For the past billion or so years, ever since photosynthesis has become a major driving factor in the earth's biogeochemical cycle, the earth has had an oxidizing atmosphere. Anything placed in that atmosphere will be changed by it. Basalt weathers, wood rots and hydrocarbons combust rapidly. And living things, besides a few prokarya living in the bottom of oxygen-starved marshes, have become accustomed to that. Most living things have adapted to living in an oxygen rich atmosphere, which both assaults their cells and structures, and also enables them to make energy.
All animals live by combining atmospheric molecular oxygen with carbon chains. There are more steps to it than that: some carbon chains are stored as fats, while others are the water soluble sugar. Many animals have quite a bit of metabolic machinery to transform various hydrocarbons into a form that is more easily transported and metabolized. But in short, all animals do is combine carbon chains with oxygen molecules to produce energy, carbon dioxide and water.
All of this is a bit of a run-up before explaining what high fructose corn syrup is. High fructose corn syrup is produced from corn starch, and is a disaccharide, consisting of two smaller sugar molecules. These molecules are glucose and fructose. Fructose is sweeter than glucose, so a smaller amount of high fructose corn syrup can produce more sweetness. And since corn is raised in overwhelming abundance in the United States, producing sweetener from corn starch is more economical than sugar from cane or beets. Fructose, in various forms, is natural to different fruits, in amounts from a trace to a good proportion. Human's natural fuel is glucose, but in whatever long years of evolution we lived in "the wild", we couldn't be picky about what type of energy we were taking in. Fructose would have come to us in a steady stream, although perhaps small. Our bodies had one way or another to process fructose, both to make sugar and to clear it out of our systems. Fructose is, after all, another sugar, and our body has the machinery to deal with sugars more exotic than fructose, such as sorbitol or xylitol.
High fructose corn syrup has become somewhat of a bogeyman in health circles as of late. There is some thought that it can be linked to any number of health problems, such as diabetes and obesity. And there is probably someone out there on the wide internet who has linked it to autism spectrum disorder or restless leg syndrome. I don't know if any of those are true. The human body has a complicated array of metabolic pathways, and they all differ from person to person, and they can change easily based on circumstances. An experiment to see how our body processes food would probably be a poor model with what actually goes on in our cells. But I do know that fructose is a relatively simple molecule, made out of carbon, oxygen and hydrogen, the elements that our metabolic machinery has been processing for several billion years. There is nothing particularly odd about fructose. No sulfur bonds, no heterocyclic rings, no cheleated metal atoms. Just the basics.
Which makes me somewhat skeptical of claims that high fructose corn syrup is an insidious and mysterious poison. Especially since most foods high in high fructose corn syrup are not exactly cavalcades of nutrition. A person who spends their weekends eating Little Debbie Nutty Bars on the couch while watching television isn't getting fat because high fructose corn syrup is a mysterious goblin. They are getting fat because of the simple physics of energy taken in as food being greater than energy expended in exercise.
Although, as mentioned, the body's cellular mechanisms are hard to understand completely, the spotlighting of high fructose corn syrup as a mysterious insidious culprit in health problems seems to be overkill, when quite simpler principles of diet and exercise account for fitness in a much more obvious way.