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What is the evidence that if works? The standard model 10 years ago was that it didn't.

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?

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Answer

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:

  1. Fasting can help sensitize our leptin receptors
  2. Leptin is know as the metabolism regulator but actually what it does is that it causes TRH to increase which leads to TSH being increased, which leads to more T4 to be made. This leads to more T3 being converted provided that one does not have ESS or Euthyroid Sick Syndrome(which I am still exploring and its relation to leptin resistance), which leads me to
  3. Leptin resistance. Is has been found in individuals with obesity that they are leptin resistant and leptin resistance is analogous to insulin resistance because the two rises and drops somewhat together and that just like any other resistance due to the downregulation of receptors for the hormones or even neurotransmitters like dopamine, too much leptin chronically(overfeeding, constant feeding, barely any window in between) can lead to the body’s leptin receptors being blunted, which leads back to
  4. Lowered leptin “visible” to the brain leads to lower thyroid production which can lead to less T4 or even if they have normal T4 which is found to be true, they have higher rT3 which is a mirror of the active T3 which makes the cells ramps up their energy metabolism. rT3 hijacks the T3 receptors making the cells “blind” to the actual T3 and rT3 has been found higher in people with obesity and I saw a number this week that it was 27% higher than the control group. This is ESS, which is not categorized as either primary or secondary hypothyroidism but is more common than Hashimoto’s.

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.

Answer

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?

Answer

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).

Answer

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.

Answer

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.

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