Fasting

Before it became a productivity tool, a longevity hack, or a branded protocol with a trademarked name, fasting was just something we did. Not by choice, but by design. Before Uber Eats and grocery stores, before caloric abundance and 24/7 availability, human beings went without food all the time.

What’s now marketed as elite discipline was once the baseline. What’s new is the assumption that eating constantly is normal.

The Cultural Inheritance

Look across the major religious and cultural traditions, and you’ll find fasting almost everywhere. Muslims fast from dawn to sunset throughout Ramadan, a month-long practice observed by roughly two billion people each year. Christians fast through Lent, with the 40-day duration tracking the time Jesus reportedly fasted in the wilderness. Jews fast on Yom Kippur, Tisha B’Av, and several other days, with Yom Kippur involving 25 hours of total abstention from food and water. Hindus observe Ekadashi twice a month (the 11th day of each lunar fortnight) along with Navratri, Karva Chauth, and numerous other fasting days. Buddhists practice various forms of restriction, including the monastic tradition of not eating after midday. Jains have some of the most rigorous fasting traditions of any major religion, with practices ranging from one-day fasts to the extended fasts of Paryushan. Indigenous traditions across the Americas, Africa, Asia, and Oceania include vision quests, coming-of-age rites, and seasonal restriction practices, almost all of which involve substantial periods without food. Greek Orthodox Christians fast approximately 180 days per year if observing the full calendar.

The recurrence isn’t a coincidence. When unrelated cultures across thousands of years independently arrive at similar practices, the practice is doing something. The specific frameworks the traditions use to explain why they fast vary enormously (spiritual purification, devotion, discipline, atonement, communion with ancestors, alignment with seasonal cycles), but the underlying physiological state being induced is the same one our biology is designed to expect.

For most of human history, this wasn’t a question of religious observance or health optimisation. It was a question of food availability. Hunter-gatherer populations went days without successful hunts. Agricultural populations went through seasonal lean periods between harvests. Famine years were common enough that most adults experienced several across a lifetime. The cycling between feast and famine, abundance and scarcity, was the default human experience. The biological adaptations to handle it (efficient fat storage during plenty, metabolic flexibility during lack, the capacity to maintain function across days without food) are deeply built into our physiology.

What Changed

Three or four generations ago, the modern relationship with food began to develop. Refrigeration made it possible to keep perishable food fresh for weeks rather than days. The supermarket and global food distribution made any food available at any time of year. Industrial food processing produced shelf-stable, hyper-palatable products that didn’t exist in any human evolutionary context. And cultural messaging, much of it driven by the food industry’s commercial interest in selling more product more often, shifted the baseline expectation from “two or three meals a day, perhaps less” to “three meals plus snacks plus dessert plus late-night options.”

For decades, the nutritional mainstream told us that not eating would slow our metabolism, cause muscle loss, or send the body into “starvation mode.” Advice to eat three to six small meals a day was pushed in fitness circles, corporate wellness campaigns, and diet magazines alike. Intentionally not eating was framed as disordered or dangerous, unless it was accidental or due to poverty. The idea that abstaining from food could improve health was treated like heresy.

Most of this advice traces to limited research, industry marketing, or pure cultural inheritance from an era when “more food more often” was associated with health and prosperity. It also coincided with the rise of metabolic disease (obesity, type 2 diabetes, fatty liver disease, cardiovascular disease) to historically unprecedented levels in the populations that adopted the advice most thoroughly. The correlation isn’t proof of causation, but it should at minimum make us question the assumption that constant feeding is the optimal default for a species whose entire evolutionary history involved cycling between fed and fasted states.

Now that fasting has returned to popularity through biohacking blogs and Silicon Valley podcasts, it’s easy to forget that it’s not a new trend, and depending on how you use it, it can either restore balance or deepen dysfunction. As with everything, the dose makes the poison.

The Two States

Underneath the cultural, religious, and commercial framings, the actual physiology is straightforward. Your body has two governing metabolic states:

• The fed state is what happens after you eat. Blood sugar rises, insulin is released, nutrients are absorbed, and energy is partitioned into immediate use or storage. Growth pathways activate. Anabolic signalling (build, repair, replicate) goes up. The body is busy processing what just came in.
• The fasted state is what happens once the body has finished processing the last meal and starts drawing on stored reserves. Insulin drops. Glucagon rises. Stored glycogen is mobilised. Within roughly 12-18 hours, glycogen stores are largely depleted, and the body shifts toward burning fatty acids and producing ketone bodies. Cellular cleanup pathways (autophagy) ramp up. Anti-inflammatory signalling rises. The body is busy maintaining and repairing rather than processing and storing.

Both states are necessary. Spending too much time in the fed state means the body never gets to maintenance work, which appears to contribute to the slow accumulation of cellular damage we recognise as ageing and metabolic disease. Spending too much time in the fasted state means the body can’t grow, repair, or build the tissue it needs, which produces its own problems (muscle loss, hormonal dysregulation, reproductive issues, immune suppression at the extreme). The body expects to cycle between the two.

Constant low-grade snacking keeps insulin elevated and prevents the metabolic switch into the fasted state. This isn’t a moral failing on the part of snackers; it’s the predictable outcome of a food environment engineered to make snacking effortless and constant. The environment has changed without permission from biology.

What Fasting Is

At its simplest, fasting is a state where the body transitions from using immediate energy (glucose from food) to mobilising stored energy (fatty acids and ketones from your own tissue). It is the activation of the fasted state we just described, sustained long enough for the body to access its full set of fasted-state adaptations.

During this transition:

• Hormones shift (insulin down, glucagon up, growth hormone elevation, catecholamine release)
• Cellular repair ramps up (autophagy, the process by which cells dismantle and recycle damaged components, increases)
• Appetite hormones recalibrate (ghrelin patterns normalise rather than rising on a schedule)
• Cognition often sharpens (a combination of catecholamine release and the brain’s adaptation to ketone fuel)
• Inflammation typically decreases (multiple anti-inflammatory pathways activate)

All of this depends on the starting state of your body and mind. A metabolically healthy person fasting for 16 hours has a very different physiological experience from a metabolically compromised person attempting the same protocol. A person with adequate sleep, low chronic stress, and a balanced relationship with food responds differently from a person who’s chronically sleep-deprived, anxious, or has a history of disordered eating. The same intervention, deployed in different contexts, produces different outcomes.

What Fasting Isn’t

• Fasting isn’t spiritual enlightenment, permanent fat loss, or instant health. Those are marketing side effects.
• Fasting isn’t appropriate for everyone. Pregnant women, breastfeeding women, children, people with a history of eating disorders, people with type 1 diabetes, and people taking certain medications all have legitimate reasons to avoid or modify fasting practices. We’ll cover this in detail in the Game Plan.
• Fasting isn’t a substitute for good nutrition. The benefits compound when the eating that surrounds the fasting is also good. Fasting your way through a junk-food diet produces worse outcomes than just eating a reasonable diet without any fasting.
• Fasting isn’t a license for chronic energy deficit. The distinction between fasting (cycling between fed and fasted states) and chronic caloric restriction (perpetual under-eating) matters substantially. Fasting done well includes refeeding phases where the body gets what it needs.
• Fasting isn’t a willpower test. The biology of hunger and metabolic switching is more interesting than the moral framing of “discipline” suggests. Most people who struggle with fasting are struggling because their metabolic flexibility is compromised, not because they lack character. The flexibility is trainable.

What You’ll Find Here

We start with fundamentals, not to dumb anything down, but to unlearn the strange mythology that’s grown around fasting. From there, we move into the practical question of how to integrate fasting into a modern life that wasn’t designed for it. We cover ketosis as the metabolic state that underpins extended fasting and certain dietary approaches. We provide a substantial reference cheatsheet for specific protocols. Then we go deeper into the research for those who want it.

Fasting, done well, is about giving the body space to do what it already knows how to do, without interference. You don’t have to treat it like a spiritual ritual or a war against food. You can just treat it like a tool, and like any good tool, it depends on the hands that hold it.