In two linked studies, British researchers found that infants with greater appetites grew faster during a 15-month span, increasing their risk for obesity. Their desire to eat more might be genetic, the findings also suggested.
"Appetite is key and it would be sensible for parents to keep an eye on children who appear to be responding to food a lot more avidly than their peers," said lead researcher Clare Llewellyn, a lecturer in behavioral obesity research at the Health Behavior Research Center at University College London.
In addition, genes appear to play an important part in the predisposition for obesity, Llewellyn said.
Body weight and obesity are strongly influenced by genes, which is surprising, she said, because the obesity epidemic has been rather recent, while people's genetic makeup hasn't changed.
The resolution of this apparent paradox may lie in the influence genes have on appetite, by reducing or not reducing the urge to eat in response to internal fullness signals, called satiety, Llewellyn said.
"Recent changes to the food environment have played a causal role in the increases in population weight," she said. As food becomes more readily available, people of all ages "who have inherited genes that make them feel less full are more likely to overeat and gain weight."
Parents should prevent excess weight gain by being strict about the foods available, keeping snacks hidden and restricting the amount of food their children eat, Llewellyn said.
The reports were published in the Feb. 17 online edition of the journal JAMA Pediatrics.
Daniel Belsky is a postdoctoral fellow at Duke University's Center for the Study of Aging and Human Development. "These provocative findings bring us a little closer to understanding why the [obesity-prone] environment we all live in makes some of us fat and others not," said Belsky, the author of an accompanying journal editorial.
Belsky, however, does not recommend that parents restrict their babies' eating as a way to prevent obesity.
"Babies are very good at what they do, which is grow and develop," he said. "More often than not, they know what they need and parents should follow their lead. If parents are concerned about their baby's appetite, they should talk to their pediatrician."
For the first study, Llewellyn and her colleagues looked closely at 800 non-identical, same-sex twins born in the United Kingdom in 2007. From these, they followed a smaller number of pairs of twins to examine their food responsiveness (the urge to eat in response to the sight or smell of food) and satiety responsiveness (the desire to eat in response to the body's "fullness" triggers).
At 6 months old, the more food-responsive twin was, on average, 1.4 pounds heavier than the other twin, the researchers found. At 15 months, the more food-responsive twin was 2.1 pounds heavier. In addition, the less satiety-responsive twin was 1.4 pounds heavier than the other twin at 6 months, and 2 pounds heavier at 15 months.
In the second study, the researchers collected data on nearly 2,300 10-year-old children born in the United Kingdom between 1994 and 1996.
They created a genetic obesity risk score, which measured each child's obesity risk by looking at 28 obesity-related genes. The higher the score, the greater the risk for obesity.
Children with higher scores were likely to have a higher body-mass index (a measurement of body fat based on height and weight) and larger waists. Moreover, these children were more likely to have low satiety responsiveness, the researchers said.
A U.S. expert, however, questioned how much focusing on genetics actually helps in addressing the problem of obesity.
"That there is genetic variation underlying human vulnerability to obesity -- and such mediating traits as hunger and satiety -- is less than [surprising]," said Dr. David Katz, director of the Yale University Prevention Research Center.
"I hope the pursuit of 'obesity genes' does not obscure the fact that our vulnerability to drowning in calories is also widely shared and more norm than exception," Katz said. "The genetic variation we find today in a world of rampant obesity was there yesterday as well when obesity was rather rare. It's not what's within us that has changed, but rather what's all around us."
For more about childhood obesity, visit the U.S. Centers for Disease Control and Prevention.