I fasted for three days. Here’s what actually happened.

Hunters could go days without a successful kill, crops could fail, and entire communities regularly experienced periods of scarcity. As a result, the human body evolved not only to survive without food for extended periods, but to adapt to it.

Many ancient cultures also fasted deliberately. The Ancient Greeks believed periods without food could improve physical and mental health, while major religions including Christianity, Islam, Buddhism, and Hinduism incorporated fasting into spiritual practice. In Portugal, fasting was also a familiar part of life for generations, particularly through Catholic traditions such as Lent and Good Friday, where periods of abstinence were observed as acts of discipline and reflection.

Unlike the people who fasted before me for religious devotion or necessity, I was driven mainly by curiosity. At around seven o’clock one evening, I finished my last meal and began a challenge I had been meaning to attempt for months.

A three-day fast had been on my list for a while, although I hadn’t planned to start that particular evening. Like many people, I had heard about the potential health benefits, including deep cellular recycling (autophagy), improvements in immune health, and longevity. While the science interested me, it wasn’t my primary motivation. More than anything, I was drawn to the challenge itself and wanted to see whether I could successfully complete three days without food.

The following morning was always going to be the easiest part. I’ve never been someone who feels the need to eat breakfast as soon as I wake up. In fact, I can comfortably go several hours without eating after getting out of bed. The real challenge usually comes later. Once I start eating, my appetite seems to wake up and my taste buds come alive.

By the time I woke up, I had already gone more than 12 hours without food. My body had processed its last meal and begun accessing stored glycogen for energy. Glycogen is the storage form of glucose in the body. When we eat carbohydrates, some of that glucose is stored in the liver and muscles, where it can later be used for energy between meals and during physical activity.

At the time, I was already cutting and eating in a calorie deficit, so my body was used to not being constantly full. Throughout the fast, I drank only water, black coffee, electrolytes, and flavoured sparkling water with virtually no calories.

The electrolytes proved particularly important. As insulin levels fall during fasting, the body releases more sodium and water through the kidneys, which is one reason people often lose weight so quickly during the first few days of a fast. Replacing electrolytes such as sodium, potassium, and magnesium can help prevent fatigue, headaches, muscle cramps, and other unwanted side effects.

As the first day continued, my body began making a more significant metabolic shift. Glycogen stores gradually became depleted and ketosis began. To meet its energy demands, the body relied more heavily on stored fat. Fat cells released fatty acids into the bloodstream, and the liver converted some of them into ketones, an alternative fuel source capable of powering both the brain and body.

Throughout the fast, I continued going to the gym and stuck to my normal routine. I also went running each day, although I kept the runs relatively short at around five kilometres. I expected my performance to drop off once I stopped eating, but that wasn’t my experience. My workouts felt largely normal, although I did feel slightly more sore than usual.

The only major difference I noticed was that I started going to bed earlier. Without dinner, there didn’t seem to be much reason to stay awake late.

The hunger I expected on the second day never really arrived. Instead, the biggest challenge was boredom. Well, boredom, and not being able to put a splash of milk in my morning coffee.

Meals divide up the day. Breakfast starts it, lunch provides a break in the middle, and dinner gives you something to look forward to at the end. Once I removed all three, I realised how much of daily life revolves around food. There was no anticipation about what I would eat next and no reward waiting for me at the end of the day.

By now, ketone production was increasing as my body became more efficient at using fat for fuel. This stage is also where fasting enthusiasts often begin talking about autophagy. The word comes from the Greek words auto (self) and phagein (to eat), literally meaning “self-eating”. That sounds alarming, but it is actually one of the body’s normal maintenance systems. You can think of it as a cellular recycling programme.

Throughout the day, cells accumulate damaged and worn-out components. During a fast, cells become more efficient and begin breaking down some of this material so it can be reused. One common analogy is that autophagy is like cleaning out a garage. Instead of buying new tools, you first get rid of broken items and salvage anything useful from old equipment.

Interest in autophagy surged after Japanese scientist Yoshinori Ohsumi won the 2016 Nobel Prize for helping explain how the process works. His research opened the door to studies exploring autophagy’s role in longevity and ageing, as well as its potential role in helping prevent cancer, Parkinson’s disease, Alzheimer’s disease, and other age-related conditions.

By the third day, I was cruising along just as comfortably as I had been on the second. Again, the challenge wasn’t hunger. It was the absence of excitement. The routine of eating had disappeared and, with it, a surprising amount of anticipation and pleasure that normally punctuates the day.

One of the most obvious effects of the fast was the weight loss. Within a couple of days, I had lost a few kilograms. While it might sound like I had suddenly shed several kilograms of body fat, that wasn’t the case. A large portion of the weight loss came from water.

As glycogen stores in the liver and muscles became depleted, the water attached to them was lost as well. The result was a rapid drop on the scales and a noticeably flatter appearance, particularly in my muscles.

Looking in the mirror, I appeared noticeably smaller, even after training. My muscles looked less full and less pumped than usual. However, once I resumed eating and completed my first workout afterwards, that fullness quickly returned. Carbohydrates replenished glycogen stores, glycogen pulled water back into the muscles, and my physique looked much more normal again. The before and after almost looked like the sort of fitness transformation that usually takes months to achieve.

By the end, I wasn’t fantasising about fast food or takeaway meals. If anything, I found myself appreciating the idea of simple, healthy food more than ever. The fast seemed to reset my cravings, and eating clean felt more appealing than it had before.

When it finally came time to break the fast, everyone online seemed to recommend bone broth as the ideal first meal after an extended fast. Unfortunately, I couldn’t find any in the supermarkets around me, so I broke my fast with soup, scrambled eggs, and some fruit instead.

Our ancestors didn’t fast because it was fashionable or because a podcast told them to. They fasted because life often gave them no choice. Three days without food offered a small glimpse into a reality that was once normal for much of humanity, and it left me appreciating both the resilience of the human body and the simple pleasure of a good healthy meal.

Read more articles from Jay Costa Owen: Exploring Gaia one kilometre at a time