Refined sugar does not cause allergic reactions — there is no immune-mediated allergy to sugar.

However, chronic high sugar intake promotes inflammation through several pathways: it drives the formation of advanced glycation end-products (AGEs) that increase inflammatory cytokines, it feeds Candida and other yeasts in the gut that can disrupt the microbiome, and it raises levels of inflammatory signalling molecules throughout the body. A clinical trial that tested sugar's acute effect on eczema by giving participants sugar versus placebo in a controlled challenge found no significant immediate effect. This suggests that any sugar-eczema link is a chronic, cumulative one rather than a direct trigger, making it more useful to focus on reducing overall processed food intake than on eliminating sugar as a single food.