d-Allulose Ameliorates Fructose-Induced Skeletal Muscle Insulin Resistance via Regulation of Ectopic Lipid Accumulation Independent of Body Weight Changes

Background/Objectives: The consumption of fructose-sweetened beverages, especially when combined with a high-fat (HF) diet, substantially contributes to obesity, diabetes, and metabolic dysfunction-associated steatotic liver disease. Ectopic fat accumulation in skeletal muscles is a critical factor in the development of insulin resistance, a key feature of these metabolic disorders. We aimed to investigate the effects of the rare sugar, d-allulose, on fructose-induced insulin resistance. Methods: Male Wistar rats were randomly assigned to fructose-free control diet (CD), HF/fructose-free diet (HF), or HF/fructose diet (HFF) groups. After 4 weeks, an intraperitoneal glucose tolerance test (IPGTT) was performed, followed by a two-step hyperinsulinemic–euglycemic clamp (HE-clamp) test at 5 weeks. Blood, skeletal muscle, and liver samples were collected after 6 weeks, and triglyceride (TG) levels were measured. Additionally, Western blot was performed on skeletal muscle samples. The same protocol was repeated for the HFF group supplemented with either 5% d-allulose or 5% cellulose. Results: Compared to the CD and HF groups, the HFF group exhibited increased blood glucose levels during the IPGTT and greater systemic and skeletal muscle insulin resistance in the HE-clamp. Furthermore, plasma, liver, and muscle TG levels were significantly elevated in the HFF group. However, d-allulose supplementation improved insulin resistance in the HFF group and reduced blood, liver, and muscle TG levels. Additionally, insulin-stimulated AKT phosphorylation and acetyl-CoA carboxylase phosphorylation were enhanced in the skeletal muscle following d-allulose administration. Conclusions: d-allulose may improve insulin resistance by reducing TG accumulation in the skeletal muscle, potentially independent of its anti-obesity properties.