A full acquaintance with the obesity research doesn’t necessarily help parents make their daily decisions. A few years ago, Dr. Lumeng got an email from another physician who had heard her speak at a meeting, and had a 6-week-old infant who was so hungry she didn’t know what to do. Should she not feed her baby, she asked?

The doctor who wrote to her was Dr. Jennifer Kerns, an obesity medicine specialist at the Veterans Affairs Medical Center in Washington, who had herself lost weight as a contestant on “The Biggest Loser” in 2006 and spent some time serving as the show’s physician. She became board certified in obesity medicine, and when she later got pregnant, she said, she worried that her child might struggle with weight as she had done.

“I was really focused on my weight gain and essentially weighed myself every day of my pregnancy,” she said. She exercised until two days before she delivered the baby. She was determined not to have a cesarean section so the baby would have a healthier microbiome from picking up the right bacteria during the passage through the birth canal. She was resolved to breast-feed.

And now she had a child who was “instantly a very dramatically hungry baby, so much so that I was unable to nurse him,” she said. “We tried for five weeks and I had five different lactation consultants. He wouldn’t be patient enough to wait for milk to let down.” Determined to give him the benefits of breast milk, she ended up using a breast pump throughout his first year of life.

Remembering a lecture Dr. Lumeng had given about voraciously hungry babies, she emailed her. “I essentially asked if she would be willing to give me any advice, any studies, what to do if your baby seems to be starving to death.”

In “a very thoughtful and kind response,” Dr. Kerns recalled, Dr. Lumeng wrote that there was no research to guide her. “She couldn’t really give me advice other than her own experience with her own children: Just feed him, trust your instincts.”

Dr. Lumeng suggested that doctors should acknowledge to parents that “modern science really doesn’t fully understand what causes obesity.” We are expecting parents to do something for their children that we adults have great difficulty doing for ourselves, she said; of those who do successfully lose weight, many gain it back within a year. “Adults can’t keep it off either, why are we expecting parents to do this?”