Fasting is important in medicine and medical teaching. Fasting is different from non fasting. The blood contents are different when we fast. Why?
To answer the question, we first need to know which pathways are active during fasting, then we can try and answer "Why".
What pathways are active during fasting?
- Glycogenolysis (this happens initially because glycogen stores are very limited)
- Protein breakdown (from the skeletal muscles that help us move)
- Fat mobilisation (after day 3 of fasting)
- Beta-oxidation (when the fatty acids enter liver mitochondria, to yield acetyl CoA)
- TCA cycle (to yield NADH and FADH2)
- Oxidative phosphorylation (ETC process, to yield ATP)
Explanation:
- Blood glucose is reduced during fasting. So alternative sources of energy need to be sought or found by the body. Remember, our body is "intelligent" - it knows where to look for energy sources, without complaining to us.
- Glycogen is broken down to yield glucose (refer to glycogenolysis lecture notes).
- Blood amino acids maybe initially increased and then reduced during fasting. The body has to use its protein stores first, i.e., mainly using our muscles - our biceps, triceps, etc. When our muscles are broken down to yield amino acids, we feel weak as we lose muscle (which is protein) and usually we cannot be as strong and energetic when we fast as our muscles are partially used to supply us energy.
- Blood fatty acids are elevated during fasting as fat mobilisation occurs. That means the body tries to breakdown its fat stores (from adipose tissues), leading to high fatty acids in the blood. These fatty acids are hydrophobic molecules (do not mix with water) and travel by way of albumin (transporter molecule) to the liver, where they undergo beta-oxidation. Refer to beta-oxidation lecture notes. Beta-oxidation has 4 steps and yield acetyl CoA (which is a 2C unit), which enters TCA cycle, and the products (NADH, FADH2) enter ETC, to yield ATP by oxidative phosphorylation. ATP is the energy we get and during fasting, it is derived from mobilised fats. This source of energy is good starting on day 3 onward, that's when fats are mobilised, till we end fasting, usually one month as in Ramadan fasting. Fat stores are the last to be used, and only after day 3 do we start using fats. So if we fast less than 3 days, fat stores are not used, and we don't hope to lose weight or fat by fasting less than 3 days. How is fat mobilised? Refer to your lipid mobilisation lecture notes. There is a hormone called HSL (hormone sensitive lipase) that is activated by cyclic AMP (c-AMP). HSL acts on TG and breaks it down to DG and further to MG and lastly to glycerol (G) and free fatty acid (FFA). So HSL is a fat buster and breaks down fat step-by-step (i.e., stepwise, like a cascade).
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