The body acquires most of its energy from the oxidative metabolism of glucose. Glucose, a simple six-carbon sugar, enters the diet as part of the sugars called sucrose, lactose, and maltose and as the major constituent of the complex polysaccharides called dietary starch. Complete oxidation of glucose yields carbon dioxide (CO2), water, and energy that is stored as adenosine triphosphate (ATP).
If glucose is not immediately metabolized, it can be stored in the liver or muscle as glycogen. Unused glucose can also be converted by the liver into fatty acids, which are stored as triglycerides, or into amino acids, which can be used for protein synthesis. The liver is pivotal in distributing glucose as needed for immediate fuel or as indicated for storage or for structural purposes. If available glucose or glycogen is insufficient for energy needs, the liver can synthesize glucose from fatty acids or even from protein-derived amino acids.2
Glucose fuels most cell and tissue functions. Thus, adequate glucose is a critical requirement for homeostasis. Many cells can derive some energy from burning fatty acids, but this energy pathway is less efficient than burning glucose and generates acid metabolites (e.g., ketones) that are harmful if they accumulate in the body. Many hormones (Table 5-1) participate in maintaining blood glucose levels in steady-state conditions or in response to stress. Measures of blood glucose indicate whether the regulation is successful. Pronounced departure from normal, either too high or too low, indicates abnormal homeostasis and should initiate a search for the etiology.3 The causes of abnormal blood glucose levels are summarized in Table 5-2.
Two major methods are used to measure blood glucose: chemical and enzymatic. Chemical methods use the nonspecific reducing properties of the glucose molecule. In enzymatic methods, glucose oxidase reacts with its specific substrate, glucose, liberating hydrogen peroxide, the effects of which are then measured. Values are 5 to 15 mg/dL higher for the reducing (chemical) methods than for enzymatic techniques because blood contains other reducing substances in addition to glucose. Urea, for example, can contribute up to 10 mg/dL in normal serum and even more when uremia exists. Several different indicator systems are used for automated enzymatic methods, yielding somewhat different normal values.4
Note also that, in the past, blood glucose values were given in terms of whole blood. Today, most laboratories measure serum or plasma glucose levels. Because of its higher water content, serum contains more dissolved glucose, and the resultant values are 1.15 times higher than are those for whole blood. Serum or plasma should be separated promptly because red and white blood cells continue to metabolize glucose. In blood with very high white blood cell levels, excessive glycolysis may actually lower glucose results. Arterial, capillary, and venous blood samples have comparable glucose levels in a fasting individual. After meals, venous levels are lower than those in arterial or capillary blood.5