After a carbohydrate-containing meal, the liver maintains plasma glucose concentration within a narrow range by taking up one- quarter to one-third of the . Molecules to metabolism Carbohydrates and lipids Proteins Plan Proteins: 4 Functions Molecules to metabolism: Amino acids 1 Catabolic processes are often oxidative in nature and energy releasing. Some, but not all of that energy is captured as ATP. Not all of the.
This paper critically evaluates the protein-energy interrelationships which occur during total starvation and hypocaloric feeding, evaluates the physiological significance of the differences that exist between lean and obese subjects, and considers the implication for nutritional support for those receiving very low calorie diets.
Although prolonged total starvation is associated with an absolute reduction in basal metabolic rate BMR , during the first 2 days of starvation there is often a small absolute increase in BMR relative to values obtained after an overnight fast Figure 1. However, a transient increase in metabolic rate may have been missed, because the results were often compared to measurement made at the end of the first day of starvation 36 hours since the last meal.
In other studies it may have been missed because the first of a series of measurements were made after a meal, as in the case of Cetti who was first studied one hour after breakfast LEHMANN et al. The classic fast reported by BENEDICT in was associated with a small increase in metabolic rate compared to values obtained after an overnight fast Figure 1 , and so were a number of other classic starvation studies in individual subjects.
The subject who did not show an increase did not have a measurement made on day 2 of starvation. More recent studies in groups of individuals have confirmed a transient early increase in BMR Figure 1. The rise in BMR could be due to either an increase in the energy equivalent of ATP gained as the body reduces the proportion of energy derived from glycogen This last cycle occurs because the liver synthesises ketone bodies from acetyl Coa AcCoA , while other tissues such as brain and muscle convert ketone bodies back to AcCoA before final oxidation.
In addition to the early increase in energy expenditure, there is also some loss of energy in the form of ketone bodies in both breath acetone and urine 3-hydroxybutyrate and acetoacetate and acetone.
In obese subjects, the loss is less, but as the circulating concentration of ketone bodies increases during more prolonged starvation, the loss also increases REICHARD et al. After the first days of starvation, the BMR becomes lower than after an overnight fast.
This reduction is partly due to loss of the metabolically active lean tissues during the first 3 days of starvation there is a loss of about g N; about 1. Although the rate of protein oxidation reflected in the rate of urine N excretion is frequently considered to decrease during prolonged starvation, especially in the obese see below , several studies have shown that there is often a transient early increase.
Examples in lean and obese subjects are shown in Figure 2 , but similar changes have been noted by a variety of workers e. In the study of ELIA et al. It would therefore appear that there is often genuine transient increase in amino acid oxidation during early starvation. TAKAHIRA suggested that this increased oxidation of protein was responsible for the temporary increase in energy expenditure that he observed in his starving subjects.
This suggestion was based on the premise that the specific dynamic action of catabolized proteins is greater than that for fat or carbohydrate. Not all of the N found in the urine during early starvation reflects protein catabolism, since some of it is due to catabolism of free amino acids, particularly glutamine. The pool of free glutamine in muscle, estimated to be about 45 g or 15 g glutamine nitrogen in a 70 kg man, almost halves between 12 and 72 hours of starvation MAGNUSSON et al.
However, the absolute changes of these other amino acids are small, relative to the loss of glutamine. After the first days of starvation, urine N excretion begins to decrease and generally continues to do so for most of the subsequent period of starvation although a premortal rise frequently occurs. The ratio of N excretion as an index of protein oxidation to BMR is variable during short-term starvation, but calculations based on the results of several studies suggest a tendency for this ratio to transiently increase during the first days of starvation.
Implications of initial body weight and fat stores on protein-energy interrelationships 3. Evidence for the first postulate of the model: Survival time in relation to body composition 3. Evidence for second postulate of the model: During prolonged starvation the contribution of protein oxidation to energy expenditure is less in obese than lean subjects 3. Starvation in man and other species. Keep track of what you input in a shopping cart Essential: Authenticate that you are logged into your user account Essential: Remember language version you selected.
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