Hearing the familiar jingle of an ice cream truck in the heat of summer may not be the only reason kids constantly crave sweet treats.
Children don't just like sugar — they are biologically hard-wired to eat it, according to scientist Julie Mennella, a researcher with the Monell Chemical Senses Center in Philadelphia.
"During periods of growth, they're attracted to foods that give us calories. In the past, it was fruits: dates, honey," she said. "Now children are living in environments where sugar's everywhere."
Children have "a very exquisite biology that attracts them to the predominant taste quality of mother's milk, and then to high-calorie foods during periods of growth," she added.
Mennella spoke to NBC Nightly News for a two-part series on sugar consumption in the United States. In response to the food and beverage industry being villianized for making sugar too prevalent in our diets, the Food and Drug Administration last month proposed a rule that would require food labels to say not just how much sugar is in a product, but what percentage of the daily recommended intake it comprises.
Sugar consumption has been linked to health issues by numerous studies, in a country where 28 percent of Americans are clinically obese. Nearly 30 percent of children and teenagers in the U.S. are obese or overweight — up from 19 percent 35 years ago.
The industry argues that calories coming from sugar don't contribute to obesity any more than calories from other sources.
"The scientific evidence just doesn't indicate that sugar, or any nutrient, is addictive," the Sugar Association has said. "Targeting of all natural sugar is an oversimplified approach to the complex problem of obesity that is simply not backed by sound science."
It added on Wednesday in a statement to NBC News, "It's common knowledge that babies are born with an innate liking of sweet that lessens over time. Scientists that study human senses, such as taste, say that one of the reasons children have a tendency to like sweet foods is because its nature's way of letting us know a food is safe to eat. Sugar can also play a positive role in the quality of a child's diet, as confirmed recently by The American Academy of Pediatrics, since sugar improves the taste of many nutritious foods."
At Menella's lab, kids sample different sugar solutions so scientists can get an idea of just how much sweetness they prefer. Philadelphia resident Jessica Mersky recently brought her daughter Sasha, 5, to participate in the research. Sasha picked the solution that was the sweetness equivalent of a can of soda — plus 11 extra teaspoons of extra sugar.
"It's super, super sweet," Sasha said, adding that she would happily drink an entire glass of the syrupy concoction.
Similar studies are being done at Yale University, where researchers examine the effects of sugar on children's brains.
Thirteen-year-old Emma Severance took part in a study there on what happens to the brain on milkshakes: After a swig of a shake, Emma underwent an MRI. The researchers watched as parts of her brain became active.
But sugar doesn't just affect young people. How much a person's brain lights up after consuming something sugary could be an indicator of a bigger problem, the researchers say.
"We think that the greater response in these regions might be a risk factor for overeating," said Dr. Barkha Patel, a Yale postdoctoral fellow.
Just the thought of food activates the brain, said Dr. David Katz with the Yale Prevention Research Center.
"It even lights up in anticipation of food when we're hungry. There's pleasure in thinking about eating," he said.
In its statement to NBC News, the Sugar Association said, "Overeating is a real and serious problem for some adults and children but the scientific evidence does not support the cause as a physiological addiction to food. Chemicals in the brain naturally respond with all pleasurable experiences, including when we are eating foods we like. The brain's response to food is just one of the many complex internal systems in the human body that regulates food intake. These facts undermine the validity of relying solely on brain activity as evidence that food is addictive or the cause of overeating."
Mersky doesn't need any scientific studies to know how sugar affects her kids. When Sasha and her brother eat sweets, the resulting sugar high is obvious.
"Hyper," Mersky said of how the kids act. "A little crazy, little hyper. And then I notice the crash more so than the hyperactivity."