Science Behind Intermittent Fasting

Science Behind Intermittent Fasting

Introduction

The science behind intermittent fasting (Triacylglycerol Metabolism). Most of you guys must have already heard of Intermittent Fasting. It is one of the trending diet people are following these days. But I know a lot of people are getting more and more intrigued to know more about intermittent fasting. Fasting is a story that not everyone buys, so this article is dedicated to the science behind intermittent fasting. The basic concept of intermittent fasting is that when you are starving yourself for about 16 hours or more your body starts to burn a lipid (fat) that is triacylglycerol to provide energy and keep you active. This is the reason we don’t faint or something by fasting for at least 16 hours. This fat is stored under our skin and anything in excess whether it is protein, carbohydrate or fat is converted into this fat- triacylglycerol and is stored under our skin. Since the skin is the largest organ you have plenty of space to store triacylglycerol.

If you are still confused about the term triacylglycerol, in a simpler language, it is a kind of fat or what a science student will call it, lipid. Now getting back to what happens when you are at the starving stage, these triacylglycerol molecules that are stored under skin (Adipositic tissue) breaks into fatty acids which are further metabolized to provide us energy. It is a better energy source than glucose as it gives us 9 kcal of energy which is more than double of energy, glucose provides. And this way you are actually burning fat during starvation. But that doesn’t mean if you will not eat for a day you will achieve your dream body. Even on intermittent fasting, you have to eat enough carbs, protein and fat for 8 hours in general. So this way you can almost eat whatever you want during your eating hours. and going daily exercise compliment this diet. It will help you stay toned. So here are some scientific terms related to all about intermittent fasting and triacylglycerol.

What is Triacylglycerol / Triglycerides?

Triacylglycerol is a simple lipid consisting of a glycerol backbone and three fatty acids molecules attached with ester bond. Triacylglycerol (TAG) is formed by condensation reaction between the hydroxyl group and Carboxyl group of glycerol and fatty acids respectively. These are esters as they are derived from alcohol and fat. Glycerol is alcohol derivative of glyceraldehyde which is acting as a backbone in triacylglycerol. The different types of fatty acid attached to glycerol backbone forms different kinds of triacylglycerol which is determined by different properties it exhibits. It is stored in anhydrous stats that means it does not require water molecules to stabilize its structure.

Triacylglycerol is hydrophobic and nature and hence it is considered better than glycogen. There are two reasons why TAG is chosen over glycogen even when they both provide energy when there is lack of glucose level in blood. First is, it is hydrophobic, so it does not require water molecules to stabilize it. Second reason is that they provide twice the energy as glycogen gives when broken into glucose. It provides 9 kcal of energy to us. It is stored in large quantity underneath our skin – adipositic cells. It is a very inert molecule that is it doesn’t react with other cell components. It is segregated into lipid droplets. It doesn’t affect the osmolarity of the cytosol.  The hormones like Glucagon, ACTH- Adrenocorticotropic hormone, Epinephrine, etc. stimulates the body to release fatty acid from adipose tissue.

Synthesis of Triacylglycerol

  • TAG is synthesized in adipose tissue and liver.
  • TAG synthesized in adipose tissue is for storage purpose, whereas in the liver it is mainly secreted as VLDL 
  • It is synthesis by esterification of fatty acyl CoA with either glycerol3 phosphate or dihydroxyacetone phosphate (DHAP).
  • Glycerol 3 phosphate is formed by either phosphorylation of glycerol or by reduction of DHAP
  • The Fatty acyl-CoA transfers the fatty acids to the hydroxyl group of glycerol which is mediated by enzyme acyltransferases.
  • There are other pathways of synthesis of TAG such as MAG pathway
  • In MAG pathway the 2-MAG absorbed is re-esterified with fatty acyl-CoA to form TAG. This pathway occurs in intestinal mucosal cells.

Fasting State

  • During fasting state, the synthesis and breakdown of TAG occurs simultaneously.
  • When a person is at fasting state the glycerol phosphate (derived from dihydroxyacetone phosphate (DHAP)) is from during gluconeogenesis. 
  • Gluconeogenesis is the phenomenon of synthesis of glucose from a non-carbohydrate source.
  • At this state the activity of an enzyme called PEPCK is enhance in liver and adipose tissue. This ensure the availability of glycerol phosphate for esterification and store the excess fatty acid mobilized.
  • Esterification of fatty Acyl-CoA and glycerol phosphate occurs to form TAG. This whole process of synthesis of TAG is much faster during fed state.
  • Then the digestion of triacylglycerol occurs to release energy when there is low level of glucose in blood that is fasting state. 

Digestion of TAG

  • Pancreatic lipase. Pancreatic lipase is an enzyme that can easily hydrolyze fatty acid esterified to form 1stcarbon and 2ndcarbon atom of glycerol forming 2- monoacylglycerol and two molecules of fatty acids.
  • Then an enzyme called isomerase comes to shift the ester bond from 2ndposition to 1st
  • The bond in the 1stplace is hydrolyzed by lipase and yields free glycerol and fatty acids.
  • Colipase then binds to triacylglycerol molecule. Colipase is secreted by pancreas in an inactive form zymogen. It is activated by action of trypsin.

Adipose tissue when person is well fed

  • The TAG is transported to adipose tissue via Chylomicrons and they are also Endogenously Synthesis in liver.
  • In a well-fed condition glucose and insulin levels are increase or sufficient.
  • The stimulant effect of insulin on the uptake of glucose by adipose tissue, on the glycolysis and on utilization of glucose by HMP pathway also enhance lipogenesis.
  • The insulin also causes inhibition of hormones sensitive lipase and hence lipolysis- breakdown of lipid is decreased. 

Adipose tissue in fasting condition

  • TAG stored in adipose tissue is mobilized under the influence of hormone glucagon and epinephrine.
  • The Cyclic AMP-mediated activation cascade enhances the intracellular hormone sensitive lipase.
  • The phosphorylated form of enzyme acts on TAG to liberate fatty acids.
  • Under starvation condition the hormones like glucagon, ACTH, glucocorticoids and thyroxine are significantly high and they have lipolytic effect.
  • The release fatty acids are taken up by the tissue as a fuel.

Diabetes Mellitus

  • Lipolysis is enhanced and the level of FFA is high in diabetes mellitus. The insulin acts through receptors on the cell surface of adipocytes.
  • These receptors are decreased that leads to insulin insensitivity in diabetes.
  • The increase mobilization of fatty acids from adipose tissue occurs.
  • The levels in presence of hyperinsulinemia stimulates synthesis of triacylglycerol.
  • The overproduction of tag leads to increase release of VLDL

Obesity

  • Obesity occurs when the fat content increases depending on the excess amount of calories taken. 
  • High level of Plasma insulin is noticed. But the insulin receptors are limited and there is peripheral resistance against insulin action leading to obesity.

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