An overweight America may be fixated on fat and obsessed with carbs, but nutritionists say the real problem is much sweeter -- we're awash in sugar.

Not just any sugar, but high fructose corn syrup.

The country eats more sweetener made from corn than from sugarcane or beets, gulping it down in drinks as well as in frozen food and baked goods. Even ketchup is laced with it.

Almost all nutritionists finger high fructose corn syrup consumption as a major culprit in the nation's obesity crisis. The inexpensive sweetener flooded the American food supply in the early 1980s, just about the time the nation's obesity rate started its unprecedented climb.

The question is why did it make us so fat. Is it simply the Big Gulp syndrome -- that we're eating too many empty calories in ever-increasing portion sizes? Or does the fructose in all that corn syrup do something more insidious -- literally short-wire our metabolism and force us to gain weight?

The debate can divide a group of nutritional researchers almost as fast as whether the low-carb craze is fact or fad.

Loading high fructose corn syrup into increasingly larger portions of soda and processed food has packed more calories into us and more money into food processing companies, say nutritionists and food activists. But some health experts argue that the issue is bigger than mere calories. The theory goes like this: The body processes the fructose in high fructose corn syrup differently than it does old-fashioned cane or beet sugar, which in turn alters the way metabolic-regulating hormones function. It also forces the liver to kick more fat out into the bloodstream.

The end result is that our bodies are essentially tricked into wanting to eat more and at the same time, we are storing more fat.

"One of the issues is the ease with which you can consume this stuff," says Carol Porter, director of nutrition and food services at UC San Francisco. "It's not that fructose itself is so bad, but they put it in so much food that you consume so much of it without knowing it."

A single 12-ounce can of soda has as much as 13 teaspoons of sugar in the form of high fructose corn syrup. And because the amount of soda we drink has more than doubled since 1970 to about 56 gallons per person a year, so has the amount of high fructose corn syrup we take in. In 2001, we consumed almost 63 pounds of it, according to the U.S. Department of Agriculture.

The USDA suggests most of us limit our intake of added sugar -- that's everything from the high fructose corn syrup hidden in your breakfast cereal to the sugar cube you drop into your after-dinner espresso -- to about 10 to 12 teaspoons a day. But we're not doing so well. In 2000, we ate an average of 31 teaspoons a day, which was more than 15 percent of our caloric intake. And much of that was in sweetened drinks.

Beyond soda

So, the answer is to just avoid soda, right? Unfortunately, it's not that simple, because the inexpensive, versatile sweetener has crept into plenty of other places -- foods you might not expect to have any at all. A low-fat, fruit-flavored yogurt, for example, can have 10 teaspoons of fructose-based sweetener in one serving.

Because high fructose corn syrup mixes easily, extends shelf-life and is as much as 20 percent cheaper than other sources of sugar, large-scale food manufacturers love it. It can help prevent freezer burn, so you'll find it on the labels of many frozen foods. It helps breads brown and keeps them soft, which is why hot dog buns and even English muffins hold unexpected amounts.

The question remains just how much more dangerous high fructose corn syrup is than other sugars.

Fructose, as the name implies, is the sugar found naturally in fruit. It can be extracted, turned into granules and used like sugar in the kitchen. It used to be considered a healthier alternative to sucrose -- plain old table sugar. It's sweeter, so less is needed to achieve the same taste. Diabetics use it because fructose doesn't stimulate insulin production, so blood sugar levels remain stable.

The process of pulling sugar from cornstarch wasn't perfected until the early 1970s, when Japanese researchers developed a reliable way to turn cornstarch into syrup sweet enough to compete with liquid sugar. After some tinkering, they landed on a formula that was 55 percent fructose and 45 percent glucose -- sweet enough and cheap enough to make most soda companies jump from liquid sugar to high fructose corn syrup by the 1980s.

The results were dramatic. -- a whopping increase of 4,080 percent.

Journalist Greg Critser lays out a compelling case against high fructose corn syrup in his 2003 book, "Fat Land: How Americans Became the Fattest People in the World." He argues that federal policies that aimed to stabilize food prices and support corn production in the 1970s led to a glut of corn and then to high fructose corn syrup. With a cheaper way to sweeten food, producers pumped up the size and amount of sweet snacks and drinks on the market and increased profits.

It's not natural

Critser writes that despite the food industry's arguments that sugar is sugar, whether fructose or sucrose, no group "has yet refuted the growing scientific concern that, when all is said and done, fructose ... is about the furthest thing from natural that one can imagine, let alone eat."

Although some researchers have long been suspicious that too much fructose can cause problems, the latest case against high fructose corn syrup began in earnest a few years ago. Dr. George Bray, principal investigator of the Diabetes Prevention Program at Louisiana State University Medical Center told the International Congress on Obesity that in 1980, just after high fructose corn syrup was introduced in mass quantities, relatively stable obesity rates began to climb. By 2000, they had doubled.

Further, the American Journal of Clinical Nutrition in 2002 published research that showed that teenagers' milk consumption between 1965 and 1996 decreased by 36 percent, while soda consumption increased by more than 200 percent. Bray argues that without calcium, which nutritionists agree can help the body regulate weight, kids got fatter. He says that he could find no other single combination of environmental or food changes that were as significant to the rise in obesity.

Other studies by researchers at UC Davis and the University of Michigan have shown that consuming fructose, which is more readily converted to fat by the liver, increases the levels of fat in the bloodstream in the form of triglycerides.

And unlike other types of carbohydrate made up of glucose, fructose does not stimulate the pancreas to produce insulin. Peter Havel, a nutrition researcher at UC Davis who studies the metabolic effects of fructose, has also shown that fructose fails to increase the production of leptin, a hormone produced by the body's fat cells.

Both insulin and leptin act as signals to the brain to turn down the appetite and control body weight. And in another metabolic twist, Havel's research shows that fructose does not appear to suppress the production of ghrelin, a hormone that increases hunger and appetite.

"Because fructose in isolation doesn't activate the hormones that regulate body weight as do other types of carbohydrate composed of glucose, consuming a diet high in fructose could lead to taking in more calories and, over time, to weight gain," he says.

However, Havel isn't convinced high fructose corn syrup is by itself the problem. That's in part because it is composed of 55 percent fructose and 45 percent glucose, which is similar to the 50-50 combination of fructose and glucose found in table sugar. Havel's studies have focused on fructose by itself and not as part of a high fructose corn syrup mixture.

"Whether there is an important difference in the effects of consuming beverages sweetened with a mixture of 55 percent as opposed to 50 percent fructose would be hard to measure," he says. "Additional studies are needed to better understand the nutritional impact of consuming different types of sugars in humans."

Still, other researchers are finding new problems with high fructose corn syrup. A study in last month's Journal of the National Cancer Institute suggests that women whose diet was high in total carbohydrate and fructose intake had an increased risk of colorectal cancer. And Dr. Mel Heyman, chief of pediatric gastroenterology and nutrition at UCSF, is seeing sick children whose bodies have been overloaded with fructose from naturally occurring fructose in fruit juice combined with soda and processed food.

"The way the body handles glucose is different than fructose," he says. "It can overload the intestines' ability to absorb carbohydrate by giving it too much fructose. That can cause cramps, bloating and loose stools."

The jury's still out

Like others in the field, he says there is much to discover in how sugar works, but he disagrees that high fructose corn syrup is somehow reprogramming our bodies toward obesity. Rather, he says, we're just eating too much of it.

Nutrition theory holds that the basic make-up of fructose-laced corn syrup is not much different than table sugar. They react about the same in the body, says Dr. Walter Willett, a professor of epidemiology and nutrition at Harvard School of Public Health. "There are some modest differences in metabolism, but I don't think fructose per se is the culprit."

Neither do the food companies that use it in copious amounts.

Says Stephanie Childs, a spokesperson for the Grocery Manufacturers Association: "At the end of the day, how any sweetener affects your weight depends on how many calories you are taking in overall. Overemphasizing one nutrient at the detriment of others is not going to solve the problem."

Even some leading nutrition reformers aren't convinced that high fructose corn syrup is of itself the issue. The bigger battle, says Michael Jacobson, executive director of the Center for Science in the Public Interest, a consumer advocacy group, is to get added sugars listed on food labels with a percentage of daily value. That means a consumer could look at a package and see that, for example, one soda provides almost all the sugar a person should eat in a day.

"It simply comes down to this," he says. "We're eating too much refined sugars, be it sucrose or high fructose corn syrup or any other refined sugar."

A sugar glossary

Here's a rundown of the various types of sugar you'll find on product labels.

Brown sugar. Sugar crystals contained in a molasses syrup, with natural flavor and color; 91 to 96 percent sucrose

Corn syrup. Made from cornstarch. Mostly glucose. Can have maltose

Dextrose. Commonly known as corn sugar and grape sugar. Naturally occurring form of glucose

Fructose. Sugar found in fruit and honey. Sweetest natural sugar

Galactose. Sugar found linked to glucose to form lactose, or milk sugar

Glucose. Also called dextrose. The human body's primary source of energy. Most of the carbohydrates you eat are converted to glucose in the body.

High fructose corn syrup. Derived from cornstarch, usually a combination of 55 percent fructose and 45 percent sucrose. Treated with an enzyme that converts glucose to fructose, which results in a sweeter product. Used in soft drinks, baked goods, jelly, syrups, fruits and desserts

Honey. Sweet syrupy fluid made by bees from the nectar collected from flowers and stored in nests or hives as food. Composed of fructose and glucose

Lactose. Sugar found in milk and milk products that is made of glucose and galactose

Maltose. Also called malt sugar. Used in the fermentation of alcohol by converting starch to sugar

Maple syrup. A concentrated sucrose solution made from mature sugar maple tree sap that flows in spring. Mostly replaced by pancake syrup, a mixture of sucrose and artificial maple flavorings

Molasses. Thick syrup left after making sugar from sugarcane. Brown in color with a high sugar concentration

Powdered or confectioner's sugar. Granulated sugar that has been pulverized. Available in several degrees of fineness

Sucrose. Commonly called cane sugar, table sugar or simply sugar

Sugar (granulated). Refined cane or beet sugar; 100 percent sucrose

Turbinado sugar. Raw sugar that has been partially refined and washed

Awash in corn syrup

It should come as no shock to most consumers that a Pepsi or a Fig Newton has plenty of sugar - most of it from high fructose corn syrup. But what's surprising is the products where the sweetener hides out and how disguised it can be by the deceptively small serving size listed on the nutrition label. Although the numbers below show teaspoons of sugar per serving, people often eat more than one serving. The U.S. Department of Agriculture advises most people to limit themselves to 10 to 12 teaspoons of added sugars a day.

How much is too much?

The list below shows how much sugar, mostly in the form of high fructose corn syrup, is in each of these single servings.

Sunkist soda: 10 1/2 teaspoons of sugar

Berkeley Farms low-fat yogurt with fruit: 10 teaspoons of sugar

Mott's applesauce: 5 teaspoons of sugar

Slim-Fast chocolate cookie dough meal bar: 5 teaspoons of sugar

1 tablespoon ketchup: 1 teaspoon of sugar

Hansen's Super Vita orange-carrot Smoothie: 10 teaspoons of sugar

Hidden sugar The chart shows how much high fructose corn syrup hides in products. PRODUCT Servings TEASPOONS OF SUGAR PER CONTAINER Per SERVING Bean with Bacon Soup 2 1/2 1 Sara Lee Hot Dog Bun 6 1 Stovetop Stuffing 8 Almost 1 (chicken flavor) V8 Splash Smoothies 6 7 (citrus blend) (8 ounces) Oroweat English Muffins 6 1/2 Stouffer's French Bread Pizza 2 Just over 1 (pepperoni) Yoplait 99% fat-free yogurt 1 8 (strawberry-banana) Baked Lay's potatoe chips 10 1/2 (KC Masterpiece barbecue) Quaker Oats low-fat corn cakes 14 (1 cake) Just over 1 (carmel-chocolate chip) Lunchables 1 19 (turkey and American cheese with drink and dessert) Sobe Elixir 2.5 6 (orange carrot flavor) (8 ounces) Red Bull energy drink 1 Almost 7