Post-absorptive state

In the post-absorptive state with the subject at rest not less than 12 h after the last meal, protein catabolism has been completed. An RQ... 

Figure 5-5. Metabolic activities of major organs during a short-term fast. The importance of the liver in providing glucose to support the brain and other glucose-requiring organs in the post-absorptive state is illustrated. The body relies on available glycogen stores as a ready source for glucose as fuel. PPP, pentose phosphate pathway FA, fatty adds TAG, triacylglycerol.

Which one of the following is elevated in plasma during the absorptive period (compared with the post-absorptive state) ... 

BMR is the amount of energy expended while at rest in a neutrally temperate environment, in the post-absorptive state (expressed in kcal/day). BMR decreases with age and with the loss of lean body mass. Increasing muscle mass increases BMR. Illness, previously consumed food and beverages, environmental temperature and stress levels can affect overall energy expenditure as well as the BMR. BMR is accurately determined by gas analysis (direct or indirect calorimetry) an estimation can be found using the equation ... 

FA stored within the TAG of adipose ceUs constitute the vast majority of energy reserves in animals. While the TAG pool in adipocytes provides a reservoir of energy for times of prolonged restricted caloric intake, there is also a normal acute flux of FA into and out of adipose tissue in response to meals and other factors, such as insuhn [8, 9J. TAG hydrolysis in adipocytes, termed hpolysis, contributes to the increased levels of plasma NEFA by two mechanisms, one direct and the other indirect. In the post-absorptive state or during starvation FA are normally released from adipocytes directly into the plasma and transported to various tissues where they serve as a source of energy. In the insulin-resistant state this direct release of FA is exacerbated, because the reduction in the anti-lipolytic action of insuhn (see below) leads to increased hpolysis and a consequent increase in the release of FA from adipocytes into plasma. In contrast to the postabsorptive direct FA release, there is... 

The specific roles of various tissues and organs and their interdependence on amino acid metabolism are discussed here. An overview of this topic is given in Chapter 22. In the post absorptive state, maintenance of steady-state concentrations of plasma amino acids depends on release... 

Early work using the lymph fistula rat demonstrated that the amount of cholesterol appearing in intestinal lymph is directly proportional to the amount of dietary cholesterol present in the intestinal lumen [102]. Although less complete data are available in other species, this relationship also appears to be true in rabbit [103] and man [104]. Thus, in the post-absorptive state, after the intake of a diet containing substantial amounts of sterol, most cholesterol in intestinal lymph is probably derived from the diet [98]. 

Normal concentrations in the blood plasma are in the range 1200-2200 mg/1. Some 30 per cent of this is in the free state, the remainder being bound to lipoproteins. These are complexes of proteins and lipids held together by non-co-valent bonds. Each has a characteristic size, molecular weight, chemical composition and density. They are classified on the basis of their density. The five classes, of which one, the chylomicrons, occurs only in the post-absorptive state, are shown in Table 3.5. 

This replacement in the mixture of carbohydrate by fat is accompanied by a decline in the non-protein respiratory quotient, and when the theoretical value for fat (0.7) is reached it can be assumed that energy is being obtained only from body reserves. In ruminants, an additional indication that the post-absorptive state has been reached is a decline in methane production (and therefore digestive activity) to a very low level. 

Ingestion, absorption and deposition of fat into adipose tissue is considerably high (i.e. 60-100 g/day). In the post-absorptive state, 50-90% of the body s total energy needs, including that of the heart, is met by free fatty acids delivered from adipose tissue. The total movement of fatty acids from one tissue to the other is voluminous and fast, while that of cholesterol and phos-phohpids is less and rather slow. The daily turnover of cholesterol, representing the transfer from the catabohc to the anabolic sites within the body, approximates 1 g. More than 90% of the cholesterol leaves the body in the feces as bile acids and predominantly as cholic acid. 

In the post-absorptive state stored fat is presented to the tissues in four forms free fatty acids derived directly from adipose tissue triglycerides formed in the liver from fatty acids and acetoacetate and hydroxybutyrate formed by the partial oxidation of free fatty acids. Free fatty acids and ketone bodies are readily oxidizable. It may be that more than one form is necessary because free fatty acids alone, owing to their low solubility and their relative toxicity, cannot be transported in sufficient quantities to meet the fuel requirements of some tissues, such as heart muscle. The other possibility for the existence of multiple forms of this substrate is that it allows direction of particular fuels to specific organs. 

BMR Basal metabolic rate Energy expenditure in the post-absorptive state measured under standardized conditions of thermal neutrality (environmental temperature 26-30 C), awake but completely at rest... 

In the fasting state (sometimes known as the post-absorptive state, as it begins about 4-5 hours after a meal, when the products of digestion have been absorbed) metabolic fuels enter the circulation from the reserves of glycogen, triacylglycerol and protein... 

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