Whole Body Metabolism of Protein and Contributions of Individual Organs

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From the above discussion of tracers of amino acid and protein metabolism, it is clear that the body is not static and that all compounds are being made and degraded over time. A general balance of the processes occurring is shown in Figure..2.6 for an average adult. In diabetics treated with insulin, Nair et al. measured leucine and phenylalanine kinetics in the whole body as well as leucine and phenylalanine tracer balances across a leg and across the splanchnic bed ( 123). This work measured directly in humans what has been assumed from a composite of the measurements shown in Figure...2,6. They found that approximately 250 g of protein turns over in a day on the basis of the leucine and phenylalanine fluxes. Muscle protein turnover accounted for 65 g/day, and splanchnic protein turnover accounted for 62 g/day. If secreted proteins are synthesized at a rate of 48 g/day, then nonsplanchnic, nonmuscle organs account for another 75 g/day. The proportion of skeletal muscle mass in the body is consistent with skeletal muscle's contribution to whole-body protein turnover: skeletal muscle comprises about one-third of the protein in the body ( 11) and accounts for about one-quarter of the turnover.

If amino acids could be completely conserved (i.e., if none were oxidized for energy or synthesized into other compounds), then all amino acids released from proteolysis could be completely reincorporated into new protein synthesis. Obviously, that is not the case, and when there is no dietary intake, whole-body protein breakdown must exceed protein synthesis by an amount equal to net disposal of amino acids by oxidative and other routes. Therefore, we need to consume enough amino acids during the day to make up for the losses that occur both during this period and during the nonfed period. This concept becomes the basis for methods defining amino acid and protein requirements discussed below.

As shown in Figure2.6A, if about 90 g of protein are eaten in a day, of which 10 g are lost in feces, the net absorption will be 80 g. In the process, considerably more protein is made and broken down. The total turnover of protein in the body, including both dietary intake and endogenous metabolism, is 90 + 250 = 340 g/day, of which oxidation of dietary protein accounts for (75 + 5)/340 = 24% of the turnover of protein in the body per day. When dietary protein intake is restricted, adaptation occurs whereby the body reduces N losses (Fig 2:7), and protein intake/oxidation becomes a much smaller proportion of total protein turnover.

The preceding discussion defines turnover of protein in various parts of the body but does not integrate flows of material per se or highlight the relationship of amino acids to metabolites that are used for energy, such as glucose and fatty acids. Clearly, there must be interorgan cooperation to maintain protein homeostasis, simply because some tissues such as muscle have large amino acid reservoirs, yet all tissues have amino acid needs. A regular feeding schedule means that part of the day is a fasting period in which endogenous protein is used for energy and gluconeogenesis. The fed period then supplies amino acids from dietary protein to replenish these losses and provide additional amino acids that can be used for energy during the feeding portion as well. Such a normal diurnal feeding and fasting pattern causes movement of amino acids among organs, which takes on particular importance in situations of trauma and stress in which adaptation, or rather lack of adaptation, of amino acid metabolism to physiologic insults or pathophysiologic states occurs.

As Cahill and Aoki has emphasized (12.4), the first consideration of the body is to maintain and distribute energy supplies (oxygen and oxidative substrates). The caloric needs of different tissues in the body are shown in T.a.b!e..2J2. As can be seen from the table, the brain makes up only about 2% of body weight yet has 20% of the energy needs (125). The brain also lacks the ability to store energy (e.g., glycogen depots), so it depends continually on delivery of energy substrates via the blood from other organs (Fig 2..16). In the postabsorptive state, the primary energy substrate for the brain is glucose. In infancy and early childhood, when the brain makes up a significantly greater proportion of body mass, glucose production and use rates are proportionately higher ( 126). The pioneering studies of Cahill, Felig, and Wahren have provided us with a wealth of data concerning flows of amino acids and glucose from organ balance studies in humans studied over a range of nutritional states (50, 51, 127, 128 and 129). Some basic concepts may be derived from these studies.

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Table 2.12 Contribution of Different Organs and Tissues to Energy Expenditure

Figure 2.16. Interorgan flow of substrates in the body to maintain energy balance in the postabsorptive state (panel A) and after adaptation to starvation (panel B). The schematic diagrams are patterned after the work of Cahill. In all states, energy needs of the brain must be satisfied. In the postabsorptive state, glucose from liver glycogenolysis provides most of the glucose needed by the brain. After liver glycogen stores have been depleted (fasting state), gluconeogenesis from amino acids from muscle stores predominates as the glucose source. Eventually the body adapts to starvation by production and use of ketone bodies instead of glucose, thus sparing amino acid loss for gluconeogenesis. AAs, amino acids; Ala, alanine; Gln, glutamine; TG, triglycerides; FFA, free fatty acids. respectively. (Redrawn from Cahill GF Jr, Aoki TT. Partial and total starvation. In: Kinney JM, ed. Assessment of energy metabolism in health and disease, report of the first Ross conference on medical research. Columbus, Ohio: Ross Laboratories, 1980;129-34.)

As shown in Figure.2.16.A, in the postabsorptive state, the body provides energy for the brain in the form of glucose primarily from hepatic glycogenolysis and secondarily from glucose synthesis (gluconeogenesis) from amino acids. Other substrates (e.g., glycerol released from triglyceride lipolysis) may also be used for gluconeogenesis, but amino acids provide the bulk of the gluconeogenic substrate. The pathways of conversion are discussed above for those amino acids whose carbon skeletons can be easily rearranged to form gluconeogenic precursors. The remaining amino acids released from protein breakdown and not used for gluconeogenesis may be oxidized. The amino acid N released by this process is removed from the body by incorporation into urea via synthesis in the liver and excretion into urine via the kidney. Gluconeogenesis also occurs in the kidney, but the effect and magnitude are masked from A-V measurements because the kidney is also a glucose consumer. With respect to amino acids, the net effect of the kidney is the uptake and use of amino acids for gluconeogenesis ( 130, 131).

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