That’s over long periods of reduced calorie intake, that’s what the recommendation is to never go below 1200 cals a day (on avg). If you do too long of a fast consistent enough (ie. 3 day fasts biweekly or more frequently) It could have the same result, but that’s taking IF to the extreme, at the extreme, it’s just fasting not IF (intermittent is key here) and extremes usually produce bad results. And you will get thinner at these extremes, but your body and its hormones will be quite abnormal and more so as you regain weight will you put on faster as your caloric needs will be lower than expected due to the abnormal hormones.
Have you ever seen photos of famine in Ethiopia, or Liberation of concentration camps?
If the body could hoard incoming food as fat due to “starving” then we wouldn’t need to solve food insecurity.
There is a mechanism in human metabolism called adaptive thermogenesis, but that is based on the idea that your body adapts to changes in caloric intake/expenditure. There is a protective aspect in that if you modify your intake too drastically, your body will prioritize functions and reduce activity in order to work on those lower calories. Some people may experience things like temporary hair loss, a lower body temp, fatigue/lethargy, etc. when they cut too low.
Also consider that smaller bodies need fewer calories to maintain. So if someone is used to maintaining their weight on 5,000 calories per day, and they drop their intake to 2,000 calories, the body will use the stored fat as energy, causing weight loss, and at a point those 2,000 calories become maintenance. It is very common then for someone to further drop their calories, and the cycle begins again. At a certain point this is very inefficient because there is a floor; you do need a minimum daily intake for normal function. So then changing the equation by burning more calories (exercise) can create the additional caloric deficit needed to lose weight.
Bottom line: starving people are not overweight.
When you fast, insulin lowers, so once you use up your glycogen stores, your body can easily access fat. So you get a steady source of energy, it’s just food you ate a long time ago.
When you eat low cal throughout the day, you get steady hits of insulin making it harder to access stored fat. Hormonally it is different. So your body becomes low energy, and may end up cutting back on liver function or how much body heat it produces.
In the famous Minnesota starvation experiment, the participants were eating like 1400 calories a day. I’ve been on many diets where we were supposed to eat much less.