First, when you see it on the label it's a proxy for a highly processed food. HFCS is not found in corn. It results from an industrial process using centrifuges and enzymes. This process dates from the 1970s. HFCS is added to soft drinks, fruit juice blends, cereal, baked goods, candy, and almost any processed food you can think of.
What is HFCS? From Jane Brody:
High-fructose corn syrup is made by converting the starch in corn to a substance that is about 90 percent fructose, a sugar that is sweeter than the sugar that fuels the body cells, called glucose, and processed differently by the body. The fructose from corn is then mixed with corn syrup, essentially pure glucose, to produce one of two mixtures called high-fructose corn syrup: 55 percent fructose and 45 percent glucose, which is used to sweeten soft drinks, and 42 percent fructose and 58 percent glucose, which is used in products like breads, jams and yogurt.What's the worst thing HFCS does? It makes it very easy to consume a lot of calories. It's cheap, up to 70% cheaper than sugar. Manufacturers use it in many sweet beverages, which are often sold in "Big Gulp" or "SuperSize" containers. There's evidence that when you drink your calories you don't as quickly feel as if you've had enough to eat, and if you thus add lots of calories to your diet, you gain weight. The presence in the American diet of HFCS-sweetened beverages may have contributed to a rise in obesity.
Neither substance is radically different from ordinary sugar, which is 50 percent fructose and 50 percent glucose. The main difference is that in high-fructose corn syrup, the two sugar molecules are chemically separated, and in sucrose they are linked. Whether this difference is meaningful to health is still debated. [From "America’s Diet: Too Sweet by the Spoonful" by Jane Brody, NY Times, February 2009.]
HFCS calories may or may not be metabolized exactly the same way as glucose: there is a chemical difference, and fructose is “really not part of our natural diet. Fruit contains only tiny amounts of it. We’ve gone from a few grams of it a day to tablespoons of it," says Dr. Barry M. Popkin, a nutrition professor (cited in the Brody article). According to an editorial "How bad is fructose?" in the American Journal of Clinical Nutrition (October 2007):
Why is fructose of concern? First, it is sweeter than either glucose or sucrose. In fruit, it serves as a marker for foods that are nutritionally rich. However, in soft drinks and other "sweets," fructose serves to reward sweet taste that provides "calories," often without much else in the way of nutrition. Second, the intake of soft drinks containing high-fructose corn syrup (HFCS) or sucrose has risen in parallel with the epidemic of obesity, which suggests a relation. Third, the article in this issue of the Journal and another article published elsewhere last year implicate dietary fructose as a potential risk factor for cardiovascular disease.There is also evidence of HFCS being associated with heart and kidney disease, as summarized in an article from 2008:
Fructose differs in several ways from glucose, the other half of the sucrose (sugar) molecule. Fructose is absorbed from the gastrointestinal tract by a different mechanism than that for glucose. Glucose stimulates insulin release from the isolated pancreas, but fructose does not.
End-stage renal disease rates rose following widespread introduction of high fructose corn syrup in the American diet, supporting speculation that fructose harms the kidney. Sugar-sweetened soda is a primary source of fructose. ...In another article in the NY Times, "Fructose-Sweetened Beverages Linked to Heart Risks," Nicholas Balakar reports that "a controlled and randomized study has found that drinks sweetened with fructose led to higher blood levels of L.D.L, or 'bad' cholesterol, and triglycerides in overweight test subjects, while drinks sweetened with another sugar, glucose, did not. Both L.D.L. and triglycerides have been linked to an increased risk of cardiovascular disease." This study, published in April, 2009, tracked and controlled the food intake of subjects. It studied fructose, not HFCS, but the results suggest that consumption of fructose has serious risks; however, the findings "do not imply that anyone should avoid fruit, which contains only small amounts of fructose and has other important nutritional benefits."
CONCLUSIONS: Findings suggest that sugary soda consumption may be associated with kidney damage, although moderate consumption of 1 or fewer sodas does not appear to be harmful. Additional studies are needed to assess whether HFCS itself, overall excess intake of sugar, or unmeasured lifestyle and confounding factors are responsible. [From "Sugary soda consumption and albuminuria: results from the National Health and Nutrition Examination Survey, 1999-2004."]
Most of the articles that I read agree that more study is needed to make an unequivocal link between HFCS and disease. As for weight gain: it's no improvement to substitute beverages and foods sweetened with other calorie-equivalent sweeteners such as sugar. I suspect that the small additions of HFCS to packaged breakfast cereal shouldn't stop me from eating one bowl per day.
Very serious public health issues surround the continued consumption of high amounts of sweetened beverages and foods, and the cheap trick that HFCS makes things taste good has implications for the collective weight of the nation. Lots of experts have already said this, and I hope someone listens.