Sugar is not toxic. It is an essential nutrient.
Sugar is a class of carbohydrates containing many different molecules of varying complexity.
The simplest sugars are called monosaccharides. They consist of a single carbohydrate, or saccharide (a carbon ring with hydroxyl (H-O) groups and a carbonyl (C=O) attached), hence the name "monosaccharides." Glucose and fructose are monosaccharides. Glucose is the basic chemical energy source for most life forms. Fructose is the sugar that naturally occurs in fruit, vegetables, and some grains.
Disaccharides are sugars made of two saccharides bound together into a larger molecule. Sucrose, maltose, and lactose are disaccharides. Sucrose (table sugar), for example, is a single glucose joined to a single fructose.
There are also trisaccharides and polysaccharides, which are longer chains of saccharides. Polysaccharides is a rather large group that contains not only sugars, but also starches and proteins. Many of the higher sugars are not broken down by the body, instead traveling through the digestive system unchanged, until they get broken down via fermentation by bacteria in the colon.
Higher sugars, starches and proteins aren't really important to this discussion, so let's focus on the monosaccharides glucose and fructose, and the disaccharide sucrose, which constitute refined sugars like white table sugar and corn syrup.
When people talk about sugar being "toxic," they're usually referring to refined sugars, specifically sucrose and fructose. As I mentioned above, sucrose (table sugar) is a disaccharide containing both glucose and fructose. Sucrose gets broken apart by enzymes in the saliva into free glucose and fructose.
Fructose, while being an important nutrient for human health, is harmful in excessive amounts because it is not processed in the stomach and duodenum like other sugars, but is broken down by enzymes in the liver, along a similar metabolic pathway as ethanol. Thus, excessive amounts of fructose and sucrose (especially when taken as a refined food additive in the absence of sufficient amounts of fiber) place an extra burden on the liver. Besides this negative effect on the liver, fructose also does not trigger the insulin reaction that makes you feel full, so many people have a tendency to overeat foods that are high in fructose but low in fiber.
Despite excessive fructose being generally bad for you, processed foods sold in America (and exported to most of the world) contain outrageous amounts of artificially-added fructose, in the form of high-fructose corn syrup (HFCS).
Next time you're in the supermarket, have a look at the ingredient labels. You may be astounded at the number of products that contain HFCS as a major ingredient (at or near the top of the list). There are several reasons for this.
The main reason is economic, resulting from instability of the cane sugar industry in the 1970s. Prior to the mid-'70s, the US imported a large share of its sugar from in the form of cane sugar, mostly from Mexico, Brazil, and other South American countries. But in 1974, the price of sugar peaked, and the US corn producers aggressively began marketing HFCS, a cheaper and more plentiful product, as a substitute. The sugar market boomed again in 1980, and that caused an even greater shift away from sugar toward HFCS in the American food and (especially) soft drink industries. Since the 1980s, the amount of HFCS in American foods has increased dramatically.
There are many economic reasons for food companies to add HFCS. For one thing, it's cheap: In the US, HFCS sells for about half the price of white sugar. It's also heavy. The large amounts of dissolved sugars and solids in HFCS boosts the specific gravity of foods, resulting in an increased shelf weight. Since most preprocessed foods are sold by weight, higher specific gravity means more weight to divide into smaller portions, resulting in greater profits.
HFCS is also hydrophilic—very soluble in water—which makes it a cheaper and easier ingredient to work with than refined sugar. Using HFCS incurs fewer energy costs for heating and stirring to dissolve it. On an industrial scale, factors like this can make a huge economic difference. HFCS is common in canned and jarred foods because its molecules remain locked up in water, so it provides better shelf stability to foods that otherwise probably wouldn't need any added sugar.
Another obvious reason for the popularity of HFCS is because it's sweet, and people like stuff that tastes sweet. American foods tend to be very, very sweet.
Is sugar poisonous? Of course not. But like anything else, overconsumption of sugar can be very bad for your health.
One way to restrict your sugar intake is to avoid sweet soft drinks. Those things contain ridiculous amounts of sugar. Another thing you can do is stay away from preprocessed, ready-to-eat foods, and instead just get off your lazy ass and cook for yourself. That way you can be reasonably sure of what you're eating, instead of getting your meals from a can like a damn dog.
