Does anyone know what causes the energy burst & feeling of your body being light during fasting/IF?
I'm thinking it's when you start to burn fat for energy/enter ketosis?
When I approach 18+ hours fasting there's a kind of shift, I suddenly feel lighter & mentally clearer, I've also felt this quite reliably after intense exercise followed by a very cold shower, which i'd imagine both trigger fat burning. The other day I did HIIT, cold shower, and then ate some coconut oil & butter straight and it seemed to amplify the effect for a few hours, so it must be ketosis right?
The only other thing I feel it could be is related to circulation, or maybe both as ketosis increases blood flow to the brain
I know what you are describing, I have experienced this as well. It usually happens while fat reserves are not too low since the body has plenty of fuel to operate on. However, if body fat dips too low, you might feel somewhat less euphoric. If I had to place a bet I would say that you are indeed in ketosis. 18 hour fasts are usually sufficiently long enough to deplete your glycogen stores assuming that you are not overweight and you eat a low-carb diet. If you follow this by ingesting a large amount of pure fat, the you will most definitely induce ketosis.
This procedure is very similar to how they introduce epileptic children to a ketogenic diet. It is first begun by fasting, in order to reduce glucose levels, which also reduces insulin levels accordingly (the body does this in order to maintain glucose homeostasis), and then high-fat, low-carb food is introduced methodically in order to assure that the patient stays in ketosis and is not kicked out by excess carb consumption or excess protein which would trigger high glucose levels via gluconeogenesis.
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