as his guilt-ridden mother puts it, “boxed something.” Between errands and activities (Alexandra Reid is a pistol instructor, raises nine chickens, sells antiques, has a real estate license and recently built a bathroom in their scuffed 1900 farmhouse), the family had grown accustomed to meals of pizza, chicken nuggets and Hamburger Helper.

As he attacks the scallions’ white bulbs, Marshall recounts the moment that prompted his transformation into a healthier eater and an adventurous cook. On the last day of fourth grade, a child stood in front of Marshall, then 10, and said, “You’re fat.”

Marshall had been bullied about his weight for years. To fortify himself for school, he took comfort in breakfasts of cans of roast beef hash, plus biscuits and gravy. That year, the school fitness report said his body mass index was 32.3. He was emphatically obese.

But it was the student’s jeer that pushed him over the edge. As Marshall walked slowly into the house that day, he said, “Mom, let’s do the opposite of ‘Super Size Me’” — Morgan Spurlock’s documentary about a McDonald’s-only diet for 30 days — “and be healthy for a month. I’m tired of this.”

Marshall brightened, adding, “We can call it Portion Size Me.”

The Marshall plan

In theory, losing weight should be straightforward: eat healthier, subtract quantity, add exercise. In the real world, though, where each family member can have a different relationship with food, that equation can be far more complicated.

Not only did Alexandra have no time for cooking, she had a powerful aversion to it. When she was 10, because of family circumstances, the cook’s role fell to her, so “food” always equaled “chore.”

Marshall’s sister Jordan, now 15, lives on the other side of the somatotype moon: a relentless soccer player, she inhales junk food but remains thin. Marshall’s father was unable to help much. Army Lt. Col. Dan Reid was in Iraq.

After the Marshall plan was announced, the family huddled, with Alexandra posing questions to help the children create goals, such as identifying and changing poor eating habits and a sedentary life. Jordan resisted. Not her problems.

“But we realized that the amount of weight you drop isn’t the endgame,” Alexandra said. “It’s about how good you feel about yourself, about making healthier choices. That’s the reward.”

A natural fit

They decided to make YouTube videos of Marshall’s new meals, to share with his father and to keep Marshall on track: see Marshall reading labels on a can of peas at the Piggly Wiggly; discussing how to reduce fat and sugar in recipes; boasting about the taste and healthy balance of his meals.

Turns out that the same know-it-all quality that can irk a child’s teachers finds its natural habitat in how-to videos. Warming to the camera as well as his project, Marshall continued for a second month. That summer, Alexandra posted about 140 videos. Soon, Marshall had a modest fan club. Producers from CNN and “The Nate Berkus Show” called. So did a literary agent.

“Portion Size Me: A Kid-Driven Plan to a Healthy Family,” a new book written by Marshall and his mother, is as much about the support of a loving family as it is about low-fat alternatives.

Alexandra came to embrace cooking. Jordan jettisoned some junk food. Motivated by his son, Dan started jogging in Iraq, keeping it up when he returned home that fall. The book includes 110 recipes, pop-up facts and journal pages.

It has been nearly two years since that schoolyard taunt. Marshall is still large for his height. He still doesn’t like sports that make him sweat. He is a self-described “military history geek” whose outdoor activity of choice is shooting arrows from an English longbow into a bale of hay.

But now, three times a week, Marshall and his father hit the gym at 5:30 a.m., taking turns as workout boss. Marshall relishes meals rich in freshness and flavor. And he is no longer obese. Marshall’s body mass index is now 27.4. He has dropped two pant sizes.

A work in progress

While Alexandra sniffs spices, she lists the excuses that closed her eyes to her son’s weight problems. Her husband was overweight as a child, but he grew out of it; she wanted to believe that Marshall would, too. During her husband’s deployments (Egypt, Qatar, Afghanistan), she was driving Jordan an hour to practice or games, waiting nearly two hours, and driving home, at least four times a week, six months a year, always with Marshall in the car, often munching on caramel corn.

“God forgive me,” she said. “I should have gotten off my rear end to make a picnic.”

For the Reids, serving dinner family-style means managing portions by filling a plate from pots on the stove, not from autopilot help-yourself bowls on the table. Alex spreads rice on plates, then flavor-laden stew (“I can’t have it not touch, honey,” she apologizes to Jordan). Marshall proudly puts out his bowls of garnishes. As they dig in, they chat about an event the night before. The Reids have made a cause out of Portion Size Me, and they’re starting to appear at local schools and youth clubs.

This summer, before Dan Reid begins reinventing himself as a finisher of antique furniture, the family will take a long-deferred seven-week vacation, packing themselves into a used Airstream and driving 11,000 miles around the country. Alexandra has arranged some snack-war challenges along the way.

The bigger challenge is how the family will keep up their hard-won eating and exercise regimens while living on top of one another in an Airstream. Everyone has been making suggestions.

“We are,” Alexandra says as her husband set down the supper plates for the dogs, “a work in progress.”