How Fasting Changes Testosterone

A clearer breakdown on what fasting and refeeding does to testosterone.

The following is the transcript for my above video. All the links to sources are provided in this document. Also, for those of you who don’t use youtube or don’t have youtube premium and would like to listen to this with your phone in your pocket, I’ve added the audio as a narration.

In 2003, David Blaine ate nothing for 44 days fast leaving him dangerously depleted - He suffered from blurred vision, back pain and heart palpitations … but while it was finally time to eat, he was entering the most dangerous part of his fast. During World War 2, the people of the soviet city Leningrad were suffering from widespread severe starvation. For a 4 month period from 1941, the only food available to citizens was about three slices of bread a day, and that bread that was 50% sawdust. Yet tragically, when some survivors were finally given a proper meal, this nourishment somehow caused them to die from heart failure. 

On the other hand, here’s another weird phenomenon: Early last year I was pretty happy when my doctor reported that my testosterone was relatively high for my age, but several months down the line when I got my first blood test at the start of a 5 day fast, I was pretty disappointed to see my testosterone was half of what it was before. I was anxious about the fast because I knew prolonged fasting can lower testosterone, but to my relief my testosterone didn’t change. Then, I got another blood test 6 days after I started eating and I was really surprised to to see that it had doubled to its highest level yet. 

My 5 day fast is no comparison to a 44 day fast or brutal starvation, but why exactly would eating provide benefits like raising testosterone after a short fast, but literally kill people who are far more starved?

In this video, we’ll take a look at what exactly happens during what I think may be the most important part of fasting - eating again. You may have heard some opponents of fasting say it’s dangerous for reasons like:

-Testosterone goes down
-Sperm count goes down.
-Cortisol, the stress hormone, usually goes up.
-A loss in lean mass, in fact, even the organs seem to shrink during a fast.
-In my own blood results, uric acid which causes inflammation, went up while I was fasting.

But by the same logic we might say exercise is bad - it increases inflammation and increases cortisol. But after resting and recovering from exercise, your inflammation and cortisol end up even lower than before.  

Way back in 1911, Upton Sinclair published in Cosmopolitan magazine two controversial articles that would eventually become a bestselling book: The Fasting Cure. 

There he described the several health benefits of multi-day fasts … and He claimed that how you break the fast is the most important part.

This study found that while 5 days of fasting increased people’s cortisol, but after 5 days of refeeding it was even lower than normal.

For me, my blood test showed that 5 days of fasting reduced a marker of inflammation called CRP, but then my inflammation was even lower after eating again for 6 days. In fact, my uric acid unfortunately went up during the fast, but it was even lower than baseline after I started eating again. 

One study on mice found that a 24 hour fast doubled the regenerative capacity of the stem cells in their intestine and Biologist Valter Longo co-authored a study that found that while organ weight reduces in the kidneys, heart, and liver during a fast, the organs rapidly regenerate and regrow themselves upon refeeding.

Longo explains that fasting and refeeding is a process of breaking down and rebuilding anew … he calls fasting the ‘auto-repair’ mode. 

As I mentioned, I got my testosterone checked three times - right before my fast started, one hour before I ate my first meal and after 6 days of eating - all tests were done at 1PM. My testosterone went from 421.5 ng/dL to 424.8ng/dL but then it jumped to more than double at 887.1 ng/dL  For those of you wondering, my free testosterone more than doubled as well.

w, I had read about this phenomenon before - A study from the year 2000 looked at US Army Rangers who undergo a brutal 8 week training course where they exercise 7 hours a day, sleep as little as 4 hours a day and eat about 1000 calories less than they need. For 4 weeks their testosterone steadily dropped to near quote “castrate levels.” Then, when they were finally refed a sufficient amount of food, their testosterone almost tripled - shooting back up to normal levels in a matter of days.  A 1981 study had six men fast for 10 days and then refeed for 5 days. Their testosterone decreased during the 10 day fast, but once they refed, 4 of the men had their testosterone shoot up much higher than what it was before the fast. One man doubled his normal testosterone from 800 to 1600.

So what is going on?

Well fasting can “set the stage” for your body to be able to grow back stronger when you reintroduce food. 

As early as 1913, Dr. Sergius Morgulis was interested in this phenomenon where temporarily starving an animal seemed to set it up to grow rapidly once you refed them. In his paper The Influence of Protracted and Intermittent Fasting Upon Growth, he cited research that claimed chickens who were deprived of food and then refed grew to be “heavier, stronger and more solid,” while specifically noting that the increase in weight was not from fat. Now, that’s a 100 year old study, but 

Valter Longo explained on a 2018 podcast with Rhonda Patrick that there’s this growth factor called IGF-1 that helps grow muscles and brain cells. However, when you temporarily reduce this growth factors through fasting, Longo explains that this “turns on” stem cells and then when you get the IGF-1 back up by refeeding with protein, then these activated stem cells replicate and multiply themselves to stimulate the growth of various types of cells. 

• The generation of healthy cells is IGF-1 dependent… there is no doubt that when you refeed, you have to have sufficient protein to rebuild. And if you don’t, I mean you really don’t have the bricks to rebuild whatever system you partially broke down.

  -Valter Longo

Just like longo said, when I fasted for 5 days, my IGF-1 went from 194ng/ml to 111ng/ml then when I ate again, it shot back up to 177ng/ml.

So what happens with testosterone?

Where this gets interesting is what fasting may be doing to the production line of testosterone. There’s this hormone produced in the brain called Gonadotropin releasing Hormone (GnRH) that is key in testosterone production and initiating puberty. GnRH acts on a specific part of the brain, causing it to release two hormones that finally act on the testes to produce testosterone. One of these hormones is LH - Luteinizing hormone. In a rare disorder where an adult man becomes deficient in GnRH, his testosterone will be very low. Treating him with GnRH will fix this issue.

So you have this nice testosterone production line that starts with GnRH and you can in fact inject people with GnRH and this will increase Luteinizing hormone and then testosterone. Well, researchers did exactly this to find out how fasting affects testosterone production. They injected people with GnRH before fasting, 8 hours into a fast and 56 hours into a fast. What they found was that the longer the fast, the more sensitive the body was to GnRH - injecting GnRH had a stronger effect on fasting people - it raised Luteinizing hormone more and it raised testosterone more.(1, 2,3,4)

That’s great and all, but who’s going to fast and then inject themselves with GnRH? Well, remember how Valter Longo was saying that when you eat protein after a fast, it increases the growth factor IGF-1? There are several pieces of evidence from in vitro and animal studies that IGF-1 stimulates the release of GnRH.(1,2,3,4,5,6) In fact, a 2021 paper in the journal Neuroendocrinology lays out how IGF-1 is suspected to play an important role in the onset of puberty as it induces the release of GnRH.

Interestingly, people who have a genetic disease known as primary IGF-1 deficiency experience delayed puberty and short stature. 

(Further reading: IGF-1 in the brain as a regulator of reproductive neuroendocrine function - “Any factor that affects GnRH or gonadotropin pulsatility is important for puberty and reproductive function and, among these factors, the neurotrophic factor IGF-1 is a strong candidate.”)

To be totally clear it’s not firmly established that rising IGF-1 after a fast is exactly what raises your testosterone and there are other ways fasting could raise testosterone in some people: for example improving insulin sensitivity, improving mitochondrial function(2), or lowering triglycerides. 

But the point is, fasting seems to prime the body to be able to increase testosterone production in some people when they eat again. 

A 1991 study on Rhesus Monkeys found that after the monkeys fasted, the bigger the refeeding meal afterwards, the bigger the pulse in Luteinizing Hormone and the greater the raise in testosterone. In fact, they found that you got much higher pulses of Luteinizing hormone on a refeeding day compared to a normal eating day. 

So there is something about stressing the body with fasting and giving it nutrients to recover that provides many benefits. Fasting Researcher Mark Mattson points out that exercise will provoke the growth of fresh new mitochondria, but this happens during the rest period after exercise. He explains that while fasting increases our resistance to stress, it’s when we refeed after a fast that makes the body rejuvenate and grow things like new mitochondria or brain cells. 

So how should we break a fast to get the most benefits?

This phenomenon where people can die of a heart attack from eating after starving is called refeeding syndrome. Well meaning soldiers desperate to help the starving prisoners in the camps of World War 2 shared with them biscuits and chocolates from their own rations, but sadly the food quickly killed them.

In David Blaine’s case, after he was escorted away from his box by ambulance, they started refeeding him with Ensure Plus. This stuff is basically a fake milkshake made with vegetable oil with a multivitamin tossed in. Here are the first ingredients: Water, Corn Maltodextrin, Sugar, Milk Protein Concentrate, Blend of Vegetable Oils (Canola, Corn). If you’re not aware of why I find the idea of drinking vegetable oils totally nauseating, check out this video on why “heart healthy” vegetable oils are probably toxic.

Ironically, the authors of the clinical report on David Blaine themselves explain why a sugary oilshake after a fast is an awful idea:

They explain that eating causes a sudden rise in insulin and, where the body had been carefully balancing the levels of electrolytes in your blood and cells, this insulin rise causes electrolytes to rush into your cells and sodium to rush out of the cell and into the blood. The effect of this is that the blood is diluted and the volume of your blood increasing, leads to heart failure.

So in this particular situation, if a sudden rise in insulin is the problem… why not avoid feeding people a carbohydrate rich meal? After all, carbohydrates are well known to induce a much bigger rise in insulin than fat or protein. That liquid blend of sugar and vegetable oils they gave David Blaine has more carbs than a can of coke.

Dr. Bernard of the YouTube channel chubbyemu has made two different videos about cases of people being admitted to the hospital with refeeding syndrome. What did these people eat? Well first off they both simply ate way too much, but it was mostly carbs and sugars. In one case a malnourished woman suddenly ate a huge container of cookies. In another, a woman fasted for 7 days and then ate 23 bananas. In Upton Sinclair’s old book on fasting, he talks about a man who broke a 50 day fast with tons of fruit - half a dozen figs … this led to intestinal abrasions that had him lose a lot of blood. 

So the first obvious rule to breaking a multi-day fast is to eat slow. 

Along with taking it slow on the carbs, you probably want to go light on the fat as well - I broke my 5 day fast right away with a fatty ribeye steak and my stomach felt a little queasy, probably because my body wasn’t used to having to produce bile to digest fats. It wasn’t much of an issue, but I imagine it would have been worse if I didn’t have so much experience with fasting. Upton Sinclair also mentioned people suffering from “slight bilious attacks from eating over-fat beef, but they quickly recovered by eating leaner beef.”

To get the growth benefits, you’ll want to eat protein after your fast so your body can increase its growth factors to allow for rejuvenation of the body as Valter Longo explained. 

I’m not here to guarantee fasting is the secret testosterone hack: There are still lots of questions like: What’s the ideal amount of protein to eat after a fast? Can you expect a testosterone rise after refeeding after just a 1 day fast? What about a 3 day fast?  and Why did 4 of these guys get a boost in testosterone after fasting but not the other 2?

A quick note on David Blaine - his testosterone strangely rose during the fast but it seemed to go down slightly after 2 days of refeeding … but he wasn’t fed many calories (for good reason) and his liquid meal had little protein - it had 3 times as much carbs as protein. A fast this extreme of course needs proper medical supervision but why did they feed him sugar and oil?

44 days is an extremely long fast - obviously there has to be a point where you are fasting too much - you need to balance fasting with refeeding and nourishing your body. Just like you can overtrain with exercise, you can of course over-fast. Like Dr. Mark Mattson explains, fasting and refeeding are both necessary to reap the health benefits.