Functional Reformulation of Shortbread Cookies with Urtica dioica L. Powder and Erythritol: Effects on Sensory Quality, Glycaemic Index and Glycaemic Load

Background/Objectives: Reformulation of bakery products with functional ingredients and reduced-sugar alternatives is an important strategy for improving nutritional quality and lowering postprandial glycaemic response. This study evaluated the effects of Urtica dioica L. powder incorporation and sucrose replacement with erythritol on the sensory properties, glycaemic index (GI) and glycaemic load (GL) of shortbread cookies. Methods: Cookies were prepared by replacing wheat flour with 0–50% nettle powder and using either sucrose or erythritol as a sweetener. Sensory evaluation was performed by 120 consumers using a 9-point hedonic scale. Based on sensory acceptance, cookies containing 0%, 10% and 20% nettle powder were selected for in vivo determination of GI and GL in 22 healthy adults according to ISO 26642:2010. Data were analysed using one- and two-way ANOVA. Results: Increasing nettle incorporation significantly reduced sensory acceptance, particularly for colour, taste, odor and overall acceptability, whereas cookies containing 10–20% nettle powder remained well accepted. Nettle addition significantly reduced GI in both sucrose- and erythritol-sweetened cookies, from 73.77 ± 11.05 to 52.10 ± 11.02 and from 68.69 ± 12.10 to 48.10 ± 9.03, respectively. The lowest GI (48.10 ± 9.03) was observed in cookies containing 20% nettle powder and erythritol. Erythritol did not significantly affect GI (p = 0.797), but significantly reduced GL compared with sucrose (p < 0.001). The lowest GL was observed for the 20% nettle-erythritol formulation (4.16 ± 0.80). No significant interaction between nettle powder addition and sweetener type was observed for either GI or GL. Conclusions: Incorporation of 10–20% nettle powder provided the most favourable balance between sensory acceptance and reduced glycaemic impact, while higher substitution levels substantially impaired sensory quality. Erythritol further reduced GL by lowering the amount of available carbohydrates in a standard serving, although it did not significantly affect GI under the carbohydrate-standardised conditions used for GI determination. The combination of moderate nettle enrichment and erythritol may therefore provide a lower-GI, lower-GL food option with acceptable sensory properties. However, these findings are based on a small cohort of healthy young adults and should not be interpreted as evidence of a therapeutic effect or directly extrapolated to populations with impaired glucose metabolism.