In this study people gargled sugary drinks, but not swallow. They showed more willpower. This shows it isn’t an increase in blood sugar that fixes your will. Instead it is a reward. The body’s reaction to sweetness is a reward.
Citation: Hagger, M., and Chatzisarantis, N. “The Sweet Taste of Success: The Presence of Glucose in the Oral Cavity Moderates the Depletion of Self-Control Resources.” Personality and Social Psychology Bulletin, 2012
Sanders, M., Shirk, S., Burgin, C., and Martin, L. “The Gargle Effect: Rinsing the Mouth With Glucose Enhances Self-Control.” Psychological Science, 2012