Fructose was long regarded as a natural and healthy sweetness. Today it is known that fructose, which is widely used by the food industry in the form of corn syrup, is particularly easy converted into fat, promotes overweight, is considered the main cause of the rapid increase in obesity and metabolic diseases, and can damage the heart. And Marcus Goncalves, Weill Cornell Medicine, New York, USA, and colleagues have shown in mice that there is a direct molecular mechanism between fructose consumption and colon cancer.

The researchers gave mice with a genetic predisposition to colorectal cancer a 55:45 fructose-glucose solution once a day (equivalent to the sweetener used in most soft drinks). Transferred to humans, the amount corresponds to one can of lemonade per day. The mice remained of normal weight throughout the experiment. 

After two months, the researchers checked the condition of the colon of all animals. The mice did not have more intestinal tumors than control animals without sugar consumption, but the tumors were significantly larger and more aggressive.
From the mixture of fructose and glucose typical for corn syrup, both sugars accumulate strongly in the large intestine and are absorbed by the tumors. In cancer cells, an enzyme called ketohexokinase (KHK) converts fructose into fructose-1-phosphate. This molecule promotes tumor growth in two ways: (i) It facilitates the use of glucose by tumor cells as an energy source. (ii) It promotes the synthesis of fatty acids. Cancer cells need fatty acid synthesis to form cell membranes, generate and store energy, and for intracellular communication. If the researchers deactivated one of the two fructose pathways through gene blockades, tumor growth failed to occur.

Whether corn syrup and correspondingly sweetened lemonades also have this tumor-promoting effect in humans still has to be tested. The researchers believe that this is very likely.

• High-fructose corn syrup enhances intestinal tumor growth in mice,
  Marcus D. Goncalves, Changyuan Lu, Jordan Tutnauer, Travis E. Hartman, Seo-Kyoung Hwang, Charles J Murphy, Chantal Pauli, Roxanne Morris, Sam Taylor, Kaitlyn Bosch, Sukjin Yang8, Yumei Wang8, Justin Van Riper, H Carl Lekaye, Jatin Roper, Young Kim, Qiuying Chen, Steven S. Gross, Kyu Y. Rhee, Lewis C. Cantley, Jihye Yun,
  Science 2019, 363(6433), 1345–1349.
  https://doi.org/10.1126/science.aat8515