Metabolism - Starvation Flashcards
Define Starvation
Failure to ingest or absorb sufficient dietary calories to sustain normal body function resulting in behavioural, physical and metabolic changes
Summarize the hormone shifts and the effects of this in starvation
INCREASED GLUCAGON (remember - glucagon acts to increase plasma glucose)
- Increase Gluconeogenesis
- Increase Glycogenolysis (liver depleted 48 hours)
- Decrease Glycolysis
DECREASED INSULIN
- Reduced GLUT-4 removal glucose from plasma
- Decreased formation of storage molecules
- decreased glycogenesis
- decreased lipogenesis
- decreased fatty acid synthesis - Decreased inhibition of endogenous glucose production
- increased gluconeogenesis
- increased lipolysis
- increased glycogenolysis - Reduced conversion of glucose to glucose-6-phosphate during glycolysis (decrease glucokinase activity)
Describe the hormonal/metabolic events during the first 48 hours of starvation, fist few days after that and then the first few weeks
Increased glucagon and reduced insulin
< 48 HOURS
- Increased Glycogenolysis (depleted < 48 hours)
- Increased Lipolysis (released of ffa and glycerol)
- Increased beta oxidation (increased Acetyl-CoA in mitochondria and formation of ketone bodies)
NEXT FEW DAYS
- Gluconeogenesis increased as glycogen stores depleted (glycerol, lactate, amino acids used as substrates)
- Increased ketone body synthesis
NEXT FEW WEEKS
1. TIssues normalise metabolism of ketone bodies and gluconeogenesis gradually declines.
What is the minimum glucose requirement of the brain and how is this maintained during long periods of starvation?
100 - 120 g glucose per 24 hours. Gluconeogenesis is the only means of supplying this during prolonged periods of starvation.
What stimulates the release of amino acids from skeletal muscle for use in gluconeogenesis
High glucagon concentrations and other counter-regulatory hormones
What behavioural changes accompany starvation
Initially energy is conserved through reduction in unnecessary movement.
In severe starvation all but life saving movement ceases
How is the autonomic nervous system affected by starvation in acute caloric deficiency vs chronic caloric deficiency
Acute:
SNS response to hypoglycaemia –> activation of counter-regulatory hormones including glucagon / adrenalin / glucocorticoids
Chronic:
Reduced SNS activity –> difficulty in controlling temperature and blood pressure
What is the usual mode of death in starvation
Total fat stores in a 70 kg man has enough triglyceride to survive around 40 - 60 days starvation.
After TG stores are exhausted, amino acids within skeletal muscle are liberated and used for gluconeogenesis.
However, once half of the muscle mass has been catabolized, there is insufficient respiratory muscle remaining to adequately clear resp secretions –> pneumonia
What is refeeding syndrome
Severe metabolic disturbance that can occur following reinstitution of nutrition to patients who have been starved or severely malnourished.
When is a patient at risk for refeeding syndrome?
Starved for 5 days or longer.
When can the onset of refeeding syndrome be recognized and how can it be recognized
Within a few days of re-instituting food
- Hypophosphataemia
- Hypokalaaemia
- Hypomagnesaemia
- Increased ECF
What is the mechanism of refeeding syndrome
When feeding is re-established and plasma glucose concentration rises, massive increase in insulin secretion from pancreatic beta cells –> glucose, Mg, K and PO4- uptake and plasma concentrations of these substances fall dramatically.
Excessive Na and H2O retention (mechanism unknown)
Increased Respiratory Quotient (Increased CO@ production) –> Va must increase to compensate but difficult with resp. muscle catabolism.
What are the principles of management of the refeeding syndrome
- Slow institution of feeding
2. Aggressive correction of electrolytes
