When fructose is joined to glucose, it makes sucrose. Sucrose is abundant in sugar cane, sugar beets, corn, and other plants. When extracted and refined, sucrose makes table sugar. In the 1800s and early 1900s, the average American took in about 15 grams of fructose (about half an ounce), mostly from eating fruits and vegetables. Today we average 55 grams per day (73 grams for adolescents). The increase in fructose intake is worrisome, says Lustig, because it suspiciously parallels increases in obesity, diabetes, and a new condition called nonalcoholic fatty liver disease that now affects up to one-third of Americans. (You can read more about nonalcoholic fatty liver disease in a Harvard Health Letter article.)
Virtually every cell in the body can use glucose for energy. In contrast, only liver cells break down fructose. What happens to fructose inside liver cells is complicated. One of the end products is triglyceride, a form of fat. Uric acid and free radicals are also formed.
None of this is good. Triglycerides can build up in liver cells and damage liver function. Triglycerides released into the bloodstream can contribute to the growth of fat-filled plaque inside artery walls. Free radicals (also called reactive oxygen species) can damage cell structures, enzymes, and even genes. Uric acid can turn off production of nitric oxide, a substance that helps protect artery walls from damage. Another effect of high fructose intake is insulin resistance, a precursor to diabetes.
http://www.health.harvard.edu/blog/is-fructose-bad-for-you-201104262425
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