I’m sorry, I’m not sure if calorie deficit is the right word for it. I know from diets that you body adapts and burns less calories. Does the same happen with fasting multiple days?
>not sure if calorie deficit is the right word for it.
The word you’re looking for is “Resting metabolic rate” or “basal metabolic rate” and it actually increases during fasting (it decreases during a calorie reduced diet)
everything else can be read here www.google.com
and watched here www.youtube.com
Anecdotally, I’ve been fasting quite a bit in the last 5 months and I seem to be losing about the same amount of weight given the amount of time I spend in a fasted state. If my base metabolic rate has decreased, it’s only slightly, which should be the case anyway since I weigh less now than when I started. I don’t worry about permanently lowering my base metabolic rate but instead sustaining this new lifestyle, which so far seems completely doable. Once you know you can fast and aren’t at the mercy of your stomach and hormones, it becomes natural. Plus I don’t calorie count or feel guilty if I indulge in something I want to eat. Calorie counting is too much like work!
Fasting will help your body utilize the fatty acids and ketones from your stored fat. It does take 1/3 of energy from food while eating to digest the food. Eating and digesting is an energy intensive process. Not eating will burn less energy for digestion. It will take a few months for body, cells, liver to adapt to fatty acids and ketones as most people at metabolically inflexible from years of carb diets.
My (perhaps flawed) understanding is that during a normal diet for someone who’s body does not handle insulin well, lowering your calorie intake while maintaining a high insulin level causes your body to try to store calories as far even though you are running at a deficit. To resolve this, your body down regulates non-critical functions to save calories.
When you fast or your diet is ketogenic, your insulin level remains low, which triggers your body to burn fat for calories instead of trying to store any available calories as fat. This means that there is no reason to reduce background processes because there is plenty of energy available.
All that said, this is my pieced together understanding and I may not have it all correct. Bottom line, tested bmr during fasting sees a small increase in the first few days then a slow dip for a few days and then more or less stable burn aside from losses due to reduced body mass thereafter.