Ultimately, the low-sugar diet was not terribly restrictive. It was not low-carb, nor was it limited in calories. The children could eat fruit, starches and pasta, for example, and they were allowed to eat as much as they wanted. But the goal was to get their added sugar intake to less than 3 percent of their daily calories — less than the 5 to 10 percent limit for adults and children recommended by the World Health Organization.

After eight weeks, the low-sugar group had gotten their added sugar intake down to just 1 percent of their daily calories, compared to 9 percent in the control group. They also had a remarkable change in their liver health. They had a 31 percent reduction in liver fat, on average, compared to no change in the control group. They also had a 40 percent drop in their levels of alanine aminotransferase, or ALT, a liver enzyme that rises when liver cells are damaged or inflamed.

“As a practicing hepatologist, I see children weekly with fatty liver, and I would love to see this kind of improvement in my patients,” said Dr. Vos. “The exciting part was not only did the fat go down, but their liver enzymes also improved. That suggests that they also got a reduction in inflammation.”

The new study was funded in part by the Nutrition Science Initiative, a nonprofit research group that was co-founded by the science and health journalist Gary Taubes, a proponent of low-carb diets. The National Institutes of Health, the University of California, San Diego, Children’s Healthcare of Atlanta and Emory University also provided funding.

Dr. Joel E. Lavine, an expert who was not involved in the study, said it was cleverly done and demonstrated “some important points about what a major constituent of diet contributes to this problem in terms of liver fat and inflammation and cell injury.” He said the ubiquity of unhealthy foods makes such a diet difficult to follow, but that as a general rule doctors should advise patients and their families to check food labels for added sugars and to avoid or eliminate juices.

“The best diet, to make it very simple, is to shop the outside aisles in supermarkets and stay away from the middle aisles containing processed foods that come in boxes, cans and packages,” said Dr. Lavine, the chief of gastroenterology, hepatology and nutrition at Columbia University Irving Medical Center and the Morgan Stanley Children’s Hospital of NewYork-Presbyterian.

The members of the low-sugar group lost about three pounds during the study, which may have contributed to their improvements in liver health. But Dr. Jeffrey B. Schwimmer, an author of the study, said it was unlikely to account for the large changes.