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.
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. One of the most consistent autopsy findings of animals and humans who died of 'pure' starvation, is the virtual absence of depot fat, both subcutaneously and internally KEYS et al. In some studies, fat was found to be replaced by a translucent gelatinous material. In the autopsy of a man who refused all food until his death, MEYERS confirmed the absence of storage fat from most parts of the body, and noted that in those areas where small amounts of fat persisted e.
Therefore, individuals dying of starvation only have a small amount of storage fat and a small amount of structural fat, mainly in cell membranes and the brain and nerves. Since body fat is the major store reserve of energy which disappears during starvation, it is reasonable to suggest that the initial amount of body fat is a major determinant of the length of survival during starvation. Most of the weight loss during starvation in lean individuals is due to loss of lean body mass. However, the brain, gonads and skeleton appear to be preferentially preserved.
Examples of the changes occurring in humans and non-human species cats, dogs and pigeons during 'total' starvation are shown in Table 1. Some examples of massive successful weight reduction during fasting in gross obesity. It is also important to identify relevant metabolites and determine their potential effects. A substance that, for example, undergoes a rapid first pass metabolism in the liver, may not reach the target organ via systemic circulation.
On the other hand, metabolism may lead to systemically available metabolites that drive a toxicological effect. Metabolism should be considered even if a substance tests negative in an in vitro assay because it is still possible that the metabolite reaches the molecular target.
Conversely, metabolism should also be investigated if an effect occurs in an in vitro system: This might be due to the fact that the system has no metabolic capacity and in contrast to the organism, no ability for detoxification.
An understanding of the nature and the kinetics of the formation, distribution and excretion of relevant metabolites and their possible interactions with the target tissues is therefore crucial to determine the relevance of an effect observed in vitro and to the design of relevant studies also taking into consideration metabolites.
Metabolic processes or the induction of metabolising enzyme s also influence the metabolism of hormones and it is important to understand the kinetics of both processes to assess whether adverse effects can occur at relevant exposure concentrations. Is the AOP in general biologically plausible?
Is the AOP biologically plausible for the chemical under evaluation? Technical report It is also important to identify relevant metabolites and determine their potential effects. Cookies To make this site work properly, we sometimes place small data files called cookies on your device.
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Metabolism of pyridalyl in rats.
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