Earlier, we examined how complex carbohydrates are digested into biochemically useful molecules, such as glucose (Chapter 14). Glucose is the principal carbohydrate in living systems and an important fuel. In mammals, it is the only fuel that the brain uses under nonstarvation conditions and the only fuel that red blood cells can use at all. Indeed, almost all organisms use glucose, and most process it in a similar fashion. Why is glucose such a prominent fuel, rather than some other monosaccharide? We can speculate on the reasons. First, glucose is one of several monosaccharides formed from formaldehyde under prebiotic conditions, and so it may have been available as a fuel source for primitive biochemical systems. Second, glucose is the most stable hexose because the hydroxyl groups and the hydroxymethyl group are all in the equatorial position, minimizing steric clashes. Third, glucose has a low tendency, relative to other monosaccharides, to nonenzymatically glycosylate proteins. In their open-chain forms, monosaccharides contain carbonyl groups that can covalently modify the amino groups of proteins. Such nonspecifically modified proteins often do not function effectively. Glucose has a strong tendency to exist in the ring formation and, consequently, relatively little tendency to modify proteins.