Prominent Doc tries to prove fasting is great – Inadvertently shows how it kills thyroid and testosterone!

I don’t want to name the Doctor who did this but he as a podcast where he did a 3 week experiment of 1 week of water only fasting sandwiched between 2 weeks of keto. He was doing it because he wanted to see if he could see any markers of autophagy.

Since he is a doctor he can take all of his own blood and get as many tests done as he likes. Here are the baseline results as well as following each phase.

He mentions what I see as a standout detriment to fasting but glazes over it for some reason. This troubles me that he does not see this as a huge issue. He skips past it and just takes away from it that he lost weight. Wonderful. Weight loss at the expense of many other important things.

These are the two very important issues I would have addressed and is the entire reason I don’t want people fasting or restricting calories as both things happen in both situations. Women especially should take note of this as they are most susceptible to one of these issues.

1. Thyroid Function

This is the one women should really be aware of. The levels dropped slightly during the keto week but knowing this Doc he was likely caloric restricting since he is really into only eating one meal a day and mentioned in the podcast that he often forgot to eat at all during the keto week. Look at his levels after the week of fasting. Just one week.

Free T3 dropped from 3.7 at baseline to 2.5 after 1 week keto and then again to 1.8 after the fasting. The free T3 went back up to an acceptable 2.9 after the final week of keto. Likely he ate more during this week than the first keto week which is typical after a week of not eating. The body ends up trying to recoup.

This is the one that really jumps out.

Reverse T3 skyrockets from 17 after the keto week, which was only a mild jump from 11 baseline, to 38 after fasting!!!! When reverse T3 goes up it blocks the receptors for the active thyroid hormone to get into cells making any active T3 you have completely useless and causes your metabolism to take a long walk off a short pier. This is a big BIG red flag. Women are already suffering enough with thyroid issues today from years of crazy restricted diets and now “experts” are telling them to fast which is clearly devastating to the thyroid function. Keep in mind as well this is a very healthy man we are looking at here. Imagine what this does to an already metabolically damaged person! The reverse T3 reverts back to an acceptable 14 after the last week of keto.

Nobody should be doing this to themselves. Especially not women who already have a damaged metabolism and terrible thyroid issues. Absolutely crazy. What is crazier is that this Doc does not see this!

2. Sex hormones (Testosterone)

Now this will likely have the same effect on women and their estrogen but I can only speak to testosterone here as that is all that we have to discuss in the test since the Doc in question is a man.

Keto Week: Test actually takes a pretty big jump in the keto week. 764 to 920! I already knew this to be true since I was on hormone replacement for years before I went really strict keto and was able to completely come off.

Fasting week: Test takes a nose dive down to 539! In just one week of fasting. This means his body almost stopped making any new testosterone!!! How crazy is that? He didn’t see a problem with doing this to himself!!!

Keto Week 2: Test jumps back up to 843 which is still higher than his baseline of 764. Keto is a powerful stimulator of test. Got to love that.

So one week of fasting sent his testosterone levels plummeting into a range where he would be a candidate for hormone replacement. If there is anything that will kill a mans ability to lose weight it is poor test levels. Yet again, he did something fairly catastrophic to his hormones and metabolism yet glossed over it because the scale showed a lower number. Of course it did. He likely dropped the majority of that in water. I would bet a dollar to a dime that he gains every pound of the weight back within 2 weeks.

In the end he wanted to see markers of autophagy. Did he? Hard to say. He only had guesses of maybe. Was the maybe of increased autophagy worth what he did to himself in that week? I say a massive NOPE. That is just me though.

Keto On!

Coach Jack

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6 Replies to “Prominent Doc tries to prove fasting is great – Inadvertently shows how it kills thyroid and testosterone!”

  1. I’m so torn about fasting! I’m glad you’re writing things to balance out the pro-fasting point-of-view.

    I don’t know what doctor you’re referring to, but I have listened to Dr. Fung a few times and he has some persuasive arguments about the benefits of fasting.

    As a woman and at my current point of research, I am not going to do any extended fasting. However, I do feel great when I eat a huge breakfast, a medium-sized lunch, and little to no dinner. I suppose you could call that intermittent fasting. If that’s what it is, I credit you for teaching me how insulin resistance/sensitivity changes throughout the day, and that’s helped me make better decisions about when to eat.

    I appreciate your blog.

    1. That would not be considered fasting. That would be considered time restricted feeding and probably the most optimal way to honor your circadian rhythm.

      Check out The Circadian Code by Dr. Satchidananda Panda. Great stuff.

  2. Jack, are you saying that Fasting is bad? There is overwhelming evidence by so many respected and knowledgeable sources about the benefits of fasting (Im referring to long term – 24-48 or more hour fasting).


    1. Yes I am saying it is bad. There is no overwelming evidence that it is good. There is lots of underwhelming evidence. How is it good to absolutely decimate your thyroid and testosterone production? How in any way could this be good.

      After attempting fasting with about 500 people and seeing zero long term success with it and having myself fast extensively for an entire year and being heavier on day 365 than on day 1 I am 100% sure that fasting is a waste of time. There is some very flimsy and unverifiable evidence that it MIGHT increase autophagy but again, underwhelming. Just protein restriction will have the same effect.

  3. I’m on week four of keto diet. My goal has been not to eat after 6 pm. I typically go to bed around midnight and only have water until 8 or 9 the following am. Is this not healthy? I thought I was doing the correct thing. Especially since most of my binge eating occurred in evening hours, bf keto

    1. The ideal thing to do is to eat within an 8 hour window and it should start when you wake up and end 8 hours from there. You are doing it right so long as you are eating enough to provide your body energy to work correctly and doing it within daylight hours. Calorie restriction lowers thyroid function which slows metabolism and fasting is nothing but extreme calorie restriction.

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