Lisa Young's a nutritionist and a professor at New York University. And today she's in a Manhattan grocery store, following up on the whereabouts of a certain suspect—added sugar.

She grabs a bottle of teriyaki sauce.

"Ten grams of sugar," she says. "That's a lot of sugar. That teriyaki is good, I didn't realize it had so much!"

She finds her target again in the "Light Options" salad dressing: seven grams of sugar. And again in tomato sauce: nine grams of sugar.

Young's worried about what all this stealthy sugar's doing to our bodies. And her concern comes from research like that done by Kimber Stanhope, a research scientist at the University California Davis.

Stanhope explains that sugar travels from your small intestine right to your liver.

"That gives the liver first rights to all the sugar we eat," she says.

One of the main parts of the the sugar we eat—both basic table sugar and high fructose corn syrup—is something called fructose. When we have too much sugar, we overload our liver with fructose. So our liver turns some of that fructose into fat.

"And there're two problems with turning that into fat," says Stanhope. "One, when the level of fat in the liver goes up, then the liver starts sending more fat into the blood." That increases your risk for cardiovascular disease.

And two, the fat that stays in the liver causes it to have problems with insulin, which increases your risk for diabetes.

This overloaded fatty liver might sound familiar. It's what happens when you have too much alcohol—something a lot of us try to limit because we know it's bad for our livers. But here's sugar doing something very similar, and I'd bet your liver's the last thing on your mind when you reach for that second donut.

I asked Stanhope why this effect of sugar on my liver should worry me, given that I take decent care of my body and don't have a family history of diabetes.

"So even in a healthy person," she says, "who might not even gain weight, it's possible that this damage is accumulating over time."

But scientists warning us about sugar have a bit of a PR problem: nutrition history.

Sugar's not the first food to get this villain treatment. We spent the second half of the last century being warned against fat, eggs, and salt. And scientists have come back around on all three of those.

Foods we've misidentified as bad for us could fill a whole book. British people in the 1600's thought tomatoes could kill you. The Pythagoreans of ancient Greece wouldn't even touch beans.

What if sugar's just the latest example of misdirected concerns?

"I think that is a completely fair question!" Stanhope responds. "And I don't blame you a bit for asking."

She says, yes, we have gotten a lot of conflicting messages about food in the past few decades. But the evidence against sugar has been growing, and she trusts it's going to keep growing.

About 15 years ago, Nicole Avena, a neuroscientist at Mount Sinai School of Medicine in New York, noticed a lot of people talking about how they felt "addicted" to sugar.

"There really wasn't any scientific evidence," she says. "There wasn't any carefully controlled or conducted studies on this."

So, she joined a team that was starting a study—in rats, to begin with.

"We started off looking for you know just simple classic signs of addiction," she explains. "Would these rats show signs of withdrawal, or would they show signs of binging or tolerance, if we just simply let them have access to sugar solutions? And that's what we found."

She then started looking at the brains of rats who showed signs sugar addiction.

"So we started looking at different neurotransmitters," she says, "like the opioids and dopamine. And we noted that there were changes in gene expression and changes in the release of those neurotransmitters that were similar to what you'd see if animal was on a low dose drug of abuse like nicotine or even alcohol or morphine."

These sugar-hooked rats' brains looked like drug addicts' brains. So in the past few years she's been working with human subjects, and finding similar things. People who show addiction-like behaviors towards sugar also show drug-addiction-like changes in their brain.

To be clear, we're mainly talking here about foods with added sugar, not healthy foods with natural sugar like apples or yogurt.

This addictive response to sugar doesn't happen to everyone, but when it does, it seems to have something to do with dopamine—the brain chemical that makes us feel good. When you're eating something for the first time, you usually get a little boost of dopamine.

"But what happens with foods after you've had them a couple of times," she says, "and your brain knows that they're safe and they're not making you sick, is that you don't release dopamine in response to eating them.

"But we're discovering when someone's addicted to a food, the dopamine release in response to candy corn, let's say, happens every single time you have a candy corn. It's no longer like a food, and it's more like what you'd see with a drug."

So maybe we aren't just weak-willed when it comes to sugar. There could be something going on biologically that compels some of us to keep eating it.

The government's nutrition policies have so far been pretty hands-off when it comes to sugar. But their new draft for dietary guidelines recommends adults get less than ten percent of their calories from added sugar. Right now the average American gets sixteen percent. And the Food and Drug Administration has proposed changes to their nutrition labels.

"Right now," says Lisa Young, the nutritionist and NYU researcher, "the labels are totally unclear, you're in the dark."

Current food labels just have one line for sugar, regardless of where it comes from. So if you're choosing foods based on sugar content, that cup of plain yogurt, which has good amount of naturally occuring sugar from lactose, it looks just as healthy as that small can of soda, which is all added sugar, and no nutritional value.

"There's been a big push to have a separate section for added sugar," Young says.

The FDA's proposing to give added sugar its own line on nutrition labels, and they've put the total cost for this relabeling at over $2 billion. But they expect that the effect of the new labels, a better awareness of added sugar, will lead to $20-30 billion dollars in healthcare savings over the next two decades.