Long fasting can slow down the metabolism for an undetermined period of time.

When you exceed a certain number of hours of fasting, around 20 hours, your body starts producing something called Reverse T3, which is used to reduce metabolism. To understand what reverse T3 is, we need to know what metabolism is and how it works. Metabolism is the generation of energy into your body, more precisely, into your cells.

In order for your cells to produce energy, they need 3 minerals: sodium, potassium, magnesium, plus glucose, plus water and oxygen (and also other things). If the cell can’t get any of these things, you are busted. There is a gland called thyroid. Thyroid produces a hormone called T3. This hormone regulates how much oxygen enters the cells. Without oxygen there can be no combustion, thus the cell cannot produce energy.

When you’re fasting and your body starts noticing glycogen stores are running low, body produces some hormones like cortisol, adrenaline and human grow hormone, so that the body can continue to have energy to be able to go hunting or gathering food and avoid losing too much muscle. So in a short term, fasting increases metabolism in exchange for biological stress.

If after your fasting you eat well, with enough micronutrients, aminoacids, vitamins, fats, proteins and carbs, and you mantain your daily/weekly maintenance calories, your body will be able to get rid of the excess weight because the low insulin levels are making easier for your body to use fat as energy, and you will start loosing weight even if you don’t reduce calorie intake, because you’re improving your hormonal profile and your metabolism is activated.

Once you reach your fat percentage levels desired by your body, your weight won’t go lower than that, even if you keep fasting to try to lower your insulin levels further to try to improve more insulin sensitivy. Here comes the moment when many people do longer fasts, or combine fasting with a caloric deficit.

It happens that both prolonged fasting and caloric deficit (produced by reducing intake below maintenance levels) forces body to create something called reverse T3. Reverse T3 blocks T3. Without T3, cells cannot get enough oxygen to create energy, and therefore, there is not as much energy as before, AKA, your metabolism slows down. That’s how body reduces metabolism in order to prolong life in a situation of food shortage. And if the metabolism slows down, weight loss stagnates and there is a rebound effect on your weight.

If you are not overweight, and you are fasting because you are looking to lower your body fat levels even more in order to have a sixpack, fasting can’t give you that. Fasting will improve your hormonal profile and makes you get rid of overweight. And the good thing is that you don’t need to go beyond the 24 hours mark to get that. The body doesn’t need to completely empty the glycogen stores in the liver and muscles to start using fat as energy. It only ask you to lower insulin and cortisol to be able to burn fat. You don’t have to push your body at the point that the body is in a stress hormones coctail because of too low available glucose. You only need to gently reduce insulin by doing short fasting periods.

The only thing that can make your body lose more fat once you’re not overweight anymore so you can get a six-pack, is a caloric deficit by increasing physical activity, but never by ingesting fewer calories than your body needs to be happy on a sedentary day. It’s okay if one day you don’t manage to ingest your daily calories, but the weekly balance should be neutral. “Lying” to your body one day by not eating enough or fasting for two days and pretending your body won’t notice there is not enough food, won’t work. The caloric deficit should only come from caloric expenditure in activities. Otherwise, you will start to have low energy, you will be very, very focused on food, your body will generate a lot of hunger hormones such as ghrelin and decrease satiety hormones such as leptin, and finally you’re gonna eat your way back to where you were.

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Sources:

1. Prominent Doc tries to prove fasting is great – Inadvertently shows how it kills thyroid and testosterone!
2. Effect of Small Doses of Iodine on Thyroid Function during Caloric Restriction
3. Caloric Restriction is PROVEN to lower thyroid function
4. Personal testing by prolonged fasting periods (from 24 to 76 hours).