The need for ATP in living cells is constant and immediate, leading to the continuous synthesis of ATP. Humans produce approximately the same amount of ATP as their body weight on a daily basis. However, as humans do not eat continuously throughout the day, there is an intermittent supply of fuel to meet the constant cellular demand for ATP. Diet-derived fuels are used for ATP production, stored for future use, or converted to other fuels based on the metabolic phase, fuel type, and cell type involved.

To understand energy metabolism, it is important to consider metabolic phases, which are determined by the availability of specific fuels and regulated by phase-specific hormones. There are three main metabolic phases: the digestive or absorptive phase, occurring during the digestion of a meal; the interdigestive or postabsorptive phase, which occurs between meals; and the fasting phase, typically experienced between the last meal before bedtime and breakfast.

Fuel sources during these phases include diet and the release of stored fuel, especially during sleep or periods of fasting or prolonged exercise. ATP is constantly needed for various cellular functions such as structural integrity, differentiated function, growth and division, response to stimuli and stress. The resting metabolic rate accounts for 60-70% of energy output, while movement contributes to 25-30%. Glucose, free fatty acids, amino acids, and ketone bodies are oxidized to produce energy, with 60% of energy released as heat.

Different cell types, particularly hepatocytes, adipocytes, and skeletal myocytes, have a significant impact on whole-body metabolism. The metabolic actions of these cells differ and are crucial in energy metabolism. Insulin plays a dominant role in the digestive phase, while during the fasting phase, insulin levels decrease, allowing for metabolic adaptations associated with fasting. Counterregulatory hormones such as glucagon, epinephrine, and norepinephrine stimulate metabolic pathways in response to the absence of ingested and absorbed fuels. Growth hormone also contributes to fasting-phase metabolism.
Fig. 39.1 ). This means that the need for ATP is immediate and unending, and consequently all living cells must continually synthesize ATP. In fact, humans produce about the equivalent of their body weight in ATP daily. This places a demand on the body to continually supply fuel in some form to all cells. All fuel originates from the diet, but humans do not eat in a nonstop manner all day long. Thus the constant cellular demand for fuels to make ATP and perform work is paired with an intermittent intake of fuels. Diet-derived fuels are oxidized for ATP, but some are also stored for future use, and some are converted to other fuels that can be used by other cell types, depending on the metabolic phase, type of fuel, and cell type in question.
In trying to make sense of energy metabolism, it is important to organize one’s thinking around the following: 1. Metabolic phases. Metabolic phases refer to the hourly and daily differences in fuel usage and energy metabolism, which are dictated largely by the abundance or scarcity of certain fuels and orchestrated by phase-specific hormones. In general there are three metabolic phases (
Fig. 39.2 ): (1) the digestive or absorptive phase, which occurs during the 2 to 3 hours it takes to digest a meal; (2) the interdigestive or postabsorptive phase, which normally occurs between meals; and (3) the
Sources of fuels 1.
2.
Diet—Release of stored fuel during postabsorptive stage (especially during sleep on a daily basis, or during a fast or prolonged exercise)
ATP constant amount universal not stored Structural integrity Differentiated function Growth and division Response to stimuli and stress Output 60%–70% = Resting metabolic rate 25%–30% = Movement Glucose, FFAs, AAs, ketone bodies EnergyOxidation of fuels 60% Heat 40% •Fig. 39.1 Overviewofenergymetabolism.
•Fig. 39.2 Bloodglucoselevelsduringthethreemetabolicphases:D,digestivephase;F,fastingphase;I,interdigestivephase.B,break-fast;D,dinner;L,lunch.Red arrow indicatestheupperlimitfornormalfastingglucose;green arrow indicatesthelowerlimitfornormalfastingglucose.FFB150100BloodGlucose(mg/dL)50LDDDDII fasting phase, which most commonly occurs between the last snack before bedtime and breakfast. (In fact, physicians refer to a blood value as “fasting,” e.g., “fasting blood glucose,” if the patient abstains from eating after midnight and has blood drawn about 8 AM; prolonged fasting and starvation are more extreme forms of fasting.) Physical exertion, which imposes a heightened energy demand, is another type of metabolic phase that occurs with some frequency and regularity for some individuals. This chapter primarily compares how metabolism differs between the digestive phase and the fasting phase, and how different hormones orchestrate these metabolic differences.
2, Metabolic actions of hepatocytes, adipocytes, and skeletal myocytes. All cells are involved in energy metabolism, but these three cell types have a profound impact on whole-body metabolism. Key features with respect to metabolism of these three cell types are listed in
Table 39.1 3, Insulin and the counterregulatory hormones. Metabolism during the digestive phase is orchestrated almost entirely by insulin. During the fasting phase, insulin drops to low levels, and this alone allows for some of the metabolic adaptions associated with fasting. In addition, glucagon and catecholamines (epinephrine, norepinephrine) stimulate metabolic pathways that integrate the body’s response to an absence of ingested and absorbed fuels. These hormones are referred to as counterregulatory hormones based on their opposition to insulin. Growth hormone (see ) also contribute somewhat to fasting-phase metabolism.