I was surprised when I recently saw the statistic that Americans spend about 23 percent of our grocery dollars on processed foods and sweets today-nearly double what we spent 20 years ago.

As a registered dietitian and nutrition editor for EatingWell Magazine, I know that eating too much sugar isn't healthy. But is too much sugar simply a matter of extra calories in our diet-or is it harming our health, too? One researcher in particular, Robert Lustig, M.D., a pediatric endocrinologist at the University of California, San Francisco, is spreading the message that "fructose is poison." Is he right-or is it just a false alarm?

Rachael Moeller Gorman answered those questions for me (and EatingWell Magazine) in the September/October issue. Here's the cliff notes version of what she found. (You can-and should-read the full story here.)

First, what is fructose-and why is it unhealthy?
Sweeteners like high-fructose corn syrup (HFCS), despite its name, and table sugar (sucrose) are both composed of two smaller sugar molecules, glucose and fructose, in roughly equal proportions. (HFCS and sucrose are virtually chemically equivalent, say most scientists and doctors.) While glucose is used to run the billions of cells in our body, fructose "is a chronic poison," Lustig says. "It doesn't kill you after one fructose meal, it kills you after 10,000. The problem is, every meal now is a fructose meal." HFCS is being added surreptitiously to processed foods from pretzels to ketchup.

Over time the mega-shots of fructose from added sugar uniquely gums up our inner workings and causes metabolic syndrome, the constellation of disorders, including type 2 diabetes, cardiovascular disease and fatty liver disease, that are devastating our country.

All of this happens when we're consuming too many calories-and most Americans are eating 200 to 300 extra calories a day. Lustig is quick to point out that he only means the fructose found in added sugar, the super-high levels of sucrose and HFCS we eat in processed foods and drinks (including fruit juices), not the lower levels found in fiber-rich whole fruit or the sugars (lactose) found naturally in milk.

Is there actually science to back this up?

• Studies have strongly linked consuming sugar-sweetened beverages like sodas and juices with obesity, at least in children and adolescents. (Find out here what the #1 food is that promotes weight gain, according to Harvard researchers.)

• For years researchers have been giving animals extremely high doses of fructose to create metabolic syndrome. And data from animal models is clear: when animals consume very high levels of fructose, say, 60 percent of their calories, they do seem to have the symptoms of metabolic syndrome-high levels of triglycerides in their blood, high blood pressure, insulin resistance and weight gain.

• But animal studies cannot reliably predict what will happen in humans. The researchers give the animals three times more than the highest amount of fructose most humans ever eat. Plus, animals process carbohydrates very differently from humans (their livers are naturally geared to make fat from carbohydrates).

• Human studies, at this point, don't all point to the same answer. Some studies say, "yes" fructose is harmful, while others not so much. The Nurses Health Study, one of the biggest epidemiological studies around, found that drinking a small glass of fruit juice daily (full of fructose) is associated with a higher incidence of type 2 diabetes; and, consuming one (or more) sugar-sweetened beverages daily, which also contain a lot of fructose, raises the risk of heart disease. The Framingham Heart Study showed that people who drank more than a can of soda a day were more likely to have metabolic syndrome. However, other large, epidemiological studies show a relationship that's tenuous at best between fructose and metabolic syndrome. And a 2010 paper that reviewed all the epidemiological studies said, "epidemiological studies, at this stage, provide an incomplete, sometimes discordant appraisal of the relationship between fructose or sugar intake and metabolic cardiovascular diseases."

But there's still more research to be done-and some new studies support Lustig's theory.

• Kimber Stanhope, Ph.D., a researcher at the University of California, Davis, is in the middle of a big new trial in which she is giving young, healthy people (versus overweight or obese people, who she has previously studied) what they actually can buy from convenience stores and vending machines: HFCS-sweetened drinks. The trial's first results (there's more to come) were just published: triglycerides and "bad" LDL cholesterol increased only in subjects who drank the fructose or HFCS, not glucose.

The Bottom Line: Many scientists think that focusing public health on one vilified nutrient is the wrong tack to take. Such a strategy has backfired before: for example, doctors once told us all fat is bad, so manufacturers took it out of processed foods (and, interestingly, replaced it with sugar). Today, it's recommended we eat 20 to 35 percent of calories from fat-most of which should be the healthy, unsaturated kind.

"We certainly agree we should eat less sugar, but I think [Lustig] is making a huge mistake from a public health perspective," says David Katz M.D., M.P.H., director of the Yale University Prevention Research Center and an EatingWell advisor. "We spent the last three decades doing exactly this: we thought just oat bran is good for us, so we put it in everything. Then we just needed to cut fat. Then it was carbs. So I can imagine we'll soon have a vast proliferation of sugar- or fructose-reduced junk food filled with some unhealthy substitute... I can't help but roll my eyes and ask, how many times do we have to get this wrong before we get it?"