Editor's note: Nationally recognized nutrition expert Marion Nestle answers readers' questions in Food Matters, written exclusively for The Chronicle. E-mail your questions to food@sfchronicle.com, with "Marion Nestle" in the subject line.

Q:What is the difference, metabolically speaking, between high-fructose corn syrup and other carbohydrate-based sweeteners (sucrose, fructose, honey and so on)?

A: From what I hear these days, high-fructose corn syrup (HFCS) is widely perceived as the new trans fat - something to be avoided at all costs. But, stop: HFCS is not poison. It is just sugar in liquid form, differing from common table sugar (sucrose) mainly in how it affects the texture of foods.

I can see why HFCS seems like a nutritional villain: It is a marker for junk foods. Cheaper than sucrose, it turns up in all kinds of processed foods, particularly soft drinks. And there is nearly as much of it in the food supply as sucrose - 56 pounds per year per person versus 62 pounds for table sugar.

In its new advertising campaign, the Corn Refiners Association says of HFCS, "Truth is, it's nutritionally the same as table sugar." Truth is, I'd call it almost the same.

Sucrose is a double sugar made of two single sugars - glucose (50 percent) and fructose (50 percent) - stuck together. HFCS also contains glucose and fructose, but the sugars are already separated and their percentages differ slightly. Because sucrose is quickly split by digestive enzymes, the body can hardly tell them apart. For the record, glucose is blood sugar, fructose is fruit sugar, and honey contains both.

The processing of sucrose involves boiling it down from sugar cane or beets, and washing, clarifying, filtering and drying the syrup. HFCS starts out as corn, of course, and you, too, can do what the makers of the indie movie, "King Corn" (2007), demonstrated in my favorite scene.

First, extract the starch. Use enzymes to break down the starch to glucose and to convert some of the glucose to fructose. Then do a bunch of refining, separation and evaporation steps. The resulting syrup is 55 percent fructose, with the rest composed of glucose or undigested starch pieces. The HFCS used in soft drinks has a bit more fructose than sucrose - 55 percent as opposed to 50 percent.

Whether this 5 percent difference matters at all depends on whether you are a metabolic optimist or pessimist.

If you are an optimist, you are happy that fructose - unlike glucose - does not stimulate the release of insulin, and in small amounts can be a useful sweetener for people with diabetes.

If you are a pessimist, you will fret that fructose is preferentially metabolized to fat, raising the possibility that HFCS - or any other source of fructose (but we won't worry about fruit) - could have something to do with current obesity trends.

HFCS entered our food supply in the mid 1960s, but did not really come into its own until farm subsidies encouraged farmers to grow as much corn as possible. In 1981, at the dawn of the obesity era, the United States food supply provided 23 pounds of HFCS per person per year, along with 79 pounds of sucrose - 102 pounds total.

Today, the balance is 56 to 62 (118 pounds), with the increase entirely due to HFCS. Guilt by association! Glucose corn syrups and honey add up to yet another 18 pounds, but their use has not changed much over time. All told, the food supply provides a third of a pound a day of HFCS and sucrose combined, which works out to about 600 calories a day per person, just from these two sources.

Note that these are available calories, not necessarily those eaten. Availability refers to sugars produced, plus imports, less exports. Even so, people who drink sodas all day long can get a substantial portion of their daily calories from HFCS. Like other sugars, HFCS supplies calories but is devoid of nutrients.

Although only about 6 percent of U.S. corn is used to make corn sweeteners, it is 6 percent of a large number. Corn production is subsidized, and subsidies encourage greater production. Until recently, subsidies drove the cost of corn sweeteners well below that of sucrose, which gets price supports. With corn now going for ethanol, HFCS is more expensive and has less of a price advantage.

Indeed, some food companies have already replaced HFCS with sucrose and are advertising their products as "HFCS-free." At least one grocery chain has said it will no longer carry products containing HFCS. Such events, along with concerns about the metabolic effects of fructose, saddle the Corn Refiners with a challenge - how to convince the American public that HFCS is no worse than any other sugar.

Their methods? First, they successfully petitioned the Food and Drug Administration to allow HFCS to be labeled "natural." Despite the many steps required to process cornstarch into HFCS, the FDA granted their request. Why? Because in processing the enzymes are fixed to a column and the sugars do not come in contact with the synthetic fixing agents. I'm not kidding about this.

Next, the Corn Refiners funded a $30 million counterattack. If you missed the full-page newspaper ads, take a look at the Web site, www.sweetsurprise.com.

HFCS has a big public relations problem, but I don't get this campaign. Since when is insulting the intelligence of critics an effective marketing strategy?

I cannot decide which aspects of the campaign are most offensive: The videos of inarticulate critics insulted by their HFCS-savvy friends? The slogans ("HFCS has no artificial ingredients")? The quiz questions ("Which of the following sweeteners is considered a natural food ingredient: HFCS, honey, sugar, or all of the above")? Or the irrelevant take-home message ("As registered dietitians recommend, keep enjoying the foods you love, just do it in moderation")?

I'm not a registered dietitian and maybe that is why I think moderation doesn't work for HFCS. Yes, HFCS has a place in the American diet and sometimes has cooking advantages over sucrose. And the research is still out on whether HFCS differs from sucrose metabolically. But the most sensible approach to HFCS and to sugars in general is not moderation. It is, "Eat less."