I Just Finished a 40-Hour Fast. Here’s My Experience…

lunch-timeI’ve talked quite a bit about Intermittent Fasting (IF) here on the site. Not so long ago I put up a post in which I made the case that periodic fasting was a natural part of our primal forebears’ lifestyle, and that it’s only very recently that it became normal for humans to consume 3-4 square meals a day (starting with an early breakfast). Even more recently I put up another post in which I talked about fasting as it relates to exercise, and I highlighted research showing that fasted exercise offers some unique health benefits.

I’ve also talked about the science of IF in many of my other articles. What I haven’t done, however, at least not in a detailed manner, is to describe my recent experience with fasting. Over the past couple of years, and in particular the past couple of months, I’ve been thinking quite a bit about the question of what constitutes the optimal meal pattern for us humans and why some people find that IF agrees with their bodies, while others don’t. I’ve also experimented with different IF protocols myself. In today’s article I thought I’d share some of the thoughts that this process has generated.

Breakfast: The least important meal of the day?

I generally don’t eat breakfast early in the morning. Lately, I’ve been having my first meal of the day sometime between noon and 4 P.M. Not yesterday though. Yesterday I did something I don’t do that often: I didn’t eat at all. It wasn’t planned, it just happened as I wasn’t really hungry (I ate a very large meal Friday night) and because I was occupied with other stuff (reading, writing, and socialization mostly).

When I ate my first meal today, 40 hours had passed since I last consumed anything solid. The only thing I took in during this lengthy fast was water. It wasn’t until this morning that I started paying attention to the fact that I was approaching a 40-hour fast. As I did, I got the idea that I would share some of my experiences with IF here on the site, which is what led to the birth of this article.

There are undoubtedly a lot of people out there who’ve fasted for more than 40 hours. I actually think I might have as well, in the past. But I can’t remember that experience. The point I’m trying to make with this post is by no means that doing a 40-hour fast is a remarkable feat (it isn’t). Rather, the reason I’m writing this article is simply that I have some thoughts about IF that I feel like sharing.   

My experience and thoughts: 5 key points

Note: The opinions I express below are not solely based on my experience with the 40-hour fast. Rather, they are based on my total experience with fasting, as well as some of the scientific knowledge I’ve accumulated over the years.

1) Fasting positively affects brain function

One of the things I’ve noticed over the years is that my brain function and cognitive abilities tend to be better when my stomach is empty as opposed to when it’s full. I’m sure others have experienced the same. There are many potential reasons as to why fasting has this effect on the brain. First of all, it makes sense that it was more important for our ancestors to be vigilant and mentally fit prior to eating as opposed to after eating, as they needed to be alert and smart to track down animals and get a hold of food. The behaviors of our ancestors as it relates to foraging and food intake may have contributed to shaping how the human mind operates pre- and post- food intake. Second, building on the last point, following a meal, the body allocates resources to digestive and metabolic functions. It’s less concerned about brain-related processes.

Third, meal consumption, particularly the consumption of carbohydrate-rich meals, causes a rise in blood glucose, something that could undermine brain health and function. At least I know my brain doesn’t do well on a high-carbohydrate diet. It gets sluggish. Fourth, the consumption of food is accompanied by an immune response (1). If this immune response is not well regulated, chronic inflammation may ensue, something that could obviously affect the brain via the leakage of proinflammatory compounds across the blood-brain barrier.

… but, after some time, brain function gets worse

Needless to say, brain function will eventually get worse if one doesn’t eat for prolonged periods of time. Personally, I noticed a drop in my cognitive abilities as I was approaching the 40-hour mark. I definitely felt that my brain was craving some glucose. Would this feeling have passed if I just continued the fast and my brain potentially got better at utilizing ketones for energy? Perhaps… Maybe I’ll have to go for 80 hours next time 🙂

2) Fasting can help lower inflammation, stabilize the gut microbiome, and improve appetite control

As pointed out earlier, meal consumption is associated with increased inflammation. Particularly the consumption of processed foods is problematic in this regard. The consumption of food can in some instances also perturb the gut microbiome. A lot of westerners eat a very “disordered” diet. They consume different foods and spices every day and frequently take in excessive amounts of food. These behaviors can destabilize the gut microbiota.

A fast can help remedy some of these issues and “reset” one’s body.

Perhaps needless to say, if you already harbor a degraded, unhealthy microbiota, simply fasting for 20-40 hours is not going to change that. However, it may help suppress the inflammatory fire that’s burning inside you, at least temporarily. It could also help normalize your appetite.

… but, if the fast continues for a very long time, immunity and gut health could get worse

No good thing lasts forever. Just like humans, microbes need energy to survive. If one doesn’t feed one’s gut bugs, they will eventually wither and die. A malnourished immune system is no good either.

3) Inter-individual differences in microbiota composition, diet quality, and immune status can largely account for inter-individual differences in how people respond to fasting

There’s no doubt that we humans are well-adapted to go many hours without eating. We don’t need a regular influx of food in order to keep our bodies running. Why then do some people report that they feel lousy if they fast for 14-16 hours+? Personally, I strongly believe inter-individual differences in how people respond to fasting can largely be accounted for by inter-individual differences in microbiota composition, diet quality, and immune status.

A person who eats a species-appropriate diet, harbors a stable microbiota, and has a well-functioning immune system is much better equipped to endure semi-long periods of food deprivation than someone who eats a species-inappropriate diet, harbors an unstable microbiota, and is chronically inflamed. There are many reasons why this is the case. Among other things, due to the disordered state of his gut microbiota and the chronic inflammatory processes in his body, the latter person may get regular cravings for unhealthy foods and feel sluggish and fatigued. Moreover, his liver’s ability to produce glucose may be compromised.

Personally, I’ve noticed that there is a close connection between my microbiota/immune system and my food cravings/dietary behaviors. During periods when I don’t feel so great, my body is markedly less willing to go long periods without food.

Many contemporary people are used to consume a very carbohydrate heavy diet. Hence, their bodies’ ability to utilize non-glucose nutrients for fuel is compromised. These people may find it difficult to go long periods without eating, particularly if they are used to always consuming a carbohydrate-heavy breakfast early in the morning.

4) It feels weird to eat after a 40 hour fast

One of the things I noticed as I broke the 40-hour fast with a large meal right before I sat down to write this article is that it felt a little strange to eat again. It’s like my gut and brain didn’t really have a clear idea as to how much food I should take in. I could definitely feel that the fast had had an effect on my body.

5) It’s not healthy to eat all day long

When I was younger, I always ate shortly after I woke up in the morning. Not because I was always hungry in the morning, but rather because I had been misled by conventional wisdom into thinking that it’s important to get some nutrients into one’s system early in the day. If I wasn’t hungry, I pretty much forced some food into my body.

Perhaps needless to say, this approach didn’t get me very far. It’s truly perplexing that we humans have somehow gotten the idea that it’s healthy to eat a bowl of cereals at 8 A.M. every day. From an evolutionary perspective, this is a very abnormal practise.

I can definitely feel that my body functions better if I give my digestive system a rest every now and then. I’m not sure I’m going to do another 40-hour fast any time soon though, as I’m so fond of eating. If I am to do a really long fast, I like to have a lot of stuff to do so that I don’t get bored and start thinking about food. Another thing that keeps me from doing 24-hour+ fasts on a regular basis is that my body doesn’t always feel up to such a task.

Now I want to hear from you: When do you typically eat your first meal of the day? What’s the longest fast you’ve ever done?


  1. 8 days on juice, 5 days on water. Two seperate fasts. Both killed me in terms of the constant craving for high fat foods. Torture! I don’t do well, like some of my friends do (30 days happily and easily on juice), for me it’s a battle as I crave and think about eating ALL the time! Maybe this one will be better. Just at the end of day two, but to make it easier and more likely ill go the distance I’m adding bone broth to the equation – juice and bone broth with water fasting days in between. So no solid foods just liquid… More of a reset than a fast this time.

    • Lucy, you’d be much better off eliminating juice from your fast. Fruit juice is very, very high in fructose (Google sugar content in juice) and contains no fiber to slow absorption into your blood stream. This causes your blood sugar to spike which your body has to work very hard to counteract by dramatically increasing your insulin levels. Your pancreas will often overshoot insulin production (reference: http://www.nutritiontotheedge.com/how-blood-sugar-affects-your-health-and-energy/) which will then cause your blood sugar to go too low. This is known as a sugar crash or a carb crash (this happens with breads, pastas and other refined carbs, too). This will cause cranky Lucy to come out to play. And nobody likes cranky Lucy. Next time you fast, go water only or water and bone broth. You should find fasting to be much easier. Best of luck to you.

  2. I fast 14 to 16 hours on a daily basis. I usually eat breakfast around 10 or 11 a.m. and dinner around 6 p.m. (sometimes earlier) with nothing afterward. I usually eat twice a day and have no desire to snack between meals. I eat this way because it feels right to me. I have no desire to fast for longer periods of time. Everyone is different, but for me personally, I don’t feel it would be of any benefit.

    • That’s just about exactly what I do and I function very well on that routine. Even on a Saturday morning — I play volleyball from 9:00 am until 12:00, so only eat about 1:00 pm. But I feel great. In retrospect, though, I guess I am actually not completely fasting until lunch, because I do have a cup of coffee with a giant tablespoon of coconut oil. It still works well, though.

    • Sounds like a great set-up, Shary!

  3. Same here 🙂
    My first meal is lunch between 12 – 1 pm, then I have one or two other small meals until 8 pm latest, mostly it is 6 pm.
    I do this since the end of last year, and I do no ‘binge-eating’ anymore. I had such attacks very often before, and it was not nice.
    Now I lost 14 pounds (is that right? We say 7 kg in Germany) without eating healthy “rabbit food” only. 😉
    I still do eat what I want, even some fries or cake sometimes. But I don’t have as much craving for sweets or snacks as I had before. Now I have more vegetables and fibre, because I really WANT it. I don’t have to force me to do that, it came automatically.

    I feel definitely better with intermittent fasting than before, it changed my eating behaviour and I lost weight. This is my personal experience.
    I would not claim that it would be good for everyone else, but if you think it could, give it a try. 🙂

  4. I have in the past fasted for a couple of days without too much trouble. It helps that when I did it I was pretty well fat adapted by having been quite strict Paleo for a number of years, so I never really craved carbs. Another vital thing was that I was very well rested (two of these fasts took place on holiday) and I kept myself busy. If I ate at 8 pm one evening, it actually wasn’t that hard to go on all day and sleep that night, and the next day it was both a battle of boredom and will power to see how long that day could last. I think I might have managed going through a second day too. It was always more a mental issue than a physical one – it felt a bit weird, to be honest, not to eat.

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