Fasting causes the body to utilize different energy sources over time. Initially, glycogen stores are broken down for glucose. After 1-2 days, gluconeogenesis produces glucose from amino acids. After 2-3 days, ketosis begins, where fatty acids are broken down into ketone bodies for energy. After 5 days, protein stores are conserved while fatty acids and ketones continue to provide energy. Growth hormone and adrenaline increase during fasting to preserve muscle mass and increase fat breakdown.

1. PHYSIOLOGY OF
FASTING
Dr.NIKITA

2. WHAT IS PHYSIOLOGY
Branch of biology which deals with the normal functioning of living
organism.
Including of physical and chemical phenomena involved.
Human Physiology - how human body functions in helalth and disease.
Dr.NIKITA

3. Facts
Source of energy in body - Glucose & fat
No glucose body switches to fat ! Naturally
Fasting depends on 3 energy metabolism - Glycogen, Lipid and Amino
acid
Body knows how to adapt in low food or no food situation
Feeding/ eating - food increases insulin level in blood which helps in
utilisation of glucose at tissue level for brain & muscle use and excess is
stored.
Dr.NIKITA

4. Phases during fasting
6-24 hours after last meal/fast - first phase is k/a post-absorptive phase
Insulin decreases
Glycogen breakdown —- glucose
This continues for approx. 24 hours
Dr.NIKITA

5. Second phase starts after 24 hours and lasts upto 2 days
Gluconeogenesis = source of energy (AA to glucose)
Fall in blood glucose level but within range of normal
Dr.NIKITA

6. Third phase - Ketosis
Begins after 2-3 days of fasting
Low insulin levels now helps in lipolysis to gain energy
Triglycerides (stored form of fat) is broken down Into glycerol and 3 FFA chains
Again glycerol initiates gluconeogenesis to produce glucose
Tissue other than brain can derive energy from FFA directly
Although ketone bodies derived form FFA can cross Blood-Brain-Barrier & can be
utilised by brain
Ketone bodies - beta hydroxy butyrate,acetoacetate, acetone
Dr.NIKITA

7. Fourth phase -Protein conservation
Starts after 5 days
Energy source - FFA & ketones
Increased levels of GH (to maintain muscle mass)
Increased adrenalin/norepinephrine (prevents fall in BMR)
Dr.NIKITA

8. Insulin
Fasting helps in decreasing insulin
Within 24-36 hours of fast level starts decreasing
Improves insulin sensitivity (helps in weight loss)
Low insulin level helps in diuresis (removal of excess salt & water) through
kidney (similar effect in Atkins diet which is helpful in weight loss)
Diuresis helps in bloating, feeling of light body, lower B.P.
0.9kg/day weight loss have been reported in some studies
Dr.NIKITA

9. Growth Hormone
Increased during fasting
Helps in fat availability for energy
Preserves muscle mass, bone density
Key factor of anti-aging effect (of fasting)- as decreases with age
Dr.NIKITA

10. Electrolytes & nutrients
Mg, Ca and P are stable during fast
Though their is no deficiency of macronutrients during the fast
Potassium level falls but not below 3mmol/L (Normal- 3.5 -5 mmol/L)
Dr.NIKITA

11. Adrenaline
Increased during fast
Helps in fat breakdown
Natural protective mechanism of body to supply more energy to search
for food
This increases BMR upto 14% in 4 days of fast in resting state.
Dr.NIKITA

12. 3 Energy
Metabolism of
Fasting in Detail
Dr.NIKITA

13. Glycogen
Fall in blood glucose due to which
Pancreas release more glucagon = in turn reduces insulin secretion =
decreases glycogen
Dr.NIKITA

14. Lipid
Increased plasma levels of (during fasting) =glucagon, epinephrine, cortisol, GH = activates hormone
sensitive lipase through different pathways
Helps in breakdown of triglycerides due to activation of HSL (Hormone sensitive lipase)
After activate HSL works with adipose triglyceride lipase to break FA from triglyceride (making it
diglyceride)
Later HSL & monoacylglycerol lipase break the other two (present in diglyceride) FA resulting = 1
glycerol + 3FFA
Later this glycerol is converted till dihydroxyacetone phosphate to get metabolised in glycolysis
pathway.
Fatty acid into - fatty acyl CoA = energy production through beta oxidation & ketogenesis
Later ketone bodies esp. acetoacetate converted into acetone & beta-hydroxybutyrate therefore
these 2 increases 70 fold during fasting.
Dr.NIKITA

15. Amino Acid
AA are used generally for synthesis of physiologically important metabolites in either
state (fasting or feeding)
Metabolised as per category (only in liver as no other organ can metabolise them)
Glucogenic AA (Alanine, arginine, asparagine, aspartate, cysteine, glutamate, glutamine,
glycine, proline, serine, histidine, methionine, valine)- into Kreb’s cycle (intermediated or
as pyruvate)
Ketogenic AA (Leucine, lysine)- into acetoacetate or acetyl CoA
AA with both features of above (isoleucine, phenylalanine, threonine, tryptophan,
tyrosine)- metabolised in either pathway
Dr.NIKITA