Describe the changes that occur during starvation

Starvation is defined as the failure to ingest or absorb sufficient dietary calories to sustain normal body function, resulting in behavioural, physical and metabolic changes.

During starvation, the body must survive partially or totally on endogenous stores. As starvation progresses, the balance of the hormones insulin and glucagon alters: insulin concentration decreases to very low levels, whilst glucagon concentration increases. This, in turn, is responsible for the major metabolic changes that occur during starvation.

Describe the biochemical changes of starvation

During periods of starvation, the body’s main concern is maintaining plasma glucose concentration; some tissues, particularly the brain and red blood cells, are dependent on glucose for their metabolism.

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  During the first 24–48 h, a reduction in plasma glucose concentration causes a fall in plasma insulin concentration and a rise in glucagon concentration:

  
  
  
  
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     – Glycogenolysis in the liver is promoted by glucagon, releasing glucose into the circulation. The liver glycogen store is exhausted after 48 h.

     
     
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     – Lipolysis is promoted by high glucagon concentration and low insulin concentration, resulting in the liberation of free fatty acids and glycerol from stored triglyceride.

     
     
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     – β-oxidation of fatty acids is promoted by low plasma insulin concentration. Most of the energy produced in the early stages of starvation comes from β-oxidation.

     
     

     This process leads to high acetyl-CoA concentration within the mitochondria, resulting in the formation of ketone bodies.

  
  
  
  
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  Over the next few days:

  
  
  
  
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     – Gluconeogenesis increases: the substrates for gluconeogenesis are glycerol, lactate and amino acids.

     
     
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     – Plasma insulin concentration becomes very low, which increases ketone body synthesis.

  
  
  
  
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  Over the next few weeks:

  
  
  
  
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     – Gluconeogenesis gradually declines as tissues adapt to metabolise ketone bodies. Plasma ketone concentration rises as high as 7 mmol/L.

     
     
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     – Basal metabolic rate (BMR) decreases.

  
  

The brain still requires 100–120 g of glucose per day, and gluconeogenesis is the only means of supplying this demand. Whilst glycerol released during lipolysis remains the main substrate for gluconeogenesis, amino acids are increasingly used; high glucagon concentration stimulates the release of amino acids from skeletal muscle.

What other changes occur during starvation?

Along with biochemical changes, starvation induces behavioural changes:

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  Initially, energy is conserved through a reduction in unnecessary movement.

  
  
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  In severe starvation, all but life-saving movement ceases.

There are also some additional physiological changes: the activity of the sympathetic nervous system is reduced in severe starvation, leading to difficulty with temperature and blood pressure control.