Steady glucose levels help support steady energy levels. When your glucose is oscillating up and down like a roller coaster, this can cause feelings of fatigue. Additionally, unsteady glucose can impair sleep, which can further contribute to fatigue.
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Learn more:
Breymeyer, K. L., et al. (2016). "Subjective mood and energy levels of healthy weight and overweight/obese healthy adults on high-and low-glycemic load experimental diets." Appetite 107: 253-259.