High fructose diets may harm the immune system Written by Beth JoJack on February 26, 2021 — Fact checked by Zia Sherrell, MPH Excessively high amounts of the sugar fructose, particularly in manufactured sweeteners, may pose a threat to the immune system. Thanyathep Eakphaitoon/Getty Images A new study suggests that eating a diet high in the sugar fructose may cause the immune system to become inflamed. This process produces more reactive molecules, which are also associated with inflammation. Inflammation can damage cells and tissues and lead to disease. A new study suggests that a diet high in the sugar fructose may prevent the immune system from functioning properly.