--- author: Sam Collins's Weblog date: 2026-07-03 category: Health, Family & Life source: https://samcollins.blog/fasting --- # Why Fast A friend of mine asked for my notes on why I fast. Looking at my smattering of bookmarks and notes, I decided to just take 20 mins to write this up. I'll make it better, this is a v1. I did my first fast in 2017 and since then I've done at least a 3 or 5 day fast once a year, sometimes fasting 3-4 times a year. I've read up a bunch on it, but I'm no expert. Here's a few of my drivers in my own words and pointers to resources that your LLM can summarize. ## Why do I fast Originally, it was pure curiosity - I was reading it was healthy and I like trying out hard new things that can be profoundly good for oneself, ESPECIALLY when emotional mind squirms at the idea of it or have I a sense of discomfort about doing it. But here's the short list of reasons I'd say for why I regularly do it, no in order: 1. **Autophagy.** Fasting accelerates a natural process called autophagy. The way I understand this is: once essential minerals stop arriving via food, your body goes into survival mode, and damaged or useless cells that still contain useful ingredients get recycled so you body can consume those instead. 2. **Overcoming hunger.** Instead of feeling like I'll die if I don't eat, I've learned that hunger is just another sensation in my body, when a chemical called ghrelin is produced on schedule to motivate this living organism to eat. You won't die if you don't eat today, and tomorrow. You do need water, and some salt, but you have enough calories stored in body fat to survive for weeks without eating food. 3. **Ketosis.** Your brain can run on two types of fuel, remarkably. Glucose or ketones. On low carb diets or when fasting, your body isn't consuming enough carbs/sugars to generate enough glucose, and instead your brain switches to its backup fuel (ketones). When you brain is running on ketones, it feels different. I feel calmer, clearer, and I generally prefer my mood in this state. 4. **Beating addictions.** I found a trick to quitting cigarettes back in the day, and I also do this when I want to reset my diet and get back to a lean low carb, low calorie diet. I fast for 3 days. During that time, my body wants food more than anything else. Not cigarettes, not coffee, not pizza. My body craves just eating a crunchy apple or a piece of perfectly cooked steak. As I come out of the fast, I allow myself to slowly consume just the minimum to satiate that craving - I get to eat carrots and zuchini cooked in bone broth. I never realized lettuce had flavor until I started fasting, but when you haven't eaten in days, a piece of lettuce is delicious. This process gives me a few days to get some distance from the real enemy (smoking, junk food) and while it doesn't guarantee success, it's set me on well on my way. 5. **Longevity.** Caloric restriction and fasting is probably the biggest known lever to improve your life span. Perhaps counter-intuitive to your emotional reaction, but it is actually unhealthy to eat well every day, and healthier to generally be hungry. ## What do I advise to people starting out? 1. I say do a 48 hour fast minimum, but ideally 3 days. Best to aim for 2 days, and tell yourself if you're feeling good you'll put in another day. 2. I don't recommend a 24 hour fast - you can be in peak discomfort around there - and many of the benefits really start to appear in the second and third days. You really want to punch through the 24-36 peak difficulty and see how it gets easier beyond that. 3. The goal is zero calories (but also no zero calorie artificial sweeteners) so there is no insulin reaction/digestion process started. So water and salt are fine (and necessary), but so too is black coffee or herbal tea etc. Just no milk etc. 4. You will need a lot more water than normal. Your body doesn't hold on to it as well, so be prepared to sip water regularly. I drink 2-4 litres per day, and you will need 1-2 teaspoons of salt per day. You will lose a lot of weight, but a lot of it is just your body no longer holding as much water, and that will come back once you start eating again, so don't get too excited by the weighing scales. 5. When you resume eating, don't just eat whatever you want . Your digestive system has essentially gone asleep and needs time to wake up. It's worth knowing that you can actually die if you screw this part up (e.g. if fast for 10 days and then eat a ton of carbs/sugar). I always start with a cup of bone broth, then 3-6 hours later, a bowl of bone broth soup with carrots and zuchini and a boiled egg or two. Only the following day am I eating more regular food, but I also try to return to a healthier diet of mostly vegetables and protein, usually skipping breakfast and only eating between 2-6pm. 6. When you fast, you want to avoid high cardio activity (where it needs carbs) and keep your heart rate in zone 1 and 2 (where it will be burning fats). Lots of long walks and slow bike rides are wonderful. 7. When I fast for 5 days, I do continue strength training. I try to keep the intensity high (lifting heavy) but I reduce the volume of work (2 sets instead of 3 or 4) and duration of workout I do (e.g. 30 mins instead of 60). 8. Surprisingly, my sleep when I'm fasting is actually terrible, which really sucks, so be prepared for this, and have plenty of books and a reading light beside your bed :) ## What did I read them helped me figure out what do? A bunch, here's some pointers I've pulled together quickly. I obviously wasn't convinced by these quotes, but by reading what these people had read to arrive at such conclusions. These are just the signposts: * "I am discovering from the literature (under Art De Vany's guidance and based on his ideas on metabolic switches) that three meals a day is for morons --we need episodes of hunger punctuated brief by periods of replenishing" **—Nassem Taleb:** https://fooledbyrandomness.com/notebook.htm * "Hungry animals become more resilient and better able to survive ... Excess weight is a leading risk factor for both cancer cases and deaths, second only to smoking ... Caloric restriction is the low-hanging fruit of cancer prevention, right up there with quitting smoking ... Fasting, or a fasting-like diet, increases the ability of normal cells to resist chemotherapy, while rendering cancer cells more vulnerable to the treatment." **—Petter Attia**, Outlive; notes by **Derek Sivers**: https://sive.rs/book/Outlive * "In the 1930s, investigators wanted to do an experiment to see if stunted growth rates during the Great Depression might impact lifespan. They tested this in rats by feeding them less food than they would normally eat. To their surprise, this actually made the rats live longer! This was a seminal discovery. For the first time, we changed the environment of an animal to make it live longer than it normally would." **—Laura Deming:** https://www.ldeming.com/longevityfaq#caloricrestriction * **Andrej Karpathy**'s own research and experimentation: https://karpathy.github.io/2020/06/11/biohacking-lite/ * Normal folks on HackerNews sharing their own personal anecdotes on fasting: https://news.ycombinator.com/item?id=34324567 * Peer reviewed research papers, e.g. https://pubmed.ncbi.nlm.nih.gov/24905167/