Analysis of a plasma sample indicated elevated levels of Alanine, Acetoacetate, β hydroxy butyrate, and blood urea nitrogen(BUN). However her plasma glucose concentration was low (55mg/dL) . She was hospitalized, given intravenous feeding, antidepressant medications and subsequently shifted to an 1800 Cal (7500 kJ) diet. Her recovery was uneventful.
How was the patient obtaining energy during the time when she was not eating?
How could patient maintain her plasma glucose within normal limits even though she was not eating?
What is the significance of elevated plasma Alanine level?
Why is BUN elevated?
What is indicated by the fact that the plasma Acetoacetate and β– hydroxy butyrate levels are elevated?
It is a case of starvation. The high blood Alanine level signifies the catabolic state. Alanine in excess is released during starvation from muscle to serve as a substrate for glucose production in liver. Acetoacetate and β hydroxy butyrate are ketone bodies which are used as alternative fuel during conditions of glucose deprivation . High BUN signifies protein degradation; the carbon skeletons of amino acids are utilized for glucose production while amino groups are converted to urea.
Prolonged fasting may result from an inability to obtain food, from the desire to lose weight rapidly, or in clinical situations in which an individual can not eat because of trauma,surgery, neoplasms, burns etc or even in depression (As in the given case) . In the absence of food the plasma levels of glucose, amino acids and triacylglycerols fall, triggering a decline in insulin secretion and an increase in glucagon release. The decreased insulin to glucagon ratio, and the decreased availability of circulating substrates, make this period of nutritional deprivation a catabolic state, characterized by degradation of glycogen, triacylglycerol and protein. This sets in to motion an exchange of substrates between liver,adipose tissue, muscle and brain that is guided by two priorities (i) the need to maintain glucose level to sustain the energy metabolism of brain ,red blood cells and other glucose requiring cells and (ii) to supply energy to other tissues by mobilizing fatty acids from adipose tissues and converting them to ketone bodies to supply energy to other cells of the body.
A typical well-nourished 70-kg man has fuel reserves totaling about 161,000 kcal (670,000 kJ). The energy need for a 24-hour period ranges from about 1600 kcal (6700 kJ) to 6000 kcal (25,000 kJ), depending on the extent of activity. Thus, stored fuels suffice to meet caloric needs in starvation for 1 to 3 months. However, the carbohydrate reserves are exhausted in only a day.
Energy supply during starvation
During starvation the energy needs are fulfilled by three types of fuels, glucose, fatty acids and ketone bodies.
a) Glucose supply during starvation (Gluconeogenesis)
Energy needs of brain and RBCs
Even under conditions of starvation, the blood-glucose level has been maintained above 2.2 mM (40 mg/dl). The first priority of metabolism in starvation is to provide sufficient glucose to the brain and other tissues(such as red blood cells) that are absolutely dependent on this fuel.However, precursors of glucose are not abundant. Most energy is stored in the fatty acyl moieties of triacylglycerols. Fatty acids cannot be converted into glucose, because acetyl CoA cannot be transformed into pyruvate. The glycerol moiety of triacylglycerol can be converted into glucose, but only a limited amount is available. The only other potential source of glucose is amino acids derived from the breakdown of proteins. However, proteins are not stored,and so any breakdown will necessitate a loss of function.
Thus, the second priority of metabolism in starvation is to preserve protein,which is accomplished by shifting the fuel being used from glucose to fatty acids and ketone bodies by cells other than brain cells and the cells lacking mitochondria.
It is a biological compromise to provide glucose to these cells as a priority. During prolonged starvation , when the gluconeogenic precursors are not available, proteins are however broken down to use carbon skeleton of glucogenic amino acids for glucose production.
b) Fatty acid oxidation
Energy need of liver
The low blood-sugar level leads to decreased secretion of insulin and increased secretion of glucagon. Glucagon stimulates the mobilization of triacylglycerols in adipose tissue and gluconeogenesis in the liver. The liver obtains energy for its own needs by oxidizing fatty acids released from adipose tissue. The concentrations of acetyl Co A and citrate consequently increase, which switch off glycolysis. Thus glucose utilization is stopped in liver cells to preserve glucose for priority cells
Energy need of muscles
The uptake of glucose by muscle is markedly diminished because of the low insulin level, whereas fatty acids enter freely.Consequently, muscle shifts almost entirely from glucose to fatty acids for fuel.The beta-oxidation of fatty acids by muscle halts the conversion of pyruvate into acetyl CoA, because acetyl CoA stimulates the phosphorylation of the pyruvate dehydrogenase complex, which renders it inactive. Most of the pyruvate is transaminated to alanine, at the expense of amino acids arising from breakdown of “labile” protein reserves synthesized in the fed state. The alanine, lactate and much of the keto-acids resulting from this transamination are exported from muscle, and taken up by the liver, where the alanine is transaminated to yield pyruvate. Pyruvate is a major substrate for gluconeogenesis in the liver.(Figure-1)
Figure- 1-showing Glucose Alanine and Cori’s cycle
In adipose tissue the decrease in insulin and increase in glucagon results in activation of intracellular hormone-sensitive lipase.This leads to release from adipose tissue of increased amounts of glycerol(which is a substrate for gluconeogenesis in the liver) and free fatty acids,which are used by liver, heart, and skeletal muscle as their preferred metabolic fuel, therefore sparing glucose.
Loss of muscle mass
During starvation, degraded proteins are not replenished and serve as carbon sources for glucose synthesis. Initial sources of protein are those that turn over rapidly, such as proteins of the intestinal epithelium and the secretions of the pancreas. Proteolysis of muscle protein provides some of three-carbon precursors of glucose. The nitrogen part of the amino acids is converted to urea (BUN)
Energy need of peripheral tissues
After about 3 days of starvation, the liver forms large amounts of acetoacetate and beta- hydroxybutyrate. Their synthesis from acetyl CoA increases markedly because the citric acid cycle is unable to oxidize all the acetyl units generated by the degradation of fatty acids. Gluconeogenesis depletes the supply of oxaloacetate, which is essential for the entry of acetyl CoA into the citric acid cycle. (Figure-2) Consequently, the liver produces large quantities of ketone bodies, which are released into the blood. At this time, the brain begins to consume appreciable amounts of acetoacetate in place of glucose. After 3 days of starvation, about a third of the energy needs of the brain are met by ketone bodies. The heart also uses ketone bodies as fuel. After several weeks of starvation, ketone bodies become the major fuel of the brain.
Figure- 2-fatty acid oxidation and ketosis during starvation.
In essence, ketone bodies are equivalents of fatty acids that can pass through the blood-brain barrier. Only 40 g of glucose is then needed per day for the brain, compared with about 120 g in the first day of starvation. The effective conversion of fatty acids into ketone bodies by the liver and their use by the brain markedly diminishes the need for glucose. Hence, less muscle is degraded than in the first days of starvation. The breakdown of 20 g of muscle daily compared with 75 g early in starvation is most important for survival.
A person’s survival time is mainly determined by the size of the triacylglycerol depot.
What happens after depletion of the triacylglycerol stores? The only source of fuel that remains is proteins. Protein degradation accelerates, and death inevitably results from a loss of heart, liver, or kidney function.Please help "Biochemistry for Medics" by CLICKING ON THE ADVERTISEMENTS above!