Melanie asked me in person:
"What's the difference between corn syrup and high fructose corn syrup?"
Before I answer this, let's talk a little bit about sugars. There are three major types of sugar: sucrose, glucose, and fructose. Sucrose is table sugar. It's a disaccharide, meaning that there are two sugar molecules joined together in a single molecule. Those two sugar molecules that make up sucrose are monosaccharides, and they are glucose and fructose.
Corn syrup is, as the name implies, made from corn. This makes it cheaper to produce than sugar, because the US government spends a lot of money to make corn growth a relatively inexpensive thing to do.
Fructose is sweeter than glucose.
Regular corn syrup, the kind you buy for candy making, is primarily glucose suspended in water. You use it in your cooking because you want to keep crystals from forming in your candy or you need something chewy instead or hard or similar.
When manufacturers use corn syrup, they sometimes need it for the same reasons home cooks need it, but more often they use it to save money. And if substituting a less expensive ingredient for an expensive ingredient is one good way to save money, another good way is to use less of the ingredient instead of more. So they treat the corn syrup (with enzymes, apparently) in order to have more fructose than glucose. Because the fructose is sweeter than glucose, they don't need to use as much of it in order to sweeten food.
There is a lot of controversy going on currently about high fructose corn syrup, about how bad it is for you, or about how it's fine in moderation like sugar. Both of these things can be true at the same time.
Current theory on obesity trends starts with an evolutionary concept: we, as a species, adapted to our environment by eating certain foods, by having to work a certain amount to get those foods, and to occasionally get a bonus in high-energy foods that come irregularly. Fats and sugars were infrequent treats that we tried to store away. Vegetables and fruits were relatively easy to get, and animals were not too bad after some work, if we were lucky.
That all changed somewhat when we started farming, but instead of spending energy on hunting or gathering, a similar amount of energy went into raising the crops or animals. For a while, this just evened out the food supply and made it more predictable, which was great. But the energy or, eventually, monetary costs of raising these foods were fairly similar to the energy costs associated with hunting and gathering, so the proportions of foods of different types were still about the same (light on meat, heavy on vegetables, sugar and fat in low supply).
The prevalence of inexpensive corn has changed that landscape significantly, many argue. After all, if sugar, which is pretty expensive to create, can be made both sweeter and cheaper, then it will be put into many more foods than it would if we didn't have the inexpensive supply. That, more than any inherently dangerous property of high fructose corn syrup, is the problem: it is insidious, and it is omnipresent.
If high fructose corn syrup costs, for example, 1/2 of what sugar costs for the same amount of sweetening, and it's a flavor that we, evolutionarily, have come to associate with "eat as much of this as you can because you're not going to get any food for a week or two," then we are going to be putting twice as much of it into our foods, which will result in that much weight gain.
So, yes, if you eat high fructose corn syrup in moderation, it probably will be fine. But it's everywhere and in so many processed foods that the way to avoid it is to avoid the foods that use it. If you are on a budget, as many of us are, then buying foods that aren't sweetened by high fructose corn syrup will have a better chance of reflecting their real costs, which will encourage you to eat in a way that matches how we've evolved to eat.
Of course, it's not that simple, because not only is the corn subsidy reach into other aspects of food (especially meat), but it's not the only subsidized food. But, if you want to try to control your weight and become healthier, avoiding commodity foods is a really good way to start. Pay for things what they really cost, and things will start to line up properly.