Glucose evolved to be the principal sugar in mammals because it is the least chemically reactive and, therefore, the least harmful in modifying other molecules when it is at a higher concentration than other sugars. In blood, it has a concentration near 5 mM, with a range of 3.9–7 mM considered to be normal. Glycosylation of DNA can lead to mutations. Explanation of glycogen synthesis and the conversion of glycogen back to glucose, when stimulated by hormones. Glucose is stored as glycogen to minimize the water needed to solubilize many individual glucose molecules. Activation of glycogen breakdown to glucose is initiated by adrenaline or glucagon. Within liver or muscle cells, a cascade of protein kinase enzymes leads to the activation of glycogen phosphorylase, which produces the needed glucose-6-P; in liver cells, this is dephosphorylated and transported into the blood as free glucose, to be available to all tissues, but mainly to be used by the brain and red blood cells. The two key regulatory enzymes in this process are regulated by ligands that have a short lifetime, thereby controlling the duration of the process. Description of gluconeogenesis from amino acids and ketones. Description of different glucose transporters and of the four hexokinase isozymes. Lactate dehydrogenase converts NADH back to NAD+ during amplified glycolysis. The main regulatory enzymes controlling glycolysis are phosphofructokinase, pyruvate kinase, and lactate dehydrogenase. Brief description of the citric acid cycle and oxidative phosphorylation. Discussion of energy sources during short-term starvation and long-term starvation. A brief description of fatty acids and triglycerides.