2014A14. Compare and contrast the physiological effects of a six hour fast of
food and fluid with a twenty four hour fast in a healthy adult.

 

List:

·         Phases of fasting

·         Energy stores

·         Metabolic pathways and key enzymes

·         Food fast

·         Fluid fast

 

Phases of fasting:

Absorptive

·  0-6 hours

Glycogenolytic

·  6-24 hours

Gluconeogenic

·  1-7 days

Protein conservation

·  7 days +

 

Energy stores:

Absorbed meal

·  e.g. 8000/3 = 2600kJ

Glycogen

·  50-100g liver, 300-400g skeletal muscle

·  Enough for ~24 hours (but some hepatic glycogen always preserved in case of emergency)

Protein

·  6kg = 96,000kJ

·  Enough for ~12 days (but vital lean tissue)

Fat

·  >200,000kJ

·  Enough for ~4 weeks

 

Metabolic pathways and key enzymes:

Glycolysis (GL)

·  Hexokinase

Gluconeogenesis (GNG)

·  From pyruvate, lactate, fructose, amino acids, glycerol

·  Note glucose 6 phosphatase mainly expressed in liver > renal cortex

Glycogenolysis (GGL)

·  Glycogen phosphorylase

Glycogenesis (GG)

·  Glycogen synthase

Lipolysis (LL)

·  Hormone-sensitive lipase

De novo lipogenesis (DNL)

·  Lipoprotein lipase, acetyl-CoA carboxylase, fatty acid synthase

Ketogenesis (KG)

·  Acetyl-CoA thiolase

 

Food fast:

 

6 hours

24 hours

Overall

·  Well tolerated

·  Exception: children (faster MR), diabetics (poor glucose homeostasis)

·  Poorly tolerated

·  Hunger, lethargy, irritability

·  Brain and RBC are obligate glucose users and stores are limited

·  High rate of protein breakdown

·  Increase risk of wound breakdown, infection

Hormonal profile

·  ↑Insulin, ↓glucagon

Progressive changes:

·  ↓Insulin, ↑glucagon

·  ↑Catecholamines: if ↓BGL (? >24 hours)

·  ↑Cortisol, growth hormone: if significant and prolonged ↓BGL (?>24 hours)

Liver

·  (GL), (GG), (DNL)

·  (GGL) > (LL) > (KG)

·  (LL) upregulated over days 1-3

·  (KG) upregulated over days 1-7

Muscle

·  (GL), (GG)

·  (GGL) -> Cori cycle >> (LL)

·  Proteolysis -> glucose-alanine cycle

Adipose

·  (DNL)

·  (LL) upregulated over days 1-3

Brain

·  Uses glucose only

·  Uses glucose >> ketones
(KG) upregulated over days 1-7

 

Fluid:

 

6 hours

24 hours

Overall

·  Well tolerated (e.g. overnight)

·  Exceptions: children (faster MR)

·  Poorly tolerated

·  Thirst, lethargy, dry mouth, postural dizziness

·  Increased blood viscosity

·  Increased risk of organ dysfunction e.g. AKI

% Volume lost

·  1.5%

·  Resp 0.4L

·  Skin 0.4L

·  GIT 0.2L

·  Kidneys 1.5L

·  Total 2.5L = 6% water loss

RAAS response

· 

·  ↑ (↓afferent arteriolar stretch, ↓NaCl load to macula densa)

ADH response

· 

· 

·  Due to ↑osmolality (sensitive to ↑1-3%)

·  Not due to ↓volume (sensitive to ↓10%)

ANP response

·  ↑ (right atrial stretch)

· 

Urine

·  Dilute

·  Increasingly concentrated, max 1200mOsmol/L

 

 

 

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