The brain, eye, and kidney are some of the organs that have glucose as the sole metabolic fuel source. Prolonged fasting or vigorous exercise depletes glycogen stores, making the body switch to de-novo glucose synthesis to regulate blood levels of this monosaccharide. Gluconeogenesis is the process that permits the body to form glucose from non-hexose precursors, especially glycerol, lactate, pyruvate, propionate, and glucogenic amino acids.
E. et al. (Tue,) studied this question.