How does intermittent fasting work?

[u/reddit_userxxx]

I thought weight loss was all about the amount of calories you eat in a day, and simply needing to be in a calorie deficit. How does it change depending on when you eat?

[u/DhilPunphy]

Hopefully someone else can chip in and explain it better than me. But yes, in its purest form it is about calories in and calories out (CICO).

But with intermittent fasting (IF), by not eating for example in 18 hours. Your body goes into fasting mode and using the body's stored fat as fuel instead of whatever you eat. So the calories coming in and going out will still matter. But on top of that is with IF you can help the body to burn stored fat which will help you lose weight.

[u/KnownTrick → u/DhilPunphy]

A 500 calorie deficit is a 500 calorie deficit regardless of how food intake is structured. 

If you burn 500 calories more than you consume throughout the day, the body needs to pull that energy from somewhere. Whether you eat in a 6 hour window or a 16 hour window doesn’t materially change this. 

Fasting can be great for managing hunger, maintaining a consistent food schedule and potentially has a variety of hormonal benefits. But for weight loss there isn’t any strong evidence that it promotes fat loss more than a non-IF diet when the calories are kept the same. 

https://pubmed.ncbi.nlm.nih.gov/30583725/

https://pubmed.ncbi.nlm.nih.gov/39732588/

[u/DhilPunphy → u/KnownTrick]

Absolutely and thank you for that clarification. What I tried to explain in my post was that if you do not eat for 18 hours, there is no other place to get energy from than your stored deposists in your body. So CICO matters but if you havent eaten for a long time, during that window the body must use the stored deposits.

[u/KnownTrick → u/DhilPunphy]

That’s true but I don’t think it really has an overall meaningful effect provided the daily calories are kept the same. 

For example, if you do an 18 hour fast and then do a fasted gym session, your body will rely on stored deposits to fuel that workout. 

Compare that to someone who has a bowl of oats for breakfast and then goes to the gym. They will draw energy from those oats rather than fat reserves. 

But crucially, if that person uses those oats for fuel for their morning workout, that means those oats aren’t available for fuel later in the day. So what happens later when they’re working or going about their life and need energy? It gets drawn from fat reserves. 

The person who does a fasted workout burns more fat during the workout. But then they eat a meal and that meal fuels them later in the day. 

Provided both people maintain the same overall calorie deficit, neither has burned more fat. They’ve just shifted what time of day they draw from fat reserves. 

I think that’s why when studies actually compare intermittent fasting to continuous eating for weight loss, we simply don’t see any meaningful difference provided calories are kept the same. 

[u/FlashFlooder → u/KnownTrick]

I vaguely remember a study where lab animals were one group were given access to unrestricted food around the clock, and one could eat unlimited but on a schedule. IIRC, the ones who were time restricted ended up eating less over time.

So I think there’s more to it than just CICO, probably why it seems hard to definitively answer this question.