I have a book from only 2010 from a major health magazine that says that fasting creates a response of lowered metabolism, and is not a good method for fat loss. I understand they might have been wrong and sometimes long-standing conclusions are proved to have been more like assumptions. Are there clear and repeatable studies that show that not eating for 16 hours puts the body into a fat burning mode? Do the users of this method lose more fat than those who just reduce calories to the same level?
Fasting is an interesting intervention for me. From what I’ve understood from my 10 years of trying to figure this out for myself is that:
How does fasting help?
Fasting lowers leptin. Bad right? Since now the thyroid levels will also be affected hence the metabolism rate will be too. But here’s the kicker, when one fasts, their leptin resistance reduce, or their leptin sensitivity increases. So what happens next?
When they eat, their leptin don’t have to be raised as high as before for their brain to “see” the leptin hence leading to getting the TRH to stimulate the TSH leading to more T4, and then more T3.
I am still unsure about what leads to the rise of rT3 in obese patients yet but I’m still reading on that because of my own personal circumstances.
Anyway, to anyone who is more knowledgeable than me, do correct me and help this information get refined. I am no scientist and just a regular guy who’s trying to figure my life out.
There have been an absolute ton of studies in the last ten years. A decade is a LONG time when it comes to science. Are there any particular studies you’ve come across whose conclusions you’re specifically questioning, or have you just not looked for any more recent studies?
If you are referring to intermittent fasting then I recommend this recent review and if you are referring to alternate-day fasting I recommend [this recent review](https://sci-hub.tw/https://www.metabolismjournal.com/article/S0026-0495(20\)30200-6/fulltext).
The main things (in my opinion) that make fasting a valid weight management method is this (the claim is easily verifiable because it is basic biology, and so it is easy to connect the dots):
It decreases insulin uptime and secretion, making you insulin sensitive.
Insulin is the main anabolic hormone of the body. It stops the use of fats in the body to prioritize carbohydrate intake by the cells, all the while promoting glycogen and fat storage.
When your insulin is low (such as while fasting), your body uses an alternative fuel source to carbohydrates so as not to empty your muscle’s and livers glycogen stores completely. That’s where the fat loss comes in.
Fat stores are released and turned into energy in the form of ketones, until the next time you eat and secrete insulin. So the more time between each insulin secretion, the more you allow your body not only to use fat, but to not store fat either.
The insulin sensitivity part is that, in the same way that the human body builds a tolerance to coffee, alcohol or drugs, your cells build a tolerance to insulin. And so, over time, you will need x unit of insulin more for the same amount of carbohydrates ingested. The more insulin “spikes”, the more fat storage is promoted and the longer fat use will be delayed.
Fasting and chronic calories restriction are not the same kind of stressor. Chronic prolonged calorie restriction does slow your metabolism from a longevity standpoint that’s actually a benefit. The science behind fasting for short bursts is that you feel stressed out enough to need to dip into your fast stores but not permanently lower your metabolism to survive and make the most of your food.