Starvation, metabolic adaptations

Glycogen stores in liver and kidney are exhausted in about 24 hours. After this, the body must find glucose equivalents somewhere. The major metabolic adaptations of starvation are the result of having to maintain glucose levels without any direct source of it (Fig. 17-8). 

Metabolic Adaptations in Prolonged Starvation Minimize Protein Degradation... 

The metabolic adaptations in starvation serve to minimize protein degradation. Large amounts of ketone bodies are formed by the liver from fatty acids and released into the blood within a few days after the onset of starvation. After several weeks of starvation, ketone bodies become the major fuel of the brain. The diminished need for glucose decreases the rate of muscle breakdown, and so the likelihood of survival is enhanced. 

Describe the metabolic changes that occur after one and three days of starvation. Discuss the metabolic adaptations that occur after prolonged starvation note especially the shift in brain fuels and the decreased rate of protein degradation. 

Metabolic adaptations to prolonged starvation include which of the following changes relative to the metabolic picture after three days of starvation ... 

Among the difficulties caused by prolonged fasting are metabolic disorders caused by vitamin deficiencies. What vitamins are needed during starvation to ensure that cells can continue to carry out the metabolic adaptations discussed in Section 30.3.1 of the text ... 

A knowledge of normal metabohsm is essential for an understanding of abnormalities underlying disease. Normal metabolism includes adaptation to periods of starvation, exercise, pregnancy, and lactation. Abnormal metabolism may result from nutritional deficiency, enzyme deficiency, abnormal secretion of hormones, or the actions of drugs and toxins. An important example of a metabolic disease is diabetes mellitus. 

Laboratory experiments showed that the water content of the muscle of small, immature cod subjected to starvation rose (reflecting protein depletion) from 80% to around 86%, beyond which level the fish died. Larger fish, however, which had spawned several times, could be depleted until the water content of the musculature was over 95% - a remarkable adaptation to the more severe depletion imposed under natural conditions (Love, unpublished). While this phenomenon differs from that described above in fatty fish, it again illustrates a change in the metabolism of fish in response to growth. Likewise, Borisov and Shatunovsky (1973) studied the possibility of using the water content to estimate the natural mortality rate of Barents Sea cod. 

When food intake decreases, the utilization of fat and protein reserves in the body enables various essential metabolic processes to continue during the nutritional inadequacy. In the early stage of fasting or starvation, glucose requirements of the brain and nervous system are fulfilled by mobilization of glycogen in the liver. This short-term adaptation lasts only a day until glycogen stores are exhausted. Gluconeogenesis... 

Analysis of regulatory networks involved in the adaptation of the metabolism of microorganisms to various environmental conditions, such as starvation, revealed the particular importance of nucleotides and amino acids. These compounds function as signals for starvation, precursors in metabolic pathways, energy sources or are involved in enz3une activity regulation [10,32,33,42,44,110-112]. They are usually determined off -line by HPLC [8,10,32,33,42,44,112], as chromatographic or electrophoretic determinations allow the simultaneous determination of all compounds of... 

Figure 36-5. The metabolic flow during prolonged starvation. The brain adapts to use ketone bodies as a source of energy, thus decreasing its utilization of glucose.

A 70-kg human can store at most the equivalent of 6700 kJ of energy as glycogen. This source of blood glucose will be exhausted just a few hours after a meal. Because it is critical for brain function that blood glucose levels be maintained near 4.4 mM, the organism adapts metabolically during starvation to increase the use of fuels other than carbohydrate. 

Starvation ultimately results in hypoproteinemia. When an individual is fed a diet of 1,600 calories for a prolonged period, he depletes his glycogen and fat stores and loses weight. At first, the weight loss is rapid and is accompanied by loss of nitrogen in the urine. Later, the starvation victim becomes stabilized at a low weight and adapts to the low caloric intake by reducing his basal metabolism and muscular activity. 

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