Can someone clear up some stuff for me re: fasting & calorie deficit?

Despite having done so much research into intermittent fasting and calorie restriction, I’m still super unclear on some of the mechanisms of the two: i If the role of intermittent fasting in fat loss is to lower insulin so that the body switches over to “fat burning mode” then isn’t a calorie deficit still king? Before entering fasting mode, doesn’t your body first have to burn off any calories consumed? On that note, would eating the same number of calories over an eating window of 6 hours have the same results as eating over 16 hours? If not, why? ii I recently found an article about how our body can only burn a certain number of fat calories a day before switching over to muscle, based on the amount of fat we have on our body. For me, with a body fat percentage of roughly 21%, supposedly my body can only burn 950 calories from fat a day before starting to switch over to muscle mass. In that case, would time restricted eating be more effective for fat loss than alternate day fasting, as during the fasting days, of the 2000 calories I would burn (as per my tdee) only 950 would come from fat? Vs say a daily caloric deficit of 950 calories? Any info is much appreciated!

Answer

When you say ‘fasting mode’, this is basically as soon as you stop eating - your body will use up its liver glycogen stores, and then tap into fats as your body transitions to ketones. Remember, not all calories are the same - your body processed fats differently to how it would process sugars/carbs.

And with eating windows, the reason it’s ideal to eat in shorter windows, is to reduce the amount of time your insulin spikes. The more your insulin spikes, the higher your insulin resistance becomes.

Unsure about your second point, but I think the amount of calories your body burns (metabolic rate) remains unchanged - whether it’s fuelled by glucose or fats, your body will still need to find a fuel to maintain your body activities.

Also, at 21% BF, I think your body will still keep burning fats - your body won’t break down muscles until your fat stores are much lower. Hope that helps

Answer

yes, calorie deficit is still king here. your body pulls from its fat when its energy needs arent met from diet regardless of insulin presence. intermittent fasting improves insulin sensitivity, but lack of insulin at that time that isnt the cause of fat loss. that is misinformation. fat is being stored and used on a constant, simultaneous basis in greater or lesser degrees depending on what is coming in. its when less dietary energy comes in that the degree of fat usage is greater, resulting in fat loss.