And if so, why are “starvation diets” deemed unhealthy for weight loss?
Does your body “live off of itself?” During a fast?
Is “metabolism” a myth in weight loss?
For your title question, yes they do make your body use its stored fat for energy.
Your next question, starvation diets are bad for the body because they signal to the body that there’s a shortage of food. It will try to become more efficient with the energy it has coming in. It also encourages loss of muscle mass, since its maintenance is expensive in terms of energy and resources.
The body can live off itself during a fast. For those that have high body fat reserves, they have plenty of resources to use. For those with low body fat reserves, it’s a struggle and should not be done, as it can easily waste muscle mass. There are still some things that our body can’t store very well, such as electrolytes and vitamin C, generally the water soluble nutrients. These need to be considered for those doing long fasts, which is why long fasting should be monitored by a doctor.
Metabolism is not a myth in biochemistry. But it is a collection of thousands of chemical reactions going on all over the body. We can estimate the amount of energy our body needs to use each day, but we don’t really know in any minute what exactly it is doing with that energy or those resources. Talking about metabolism is somewhat reductive or unhelpful in my view, as in weight loss it’s often seen as something to fight against. In reality your body is working hard for you, every single day. It needs to be cared for and supported, not fought against.
There are good resources on YouTube, including lectures by Jason Fung if you’re curious what that’s all about. There are plenty of other resources out there about fasting if you’re interested. Consider carefully what’s right for you and remember that if you’re looking at fasting for weight loss, it’s most important to think about learning habits that will help you maintain your weight loss in the future, to avoid yo-yo dieting.
Yes, the whole premise of fasting is that when we take in calories, our body uses those calories first for energy, then uses insulin to store the excess calories in our fat cells. Whereas if we fast, the body has to rely on stored fat cells to burn for energy.